Fix oversight in previous error-reporting patch; mustn't pfree path string

[PostgreSQL.git] / src / backend / utils / sort / tuplesort.c

/*-------------------------------------------------------------------------
 *
 * tuplesort.c
 *        Generalized tuple sorting routines.
 *
 * This module handles sorting of heap tuples, index tuples, or single
 * Datums (and could easily support other kinds of sortable objects,
 * if necessary).  It works efficiently for both small and large amounts
 * of data.  Small amounts are sorted in-memory using qsort().  Large
 * amounts are sorted using temporary files and a standard external sort
 * algorithm.
 *
 * See Knuth, volume 3, for more than you want to know about the external
 * sorting algorithm.  We divide the input into sorted runs using replacement
 * selection, in the form of a priority tree implemented as a heap
 * (essentially his Algorithm 5.2.3H), then merge the runs using polyphase
 * merge, Knuth's Algorithm 5.4.2D.  The logical "tapes" used by Algorithm D
 * are implemented by logtape.c, which avoids space wastage by recycling
 * disk space as soon as each block is read from its "tape".
 *
 * We do not form the initial runs using Knuth's recommended replacement
 * selection data structure (Algorithm 5.4.1R), because it uses a fixed
 * number of records in memory at all times.  Since we are dealing with
 * tuples that may vary considerably in size, we want to be able to vary
 * the number of records kept in memory to ensure full utilization of the
 * allowed sort memory space.  So, we keep the tuples in a variable-size
 * heap, with the next record to go out at the top of the heap.  Like
 * Algorithm 5.4.1R, each record is stored with the run number that it
 * must go into, and we use (run number, key) as the ordering key for the
 * heap.  When the run number at the top of the heap changes, we know that
 * no more records of the prior run are left in the heap.
 *
 * The approximate amount of memory allowed for any one sort operation
 * is specified in kilobytes by the caller (most pass work_mem).  Initially,
 * we absorb tuples and simply store them in an unsorted array as long as
 * we haven't exceeded workMem.  If we reach the end of the input without
 * exceeding workMem, we sort the array using qsort() and subsequently return
 * tuples just by scanning the tuple array sequentially.  If we do exceed
 * workMem, we construct a heap using Algorithm H and begin to emit tuples
 * into sorted runs in temporary tapes, emitting just enough tuples at each
 * step to get back within the workMem limit.  Whenever the run number at
 * the top of the heap changes, we begin a new run with a new output tape
 * (selected per Algorithm D).  After the end of the input is reached,
 * we dump out remaining tuples in memory into a final run (or two),
 * then merge the runs using Algorithm D.
 *
 * When merging runs, we use a heap containing just the frontmost tuple from
 * each source run; we repeatedly output the smallest tuple and insert the
 * next tuple from its source tape (if any).  When the heap empties, the merge
 * is complete.  The basic merge algorithm thus needs very little memory ---
 * only M tuples for an M-way merge, and M is constrained to a small number.
 * However, we can still make good use of our full workMem allocation by
 * pre-reading additional tuples from each source tape.  Without prereading,
 * our access pattern to the temporary file would be very erratic; on average
 * we'd read one block from each of M source tapes during the same time that
 * we're writing M blocks to the output tape, so there is no sequentiality of
 * access at all, defeating the read-ahead methods used by most Unix kernels.
 * Worse, the output tape gets written into a very random sequence of blocks
 * of the temp file, ensuring that things will be even worse when it comes
 * time to read that tape.      A straightforward merge pass thus ends up doing a
 * lot of waiting for disk seeks.  We can improve matters by prereading from
 * each source tape sequentially, loading about workMem/M bytes from each tape
 * in turn.  Then we run the merge algorithm, writing but not reading until
 * one of the preloaded tuple series runs out.  Then we switch back to preread
 * mode, fill memory again, and repeat.  This approach helps to localize both
 * read and write accesses.
 *
 * When the caller requests random access to the sort result, we form
 * the final sorted run on a logical tape which is then "frozen", so
 * that we can access it randomly.      When the caller does not need random
 * access, we return from tuplesort_performsort() as soon as we are down
 * to one run per logical tape.  The final merge is then performed
 * on-the-fly as the caller repeatedly calls tuplesort_getXXX; this
 * saves one cycle of writing all the data out to disk and reading it in.
 *
 * Before Postgres 8.2, we always used a seven-tape polyphase merge, on the
 * grounds that 7 is the "sweet spot" on the tapes-to-passes curve according
 * to Knuth's figure 70 (section 5.4.2).  However, Knuth is assuming that
 * tape drives are expensive beasts, and in particular that there will always
 * be many more runs than tape drives.  In our implementation a "tape drive"
 * doesn't cost much more than a few Kb of memory buffers, so we can afford
 * to have lots of them.  In particular, if we can have as many tape drives
 * as sorted runs, we can eliminate any repeated I/O at all.  In the current
 * code we determine the number of tapes M on the basis of workMem: we want
 * workMem/M to be large enough that we read a fair amount of data each time
 * we preread from a tape, so as to maintain the locality of access described
 * above.  Nonetheless, with large workMem we can have many tapes.
 *
 *
 * Portions Copyright (c) 1996-2008, PostgreSQL Global Development Group
 * Portions Copyright (c) 1994, Regents of the University of California
 *
 * IDENTIFICATION
 *        $PostgreSQL$
 *
 *-------------------------------------------------------------------------
 */

#include "postgres.h"

#include <limits.h>

#include "access/genam.h"
#include "access/nbtree.h"
#include "catalog/pg_amop.h"
#include "catalog/pg_operator.h"
#include "commands/tablespace.h"
#include "miscadmin.h"
#include "pg_trace.h"
#include "utils/datum.h"
#include "utils/logtape.h"
#include "utils/lsyscache.h"
#include "utils/memutils.h"
#include "utils/pg_rusage.h"
#include "utils/rel.h"
#include "utils/syscache.h"
#include "utils/tuplesort.h"


/* GUC variables */
#ifdef TRACE_SORT
bool            trace_sort = false;
#endif

#define HEAP_SORT       0
#define INDEX_SORT      1
#define DATUM_SORT      2

#ifdef DEBUG_BOUNDED_SORT
bool            optimize_bounded_sort = true;
#endif


/*
 * The objects we actually sort are SortTuple structs.  These contain
 * a pointer to the tuple proper (might be a MinimalTuple or IndexTuple),
 * which is a separate palloc chunk --- we assume it is just one chunk and
 * can be freed by a simple pfree().  SortTuples also contain the tuple's
 * first key column in Datum/nullflag format, and an index integer.
 *
 * Storing the first key column lets us save heap_getattr or index_getattr
 * calls during tuple comparisons.      We could extract and save all the key
 * columns not just the first, but this would increase code complexity and
 * overhead, and wouldn't actually save any comparison cycles in the common
 * case where the first key determines the comparison result.  Note that
 * for a pass-by-reference datatype, datum1 points into the "tuple" storage.
 *
 * When sorting single Datums, the data value is represented directly by
 * datum1/isnull1.      If the datatype is pass-by-reference and isnull1 is false,
 * then datum1 points to a separately palloc'd data value that is also pointed
 * to by the "tuple" pointer; otherwise "tuple" is NULL.
 *
 * While building initial runs, tupindex holds the tuple's run number.  During
 * merge passes, we re-use it to hold the input tape number that each tuple in
 * the heap was read from, or to hold the index of the next tuple pre-read
 * from the same tape in the case of pre-read entries.  tupindex goes unused
 * if the sort occurs entirely in memory.
 */
typedef struct
{
        void       *tuple;                      /* the tuple proper */
        Datum           datum1;                 /* value of first key column */
        bool            isnull1;                /* is first key column NULL? */
        int                     tupindex;               /* see notes above */
} SortTuple;


/*
 * Possible states of a Tuplesort object.  These denote the states that
 * persist between calls of Tuplesort routines.
 */
typedef enum
{
        TSS_INITIAL,                            /* Loading tuples; still within memory limit */
        TSS_BOUNDED,                            /* Loading tuples into bounded-size heap */
        TSS_BUILDRUNS,                          /* Loading tuples; writing to tape */
        TSS_SORTEDINMEM,                        /* Sort completed entirely in memory */
        TSS_SORTEDONTAPE,                       /* Sort completed, final run is on tape */
        TSS_FINALMERGE                          /* Performing final merge on-the-fly */
} TupSortStatus;

/*
 * Parameters for calculation of number of tapes to use --- see inittapes()
 * and tuplesort_merge_order().
 *
 * In this calculation we assume that each tape will cost us about 3 blocks
 * worth of buffer space (which is an underestimate for very large data
 * volumes, but it's probably close enough --- see logtape.c).
 *
 * MERGE_BUFFER_SIZE is how much data we'd like to read from each input
 * tape during a preread cycle (see discussion at top of file).
 */
#define MINORDER                6               /* minimum merge order */
#define TAPE_BUFFER_OVERHEAD            (BLCKSZ * 3)
#define MERGE_BUFFER_SIZE                       (BLCKSZ * 32)

/*
 * Private state of a Tuplesort operation.
 */
struct Tuplesortstate
{
        TupSortStatus status;           /* enumerated value as shown above */
        int                     nKeys;                  /* number of columns in sort key */
        bool            randomAccess;   /* did caller request random access? */
        bool            bounded;                /* did caller specify a maximum number of
                                                                 * tuples to return? */
        bool            boundUsed;              /* true if we made use of a bounded heap */
        int                     bound;                  /* if bounded, the maximum number of tuples */
        long            availMem;               /* remaining memory available, in bytes */
        long            allowedMem;             /* total memory allowed, in bytes */
        int                     maxTapes;               /* number of tapes (Knuth's T) */
        int                     tapeRange;              /* maxTapes-1 (Knuth's P) */
        MemoryContext sortcontext;      /* memory context holding all sort data */
        LogicalTapeSet *tapeset;        /* logtape.c object for tapes in a temp file */

        /*
         * These function pointers decouple the routines that must know what kind
         * of tuple we are sorting from the routines that don't need to know it.
         * They are set up by the tuplesort_begin_xxx routines.
         *
         * Function to compare two tuples; result is per qsort() convention, ie:
         * <0, 0, >0 according as a<b, a=b, a>b.  The API must match
         * qsort_arg_comparator.
         */
        int                     (*comparetup) (const SortTuple *a, const SortTuple *b,
                                                                                   Tuplesortstate *state);

        /*
         * Function to copy a supplied input tuple into palloc'd space and set up
         * its SortTuple representation (ie, set tuple/datum1/isnull1).  Also,
         * state->availMem must be decreased by the amount of space used for the
         * tuple copy (note the SortTuple struct itself is not counted).
         */
        void            (*copytup) (Tuplesortstate *state, SortTuple *stup, void *tup);

        /*
         * Function to write a stored tuple onto tape.  The representation of the
         * tuple on tape need not be the same as it is in memory; requirements on
         * the tape representation are given below.  After writing the tuple,
         * pfree() the out-of-line data (not the SortTuple struct!), and increase
         * state->availMem by the amount of memory space thereby released.
         */
        void            (*writetup) (Tuplesortstate *state, int tapenum,
                                                                                 SortTuple *stup);

        /*
         * Function to read a stored tuple from tape back into memory. 'len' is
         * the already-read length of the stored tuple.  Create a palloc'd copy,
         * initialize tuple/datum1/isnull1 in the target SortTuple struct, and
         * decrease state->availMem by the amount of memory space consumed.
         */
        void            (*readtup) (Tuplesortstate *state, SortTuple *stup,
                                                                                int tapenum, unsigned int len);

        /*
         * Function to reverse the sort direction from its current state. (We
         * could dispense with this if we wanted to enforce that all variants
         * represent the sort key information alike.)
         */
        void            (*reversedirection) (Tuplesortstate *state);

        /*
         * This array holds the tuples now in sort memory.      If we are in state
         * INITIAL, the tuples are in no particular order; if we are in state
         * SORTEDINMEM, the tuples are in final sorted order; in states BUILDRUNS
         * and FINALMERGE, the tuples are organized in "heap" order per Algorithm
         * H.  (Note that memtupcount only counts the tuples that are part of the
         * heap --- during merge passes, memtuples[] entries beyond tapeRange are
         * never in the heap and are used to hold pre-read tuples.)  In state
         * SORTEDONTAPE, the array is not used.
         */
        SortTuple  *memtuples;          /* array of SortTuple structs */
        int                     memtupcount;    /* number of tuples currently present */
        int                     memtupsize;             /* allocated length of memtuples array */

        /*
         * While building initial runs, this is the current output run number
         * (starting at 0).  Afterwards, it is the number of initial runs we made.
         */
        int                     currentRun;

        /*
         * Unless otherwise noted, all pointer variables below are pointers to
         * arrays of length maxTapes, holding per-tape data.
         */

        /*
         * These variables are only used during merge passes.  mergeactive[i] is
         * true if we are reading an input run from (actual) tape number i and
         * have not yet exhausted that run.  mergenext[i] is the memtuples index
         * of the next pre-read tuple (next to be loaded into the heap) for tape
         * i, or 0 if we are out of pre-read tuples.  mergelast[i] similarly
         * points to the last pre-read tuple from each tape.  mergeavailslots[i]
         * is the number of unused memtuples[] slots reserved for tape i, and
         * mergeavailmem[i] is the amount of unused space allocated for tape i.
         * mergefreelist and mergefirstfree keep track of unused locations in the
         * memtuples[] array.  The memtuples[].tupindex fields link together
         * pre-read tuples for each tape as well as recycled locations in
         * mergefreelist. It is OK to use 0 as a null link in these lists, because
         * memtuples[0] is part of the merge heap and is never a pre-read tuple.
         */
        bool       *mergeactive;        /* active input run source? */
        int                *mergenext;          /* first preread tuple for each source */
        int                *mergelast;          /* last preread tuple for each source */
        int                *mergeavailslots;    /* slots left for prereading each tape */
        long       *mergeavailmem;      /* availMem for prereading each tape */
        int                     mergefreelist;  /* head of freelist of recycled slots */
        int                     mergefirstfree; /* first slot never used in this merge */

        /*
         * Variables for Algorithm D.  Note that destTape is a "logical" tape
         * number, ie, an index into the tp_xxx[] arrays.  Be careful to keep
         * "logical" and "actual" tape numbers straight!
         */
        int                     Level;                  /* Knuth's l */
        int                     destTape;               /* current output tape (Knuth's j, less 1) */
        int                *tp_fib;                     /* Target Fibonacci run counts (A[]) */
        int                *tp_runs;            /* # of real runs on each tape */
        int                *tp_dummy;           /* # of dummy runs for each tape (D[]) */
        int                *tp_tapenum;         /* Actual tape numbers (TAPE[]) */
        int                     activeTapes;    /* # of active input tapes in merge pass */

        /*
         * These variables are used after completion of sorting to keep track of
         * the next tuple to return.  (In the tape case, the tape's current read
         * position is also critical state.)
         */
        int                     result_tape;    /* actual tape number of finished output */
        int                     current;                /* array index (only used if SORTEDINMEM) */
        bool            eof_reached;    /* reached EOF (needed for cursors) */

        /* markpos_xxx holds marked position for mark and restore */
        long            markpos_block;  /* tape block# (only used if SORTEDONTAPE) */
        int                     markpos_offset; /* saved "current", or offset in tape block */
        bool            markpos_eof;    /* saved "eof_reached" */

        /*
         * These variables are specific to the MinimalTuple case; they are set by
         * tuplesort_begin_heap and used only by the MinimalTuple routines.
         */
        TupleDesc       tupDesc;
        ScanKey         scanKeys;               /* array of length nKeys */

        /*
         * These variables are specific to the IndexTuple case; they are set by
         * tuplesort_begin_index_xxx and used only by the IndexTuple routines.
         */
        Relation        indexRel;               /* index being built */

        /* These are specific to the index_btree subcase: */
        ScanKey         indexScanKey;
        bool            enforceUnique;  /* complain if we find duplicate tuples */

        /* These are specific to the index_hash subcase: */
        uint32          hash_mask;              /* mask for sortable part of hash code */

        /*
         * These variables are specific to the Datum case; they are set by
         * tuplesort_begin_datum and used only by the DatumTuple routines.
         */
        Oid                     datumType;
        FmgrInfo        sortOpFn;               /* cached lookup data for sortOperator */
        int                     sortFnFlags;    /* equivalent to sk_flags */
        /* we need typelen and byval in order to know how to copy the Datums. */
        int                     datumTypeLen;
        bool            datumTypeByVal;

        /*
         * Resource snapshot for time of sort start.
         */
#ifdef TRACE_SORT
        PGRUsage        ru_start;
#endif
};

#define COMPARETUP(state,a,b)   ((*(state)->comparetup) (a, b, state))
#define COPYTUP(state,stup,tup) ((*(state)->copytup) (state, stup, tup))
#define WRITETUP(state,tape,stup)       ((*(state)->writetup) (state, tape, stup))
#define READTUP(state,stup,tape,len) ((*(state)->readtup) (state, stup, tape, len))
#define REVERSEDIRECTION(state) ((*(state)->reversedirection) (state))
#define LACKMEM(state)          ((state)->availMem < 0)
#define USEMEM(state,amt)       ((state)->availMem -= (amt))
#define FREEMEM(state,amt)      ((state)->availMem += (amt))

/*
 * NOTES about on-tape representation of tuples:
 *
 * We require the first "unsigned int" of a stored tuple to be the total size
 * on-tape of the tuple, including itself (so it is never zero; an all-zero
 * unsigned int is used to delimit runs).  The remainder of the stored tuple
 * may or may not match the in-memory representation of the tuple ---
 * any conversion needed is the job of the writetup and readtup routines.
 *
 * If state->randomAccess is true, then the stored representation of the
 * tuple must be followed by another "unsigned int" that is a copy of the
 * length --- so the total tape space used is actually sizeof(unsigned int)
 * more than the stored length value.  This allows read-backwards.      When
 * randomAccess is not true, the write/read routines may omit the extra
 * length word.
 *
 * writetup is expected to write both length words as well as the tuple
 * data.  When readtup is called, the tape is positioned just after the
 * front length word; readtup must read the tuple data and advance past
 * the back length word (if present).
 *
 * The write/read routines can make use of the tuple description data
 * stored in the Tuplesortstate record, if needed.      They are also expected
 * to adjust state->availMem by the amount of memory space (not tape space!)
 * released or consumed.  There is no error return from either writetup
 * or readtup; they should ereport() on failure.
 *
 *
 * NOTES about memory consumption calculations:
 *
 * We count space allocated for tuples against the workMem limit, plus
 * the space used by the variable-size memtuples array.  Fixed-size space
 * is not counted; it's small enough to not be interesting.
 *
 * Note that we count actual space used (as shown by GetMemoryChunkSpace)
 * rather than the originally-requested size.  This is important since
 * palloc can add substantial overhead.  It's not a complete answer since
 * we won't count any wasted space in palloc allocation blocks, but it's
 * a lot better than what we were doing before 7.3.
 */


static Tuplesortstate *tuplesort_begin_common(int workMem, bool randomAccess);
static void puttuple_common(Tuplesortstate *state, SortTuple *tuple);
static void inittapes(Tuplesortstate *state);
static void selectnewtape(Tuplesortstate *state);
static void mergeruns(Tuplesortstate *state);
static void mergeonerun(Tuplesortstate *state);
static void beginmerge(Tuplesortstate *state);
static void mergepreread(Tuplesortstate *state);
static void mergeprereadone(Tuplesortstate *state, int srcTape);
static void dumptuples(Tuplesortstate *state, bool alltuples);
static void make_bounded_heap(Tuplesortstate *state);
static void sort_bounded_heap(Tuplesortstate *state);
static void tuplesort_heap_insert(Tuplesortstate *state, SortTuple *tuple,
                                          int tupleindex, bool checkIndex);
static void tuplesort_heap_siftup(Tuplesortstate *state, bool checkIndex);
static unsigned int getlen(Tuplesortstate *state, int tapenum, bool eofOK);
static void markrunend(Tuplesortstate *state, int tapenum);
static int comparetup_heap(const SortTuple *a, const SortTuple *b,
                                Tuplesortstate *state);
static void copytup_heap(Tuplesortstate *state, SortTuple *stup, void *tup);
static void writetup_heap(Tuplesortstate *state, int tapenum,
                          SortTuple *stup);
static void readtup_heap(Tuplesortstate *state, SortTuple *stup,
                         int tapenum, unsigned int len);
static void reversedirection_heap(Tuplesortstate *state);
static int comparetup_index_btree(const SortTuple *a, const SortTuple *b,
                                 Tuplesortstate *state);
static int comparetup_index_hash(const SortTuple *a, const SortTuple *b,
                                 Tuplesortstate *state);
static void copytup_index(Tuplesortstate *state, SortTuple *stup, void *tup);
static void writetup_index(Tuplesortstate *state, int tapenum,
                           SortTuple *stup);
static void readtup_index(Tuplesortstate *state, SortTuple *stup,
                          int tapenum, unsigned int len);
static void reversedirection_index_btree(Tuplesortstate *state);
static void reversedirection_index_hash(Tuplesortstate *state);
static int comparetup_datum(const SortTuple *a, const SortTuple *b,
                                 Tuplesortstate *state);
static void copytup_datum(Tuplesortstate *state, SortTuple *stup, void *tup);
static void writetup_datum(Tuplesortstate *state, int tapenum,
                           SortTuple *stup);
static void readtup_datum(Tuplesortstate *state, SortTuple *stup,
                          int tapenum, unsigned int len);
static void reversedirection_datum(Tuplesortstate *state);
static void free_sort_tuple(Tuplesortstate *state, SortTuple *stup);


/*
 *              tuplesort_begin_xxx
 *
 * Initialize for a tuple sort operation.
 *
 * After calling tuplesort_begin, the caller should call tuplesort_putXXX
 * zero or more times, then call tuplesort_performsort when all the tuples
 * have been supplied.  After performsort, retrieve the tuples in sorted
 * order by calling tuplesort_getXXX until it returns false/NULL.  (If random
 * access was requested, rescan, markpos, and restorepos can also be called.)
 * Call tuplesort_end to terminate the operation and release memory/disk space.
 *
 * Each variant of tuplesort_begin has a workMem parameter specifying the
 * maximum number of kilobytes of RAM to use before spilling data to disk.
 * (The normal value of this parameter is work_mem, but some callers use
 * other values.)  Each variant also has a randomAccess parameter specifying
 * whether the caller needs non-sequential access to the sort result.
 */

static Tuplesortstate *
tuplesort_begin_common(int workMem, bool randomAccess)
{
        Tuplesortstate *state;
        MemoryContext sortcontext;
        MemoryContext oldcontext;

        /*
         * Create a working memory context for this sort operation. All data
         * needed by the sort will live inside this context.
         */
        sortcontext = AllocSetContextCreate(CurrentMemoryContext,
                                                                                "TupleSort",
                                                                                ALLOCSET_DEFAULT_MINSIZE,
                                                                                ALLOCSET_DEFAULT_INITSIZE,
                                                                                ALLOCSET_DEFAULT_MAXSIZE);

        /*
         * Make the Tuplesortstate within the per-sort context.  This way, we
         * don't need a separate pfree() operation for it at shutdown.
         */
        oldcontext = MemoryContextSwitchTo(sortcontext);

        state = (Tuplesortstate *) palloc0(sizeof(Tuplesortstate));

#ifdef TRACE_SORT
        if (trace_sort)
                pg_rusage_init(&state->ru_start);
#endif

        state->status = TSS_INITIAL;
        state->randomAccess = randomAccess;
        state->bounded = false;
        state->boundUsed = false;
        state->allowedMem = workMem * 1024L;
        state->availMem = state->allowedMem;
        state->sortcontext = sortcontext;
        state->tapeset = NULL;

        state->memtupcount = 0;
        state->memtupsize = 1024;       /* initial guess */
        state->memtuples = (SortTuple *) palloc(state->memtupsize * sizeof(SortTuple));

        USEMEM(state, GetMemoryChunkSpace(state->memtuples));

        /* workMem must be large enough for the minimal memtuples array */
        if (LACKMEM(state))
                elog(ERROR, "insufficient memory allowed for sort");

        state->currentRun = 0;

        /*
         * maxTapes, tapeRange, and Algorithm D variables will be initialized by
         * inittapes(), if needed
         */

        state->result_tape = -1;        /* flag that result tape has not been formed */

        MemoryContextSwitchTo(oldcontext);

        return state;
}

Tuplesortstate *
tuplesort_begin_heap(TupleDesc tupDesc,
                                         int nkeys, AttrNumber *attNums,
                                         Oid *sortOperators, bool *nullsFirstFlags,
                                         int workMem, bool randomAccess)
{
        Tuplesortstate *state = tuplesort_begin_common(workMem, randomAccess);
        MemoryContext oldcontext;
        int                     i;

        oldcontext = MemoryContextSwitchTo(state->sortcontext);

        AssertArg(nkeys > 0);

#ifdef TRACE_SORT
        if (trace_sort)
                elog(LOG,
                         "begin tuple sort: nkeys = %d, workMem = %d, randomAccess = %c",
                         nkeys, workMem, randomAccess ? 't' : 'f');
#endif

        TRACE_POSTGRESQL_SORT_START(HEAP_SORT, false, nkeys, workMem, randomAccess);

        state->nKeys = nkeys;

        state->comparetup = comparetup_heap;
        state->copytup = copytup_heap;
        state->writetup = writetup_heap;
        state->readtup = readtup_heap;
        state->reversedirection = reversedirection_heap;

        state->tupDesc = tupDesc;       /* assume we need not copy tupDesc */
        state->scanKeys = (ScanKey) palloc0(nkeys * sizeof(ScanKeyData));

        for (i = 0; i < nkeys; i++)
        {
                Oid                     sortFunction;
                bool            reverse;

                AssertArg(attNums[i] != 0);
                AssertArg(sortOperators[i] != 0);

                if (!get_compare_function_for_ordering_op(sortOperators[i],
                                                                                                  &sortFunction, &reverse))
                        elog(ERROR, "operator %u is not a valid ordering operator",
                                 sortOperators[i]);

                /*
                 * We needn't fill in sk_strategy or sk_subtype since these scankeys
                 * will never be passed to an index.
                 */
                ScanKeyInit(&state->scanKeys[i],
                                        attNums[i],
                                        InvalidStrategy,
                                        sortFunction,
                                        (Datum) 0);

                /* However, we use btree's conventions for encoding directionality */
                if (reverse)
                        state->scanKeys[i].sk_flags |= SK_BT_DESC;
                if (nullsFirstFlags[i])
                        state->scanKeys[i].sk_flags |= SK_BT_NULLS_FIRST;
        }

        MemoryContextSwitchTo(oldcontext);

        return state;
}

Tuplesortstate *
tuplesort_begin_index_btree(Relation indexRel,
                                                        bool enforceUnique,
                                                        int workMem, bool randomAccess)
{
        Tuplesortstate *state = tuplesort_begin_common(workMem, randomAccess);
        MemoryContext oldcontext;

        oldcontext = MemoryContextSwitchTo(state->sortcontext);

#ifdef TRACE_SORT
        if (trace_sort)
                elog(LOG,
                         "begin index sort: unique = %c, workMem = %d, randomAccess = %c",
                         enforceUnique ? 't' : 'f',
                         workMem, randomAccess ? 't' : 'f');
#endif

        state->nKeys = RelationGetNumberOfAttributes(indexRel);

        TRACE_POSTGRESQL_SORT_START(INDEX_SORT, enforceUnique, state->nKeys, workMem, randomAccess);

        state->comparetup = comparetup_index_btree;
        state->copytup = copytup_index;
        state->writetup = writetup_index;
        state->readtup = readtup_index;
        state->reversedirection = reversedirection_index_btree;

        state->indexRel = indexRel;
        state->indexScanKey = _bt_mkscankey_nodata(indexRel);
        state->enforceUnique = enforceUnique;

        MemoryContextSwitchTo(oldcontext);

        return state;
}

Tuplesortstate *
tuplesort_begin_index_hash(Relation indexRel,
                                                   uint32 hash_mask,
                                                   int workMem, bool randomAccess)
{
        Tuplesortstate *state = tuplesort_begin_common(workMem, randomAccess);
        MemoryContext oldcontext;

        oldcontext = MemoryContextSwitchTo(state->sortcontext);

#ifdef TRACE_SORT
        if (trace_sort)
                elog(LOG,
                         "begin index sort: hash_mask = 0x%x, workMem = %d, randomAccess = %c",
                         hash_mask,
                         workMem, randomAccess ? 't' : 'f');
#endif

        state->nKeys = 1;                       /* Only one sort column, the hash code */

        state->comparetup = comparetup_index_hash;
        state->copytup = copytup_index;
        state->writetup = writetup_index;
        state->readtup = readtup_index;
        state->reversedirection = reversedirection_index_hash;

        state->indexRel = indexRel;

        state->hash_mask = hash_mask;

        MemoryContextSwitchTo(oldcontext);

        return state;
}

Tuplesortstate *
tuplesort_begin_datum(Oid datumType,
                                          Oid sortOperator, bool nullsFirstFlag,
                                          int workMem, bool randomAccess)
{
        Tuplesortstate *state = tuplesort_begin_common(workMem, randomAccess);
        MemoryContext oldcontext;
        Oid                     sortFunction;
        bool            reverse;
        int16           typlen;
        bool            typbyval;

        oldcontext = MemoryContextSwitchTo(state->sortcontext);

#ifdef TRACE_SORT
        if (trace_sort)
                elog(LOG,
                         "begin datum sort: workMem = %d, randomAccess = %c",
                         workMem, randomAccess ? 't' : 'f');
#endif

        TRACE_POSTGRESQL_SORT_START(DATUM_SORT, false, 1, workMem, randomAccess);

        state->nKeys = 1;                       /* always a one-column sort */

        state->comparetup = comparetup_datum;
        state->copytup = copytup_datum;
        state->writetup = writetup_datum;
        state->readtup = readtup_datum;
        state->reversedirection = reversedirection_datum;

        state->datumType = datumType;

        /* lookup the ordering function */
        if (!get_compare_function_for_ordering_op(sortOperator,
                                                                                          &sortFunction, &reverse))
                elog(ERROR, "operator %u is not a valid ordering operator",
                         sortOperator);
        fmgr_info(sortFunction, &state->sortOpFn);

        /* set ordering flags */
        state->sortFnFlags = reverse ? SK_BT_DESC : 0;
        if (nullsFirstFlag)
                state->sortFnFlags |= SK_BT_NULLS_FIRST;

        /* lookup necessary attributes of the datum type */
        get_typlenbyval(datumType, &typlen, &typbyval);
        state->datumTypeLen = typlen;
        state->datumTypeByVal = typbyval;

        MemoryContextSwitchTo(oldcontext);

        return state;
}

/*
 * tuplesort_set_bound
 *
 *      Advise tuplesort that at most the first N result tuples are required.
 *
 * Must be called before inserting any tuples.  (Actually, we could allow it
 * as long as the sort hasn't spilled to disk, but there seems no need for
 * delayed calls at the moment.)
 *
 * This is a hint only. The tuplesort may still return more tuples than
 * requested.
 */
void
tuplesort_set_bound(Tuplesortstate *state, int64 bound)
{
        /* Assert we're called before loading any tuples */
        Assert(state->status == TSS_INITIAL);
        Assert(state->memtupcount == 0);
        Assert(!state->bounded);

#ifdef DEBUG_BOUNDED_SORT
        /* Honor GUC setting that disables the feature (for easy testing) */
        if (!optimize_bounded_sort)
                return;
#endif

        /* We want to be able to compute bound * 2, so limit the setting */
        if (bound > (int64) (INT_MAX / 2))
                return;

        state->bounded = true;
        state->bound = (int) bound;
}

/*
 * tuplesort_end
 *
 *      Release resources and clean up.
 *
 * NOTE: after calling this, any pointers returned by tuplesort_getXXX are
 * pointing to garbage.  Be careful not to attempt to use or free such
 * pointers afterwards!
 */
void
tuplesort_end(Tuplesortstate *state)
{
        /* context swap probably not needed, but let's be safe */
        MemoryContext oldcontext = MemoryContextSwitchTo(state->sortcontext);

#ifdef TRACE_SORT
        long            spaceUsed;

        if (state->tapeset)
                spaceUsed = LogicalTapeSetBlocks(state->tapeset);
        else
                spaceUsed = (state->allowedMem - state->availMem + 1023) / 1024;
#endif

        /*
         * Delete temporary "tape" files, if any.
         *
         * Note: want to include this in reported total cost of sort, hence need
         * for two #ifdef TRACE_SORT sections.
         */
        if (state->tapeset)
                LogicalTapeSetClose(state->tapeset);

#ifdef TRACE_SORT
        if (trace_sort)
        {
                if (state->tapeset)
                        elog(LOG, "external sort ended, %ld disk blocks used: %s",
                                 spaceUsed, pg_rusage_show(&state->ru_start));
                else
                        elog(LOG, "internal sort ended, %ld KB used: %s",
                                 spaceUsed, pg_rusage_show(&state->ru_start));
        }
#endif

        TRACE_POSTGRESQL_SORT_DONE(state->tapeset,
                        (state->tapeset ? LogicalTapeSetBlocks(state->tapeset) :
                        (state->allowedMem - state->availMem + 1023) / 1024));


        MemoryContextSwitchTo(oldcontext);

        /*
         * Free the per-sort memory context, thereby releasing all working memory,
         * including the Tuplesortstate struct itself.
         */
        MemoryContextDelete(state->sortcontext);
}

/*
 * Grow the memtuples[] array, if possible within our memory constraint.
 * Return TRUE if able to enlarge the array, FALSE if not.
 *
 * At each increment we double the size of the array.  When we are short
 * on memory we could consider smaller increases, but because availMem
 * moves around with tuple addition/removal, this might result in thrashing.
 * Small increases in the array size are likely to be pretty inefficient.
 */
static bool
grow_memtuples(Tuplesortstate *state)
{
        /*
         * We need to be sure that we do not cause LACKMEM to become true, else
         * the space management algorithm will go nuts.  We assume here that the
         * memory chunk overhead associated with the memtuples array is constant
         * and so there will be no unexpected addition to what we ask for.      (The
         * minimum array size established in tuplesort_begin_common is large
         * enough to force palloc to treat it as a separate chunk, so this
         * assumption should be good.  But let's check it.)
         */
        if (state->availMem <= (long) (state->memtupsize * sizeof(SortTuple)))
                return false;

        /*
         * On a 64-bit machine, allowedMem could be high enough to get us into
         * trouble with MaxAllocSize, too.
         */
        if ((Size) (state->memtupsize * 2) >= MaxAllocSize / sizeof(SortTuple))
                return false;

        FREEMEM(state, GetMemoryChunkSpace(state->memtuples));
        state->memtupsize *= 2;
        state->memtuples = (SortTuple *)
                repalloc(state->memtuples,
                                 state->memtupsize * sizeof(SortTuple));
        USEMEM(state, GetMemoryChunkSpace(state->memtuples));
        if (LACKMEM(state))
                elog(ERROR, "unexpected out-of-memory situation during sort");
        return true;
}

/*
 * Accept one tuple while collecting input data for sort.
 *
 * Note that the input data is always copied; the caller need not save it.
 */
void
tuplesort_puttupleslot(Tuplesortstate *state, TupleTableSlot *slot)
{
        MemoryContext oldcontext = MemoryContextSwitchTo(state->sortcontext);
        SortTuple       stup;

        /*
         * Copy the given tuple into memory we control, and decrease availMem.
         * Then call the common code.
         */
        COPYTUP(state, &stup, (void *) slot);

        puttuple_common(state, &stup);

        MemoryContextSwitchTo(oldcontext);
}

/*
 * Accept one index tuple while collecting input data for sort.
 *
 * Note that the input tuple is always copied; the caller need not save it.
 */
void
tuplesort_putindextuple(Tuplesortstate *state, IndexTuple tuple)
{
        MemoryContext oldcontext = MemoryContextSwitchTo(state->sortcontext);
        SortTuple       stup;

        /*
         * Copy the given tuple into memory we control, and decrease availMem.
         * Then call the common code.
         */
        COPYTUP(state, &stup, (void *) tuple);

        puttuple_common(state, &stup);

        MemoryContextSwitchTo(oldcontext);
}

/*
 * Accept one Datum while collecting input data for sort.
 *
 * If the Datum is pass-by-ref type, the value will be copied.
 */
void
tuplesort_putdatum(Tuplesortstate *state, Datum val, bool isNull)
{
        MemoryContext oldcontext = MemoryContextSwitchTo(state->sortcontext);
        SortTuple       stup;

        /*
         * If it's a pass-by-reference value, copy it into memory we control, and
         * decrease availMem.  Then call the common code.
         */
        if (isNull || state->datumTypeByVal)
        {
                stup.datum1 = val;
                stup.isnull1 = isNull;
                stup.tuple = NULL;              /* no separate storage */
        }
        else
        {
                stup.datum1 = datumCopy(val, false, state->datumTypeLen);
                stup.isnull1 = false;
                stup.tuple = DatumGetPointer(stup.datum1);
                USEMEM(state, GetMemoryChunkSpace(stup.tuple));
        }

        puttuple_common(state, &stup);

        MemoryContextSwitchTo(oldcontext);
}

/*
 * Shared code for tuple and datum cases.
 */
static void
puttuple_common(Tuplesortstate *state, SortTuple *tuple)
{
        switch (state->status)
        {
                case TSS_INITIAL:

                        /*
                         * Save the tuple into the unsorted array.      First, grow the array
                         * as needed.  Note that we try to grow the array when there is
                         * still one free slot remaining --- if we fail, there'll still be
                         * room to store the incoming tuple, and then we'll switch to
                         * tape-based operation.
                         */
                        if (state->memtupcount >= state->memtupsize - 1)
                        {
                                (void) grow_memtuples(state);
                                Assert(state->memtupcount < state->memtupsize);
                        }
                        state->memtuples[state->memtupcount++] = *tuple;

                        /*
                         * Check if it's time to switch over to a bounded heapsort. We do
                         * so if the input tuple count exceeds twice the desired tuple
                         * count (this is a heuristic for where heapsort becomes cheaper
                         * than a quicksort), or if we've just filled workMem and have
                         * enough tuples to meet the bound.
                         *
                         * Note that once we enter TSS_BOUNDED state we will always try to
                         * complete the sort that way.  In the worst case, if later input
                         * tuples are larger than earlier ones, this might cause us to
                         * exceed workMem significantly.
                         */
                        if (state->bounded &&
                                (state->memtupcount > state->bound * 2 ||
                                 (state->memtupcount > state->bound && LACKMEM(state))))
                        {
#ifdef TRACE_SORT
                                if (trace_sort)
                                        elog(LOG, "switching to bounded heapsort at %d tuples: %s",
                                                 state->memtupcount,
                                                 pg_rusage_show(&state->ru_start));
#endif
                                make_bounded_heap(state);
                                return;
                        }

                        /*
                         * Done if we still fit in available memory and have array slots.
                         */
                        if (state->memtupcount < state->memtupsize && !LACKMEM(state))
                                return;

                        /*
                         * Nope; time to switch to tape-based operation.
                         */
                        inittapes(state);

                        /*
                         * Dump tuples until we are back under the limit.
                         */
                        dumptuples(state, false);
                        break;

                case TSS_BOUNDED:

                        /*
                         * We don't want to grow the array here, so check whether the new
                         * tuple can be discarded before putting it in.  This should be a
                         * good speed optimization, too, since when there are many more
                         * input tuples than the bound, most input tuples can be discarded
                         * with just this one comparison.  Note that because we currently
                         * have the sort direction reversed, we must check for <= not >=.
                         */
                        if (COMPARETUP(state, tuple, &state->memtuples[0]) <= 0)
                        {
                                /* new tuple <= top of the heap, so we can discard it */
                                free_sort_tuple(state, tuple);
                        }
                        else
                        {
                                /* discard top of heap, sift up, insert new tuple */
                                free_sort_tuple(state, &state->memtuples[0]);
                                tuplesort_heap_siftup(state, false);
                                tuplesort_heap_insert(state, tuple, 0, false);
                        }
                        break;

                case TSS_BUILDRUNS:

                        /*
                         * Insert the tuple into the heap, with run number currentRun if
                         * it can go into the current run, else run number currentRun+1.
                         * The tuple can go into the current run if it is >= the first
                         * not-yet-output tuple.  (Actually, it could go into the current
                         * run if it is >= the most recently output tuple ... but that
                         * would require keeping around the tuple we last output, and it's
                         * simplest to let writetup free each tuple as soon as it's
                         * written.)
                         *
                         * Note there will always be at least one tuple in the heap at
                         * this point; see dumptuples.
                         */
                        Assert(state->memtupcount > 0);
                        if (COMPARETUP(state, tuple, &state->memtuples[0]) >= 0)
                                tuplesort_heap_insert(state, tuple, state->currentRun, true);
                        else
                                tuplesort_heap_insert(state, tuple, state->currentRun + 1, true);

                        /*
                         * If we are over the memory limit, dump tuples till we're under.
                         */
                        dumptuples(state, false);
                        break;

                default:
                        elog(ERROR, "invalid tuplesort state");
                        break;
        }
}

/*
 * All tuples have been provided; finish the sort.
 */
void
tuplesort_performsort(Tuplesortstate *state)
{
        MemoryContext oldcontext = MemoryContextSwitchTo(state->sortcontext);

#ifdef TRACE_SORT
        if (trace_sort)
                elog(LOG, "performsort starting: %s",
                         pg_rusage_show(&state->ru_start));
#endif

        switch (state->status)
        {
                case TSS_INITIAL:

                        /*
                         * We were able to accumulate all the tuples within the allowed
                         * amount of memory.  Just qsort 'em and we're done.
                         */
                        if (state->memtupcount > 1)
                                qsort_arg((void *) state->memtuples,
                                                  state->memtupcount,
                                                  sizeof(SortTuple),
                                                  (qsort_arg_comparator) state->comparetup,
                                                  (void *) state);
                        state->current = 0;
                        state->eof_reached = false;
                        state->markpos_offset = 0;
                        state->markpos_eof = false;
                        state->status = TSS_SORTEDINMEM;
                        break;

                case TSS_BOUNDED:

                        /*
                         * We were able to accumulate all the tuples required for output
                         * in memory, using a heap to eliminate excess tuples.  Now we
                         * have to transform the heap to a properly-sorted array.
                         */
                        sort_bounded_heap(state);
                        state->current = 0;
                        state->eof_reached = false;
                        state->markpos_offset = 0;
                        state->markpos_eof = false;
                        state->status = TSS_SORTEDINMEM;
                        break;

                case TSS_BUILDRUNS:

                        /*
                         * Finish tape-based sort.      First, flush all tuples remaining in
                         * memory out to tape; then merge until we have a single remaining
                         * run (or, if !randomAccess, one run per tape). Note that
                         * mergeruns sets the correct state->status.
                         */
                        dumptuples(state, true);
                        mergeruns(state);
                        state->eof_reached = false;
                        state->markpos_block = 0L;
                        state->markpos_offset = 0;
                        state->markpos_eof = false;
                        break;

                default:
                        elog(ERROR, "invalid tuplesort state");
                        break;
        }

#ifdef TRACE_SORT
        if (trace_sort)
        {
                if (state->status == TSS_FINALMERGE)
                        elog(LOG, "performsort done (except %d-way final merge): %s",
                                 state->activeTapes,
                                 pg_rusage_show(&state->ru_start));
                else
                        elog(LOG, "performsort done: %s",
                                 pg_rusage_show(&state->ru_start));
        }
#endif

        MemoryContextSwitchTo(oldcontext);
}

/*
 * Internal routine to fetch the next tuple in either forward or back
 * direction into *stup.  Returns FALSE if no more tuples.
 * If *should_free is set, the caller must pfree stup.tuple when done with it.
 */
static bool
tuplesort_gettuple_common(Tuplesortstate *state, bool forward,
                                                  SortTuple *stup, bool *should_free)
{
        unsigned int tuplen;

        switch (state->status)
        {
                case TSS_SORTEDINMEM:
                        Assert(forward || state->randomAccess);
                        *should_free = false;
                        if (forward)
                        {
                                if (state->current < state->memtupcount)
                                {
                                        *stup = state->memtuples[state->current++];
                                        return true;
                                }
                                state->eof_reached = true;

                                /*
                                 * Complain if caller tries to retrieve more tuples than
                                 * originally asked for in a bounded sort.      This is because
                                 * returning EOF here might be the wrong thing.
                                 */
                                if (state->bounded && state->current >= state->bound)
                                        elog(ERROR, "retrieved too many tuples in a bounded sort");

                                return false;
                        }
                        else
                        {
                                if (state->current <= 0)
                                        return false;

                                /*
                                 * if all tuples are fetched already then we return last
                                 * tuple, else - tuple before last returned.
                                 */
                                if (state->eof_reached)
                                        state->eof_reached = false;
                                else
                                {
                                        state->current--;       /* last returned tuple */
                                        if (state->current <= 0)
                                                return false;
                                }
                                *stup = state->memtuples[state->current - 1];
                                return true;
                        }
                        break;

                case TSS_SORTEDONTAPE:
                        Assert(forward || state->randomAccess);
                        *should_free = true;
                        if (forward)
                        {
                                if (state->eof_reached)
                                        return false;
                                if ((tuplen = getlen(state, state->result_tape, true)) != 0)
                                {
                                        READTUP(state, stup, state->result_tape, tuplen);
                                        return true;
                                }
                                else
                                {
                                        state->eof_reached = true;
                                        return false;
                                }
                        }

                        /*
                         * Backward.
                         *
                         * if all tuples are fetched already then we return last tuple,
                         * else - tuple before last returned.
                         */
                        if (state->eof_reached)
                        {
                                /*
                                 * Seek position is pointing just past the zero tuplen at the
                                 * end of file; back up to fetch last tuple's ending length
                                 * word.  If seek fails we must have a completely empty file.
                                 */
                                if (!LogicalTapeBackspace(state->tapeset,
                                                                                  state->result_tape,
                                                                                  2 * sizeof(unsigned int)))
                                        return false;
                                state->eof_reached = false;
                        }
                        else
                        {
                                /*
                                 * Back up and fetch previously-returned tuple's ending length
                                 * word.  If seek fails, assume we are at start of file.
                                 */
                                if (!LogicalTapeBackspace(state->tapeset,
                                                                                  state->result_tape,
                                                                                  sizeof(unsigned int)))
                                        return false;
                                tuplen = getlen(state, state->result_tape, false);

                                /*
                                 * Back up to get ending length word of tuple before it.
                                 */
                                if (!LogicalTapeBackspace(state->tapeset,
                                                                                  state->result_tape,
                                                                                  tuplen + 2 * sizeof(unsigned int)))
                                {
                                        /*
                                         * If that fails, presumably the prev tuple is the first
                                         * in the file.  Back up so that it becomes next to read
                                         * in forward direction (not obviously right, but that is
                                         * what in-memory case does).
                                         */
                                        if (!LogicalTapeBackspace(state->tapeset,
                                                                                          state->result_tape,
                                                                                          tuplen + sizeof(unsigned int)))
                                                elog(ERROR, "bogus tuple length in backward scan");
                                        return false;
                                }
                        }

                        tuplen = getlen(state, state->result_tape, false);

                        /*
                         * Now we have the length of the prior tuple, back up and read it.
                         * Note: READTUP expects we are positioned after the initial
                         * length word of the tuple, so back up to that point.
                         */
                        if (!LogicalTapeBackspace(state->tapeset,
                                                                          state->result_tape,
                                                                          tuplen))
                                elog(ERROR, "bogus tuple length in backward scan");
                        READTUP(state, stup, state->result_tape, tuplen);
                        return true;

                case TSS_FINALMERGE:
                        Assert(forward);
                        *should_free = true;

                        /*
                         * This code should match the inner loop of mergeonerun().
                         */
                        if (state->memtupcount > 0)
                        {
                                int                     srcTape = state->memtuples[0].tupindex;
                                Size            tuplen;
                                int                     tupIndex;
                                SortTuple  *newtup;

                                *stup = state->memtuples[0];
                                /* returned tuple is no longer counted in our memory space */
                                if (stup->tuple)
                                {
                                        tuplen = GetMemoryChunkSpace(stup->tuple);
                                        state->availMem += tuplen;
                                        state->mergeavailmem[srcTape] += tuplen;
                                }
                                tuplesort_heap_siftup(state, false);
                                if ((tupIndex = state->mergenext[srcTape]) == 0)
                                {
                                        /*
                                         * out of preloaded data on this tape, try to read more
                                         *
                                         * Unlike mergeonerun(), we only preload from the single
                                         * tape that's run dry.  See mergepreread() comments.
                                         */
                                        mergeprereadone(state, srcTape);

                                        /*
                                         * if still no data, we've reached end of run on this tape
                                         */
                                        if ((tupIndex = state->mergenext[srcTape]) == 0)
                                                return true;
                                }
                                /* pull next preread tuple from list, insert in heap */
                                newtup = &state->memtuples[tupIndex];
                                state->mergenext[srcTape] = newtup->tupindex;
                                if (state->mergenext[srcTape] == 0)
                                        state->mergelast[srcTape] = 0;
                                tuplesort_heap_insert(state, newtup, srcTape, false);
                                /* put the now-unused memtuples entry on the freelist */
                                newtup->tupindex = state->mergefreelist;
                                state->mergefreelist = tupIndex;
                                state->mergeavailslots[srcTape]++;
                                return true;
                        }
                        return false;

                default:
                        elog(ERROR, "invalid tuplesort state");
                        return false;           /* keep compiler quiet */
        }
}

/*
 * Fetch the next tuple in either forward or back direction.
 * If successful, put tuple in slot and return TRUE; else, clear the slot
 * and return FALSE.
 */
bool
tuplesort_gettupleslot(Tuplesortstate *state, bool forward,
                                           TupleTableSlot *slot)
{
        MemoryContext oldcontext = MemoryContextSwitchTo(state->sortcontext);
        SortTuple       stup;
        bool            should_free;

        if (!tuplesort_gettuple_common(state, forward, &stup, &should_free))
                stup.tuple = NULL;

        MemoryContextSwitchTo(oldcontext);

        if (stup.tuple)
        {
                ExecStoreMinimalTuple((MinimalTuple) stup.tuple, slot, should_free);
                return true;
        }
        else
        {
                ExecClearTuple(slot);
                return false;
        }
}

/*
 * Fetch the next index tuple in either forward or back direction.
 * Returns NULL if no more tuples.      If *should_free is set, the
 * caller must pfree the returned tuple when done with it.
 */
IndexTuple
tuplesort_getindextuple(Tuplesortstate *state, bool forward,
                                                bool *should_free)
{
        MemoryContext oldcontext = MemoryContextSwitchTo(state->sortcontext);
        SortTuple       stup;

        if (!tuplesort_gettuple_common(state, forward, &stup, should_free))
                stup.tuple = NULL;

        MemoryContextSwitchTo(oldcontext);

        return (IndexTuple) stup.tuple;
}

/*
 * Fetch the next Datum in either forward or back direction.
 * Returns FALSE if no more datums.
 *
 * If the Datum is pass-by-ref type, the returned value is freshly palloc'd
 * and is now owned by the caller.
 */
bool
tuplesort_getdatum(Tuplesortstate *state, bool forward,
                                   Datum *val, bool *isNull)
{
        MemoryContext oldcontext = MemoryContextSwitchTo(state->sortcontext);
        SortTuple       stup;
        bool            should_free;

        if (!tuplesort_gettuple_common(state, forward, &stup, &should_free))
        {
                MemoryContextSwitchTo(oldcontext);
                return false;
        }

        if (stup.isnull1 || state->datumTypeByVal)
        {
                *val = stup.datum1;
                *isNull = stup.isnull1;
        }
        else
        {
                if (should_free)
                        *val = stup.datum1;
                else
                        *val = datumCopy(stup.datum1, false, state->datumTypeLen);
                *isNull = false;
        }

        MemoryContextSwitchTo(oldcontext);

        return true;
}

/*
 * tuplesort_merge_order - report merge order we'll use for given memory
 * (note: "merge order" just means the number of input tapes in the merge).
 *
 * This is exported for use by the planner.  allowedMem is in bytes.
 */
int
tuplesort_merge_order(long allowedMem)
{
        int                     mOrder;

        /*
         * We need one tape for each merge input, plus another one for the output,
         * and each of these tapes needs buffer space.  In addition we want
         * MERGE_BUFFER_SIZE workspace per input tape (but the output tape doesn't
         * count).
         *
         * Note: you might be thinking we need to account for the memtuples[]
         * array in this calculation, but we effectively treat that as part of the
         * MERGE_BUFFER_SIZE workspace.
         */
        mOrder = (allowedMem - TAPE_BUFFER_OVERHEAD) /
                (MERGE_BUFFER_SIZE + TAPE_BUFFER_OVERHEAD);

        /* Even in minimum memory, use at least a MINORDER merge */
        mOrder = Max(mOrder, MINORDER);

        return mOrder;
}

/*
 * inittapes - initialize for tape sorting.
 *
 * This is called only if we have found we don't have room to sort in memory.
 */
static void
inittapes(Tuplesortstate *state)
{
        int                     maxTapes,
                                ntuples,
                                j;
        long            tapeSpace;

        /* Compute number of tapes to use: merge order plus 1 */
        maxTapes = tuplesort_merge_order(state->allowedMem) + 1;

        /*
         * We must have at least 2*maxTapes slots in the memtuples[] array, else
         * we'd not have room for merge heap plus preread.  It seems unlikely that
         * this case would ever occur, but be safe.
         */
        maxTapes = Min(maxTapes, state->memtupsize / 2);

        state->maxTapes = maxTapes;
        state->tapeRange = maxTapes - 1;

#ifdef TRACE_SORT
        if (trace_sort)
                elog(LOG, "switching to external sort with %d tapes: %s",
                         maxTapes, pg_rusage_show(&state->ru_start));
#endif

        /*
         * Decrease availMem to reflect the space needed for tape buffers; but
         * don't decrease it to the point that we have no room for tuples. (That
         * case is only likely to occur if sorting pass-by-value Datums; in all
         * other scenarios the memtuples[] array is unlikely to occupy more than
         * half of allowedMem.  In the pass-by-value case it's not important to
         * account for tuple space, so we don't care if LACKMEM becomes
         * inaccurate.)
         */
        tapeSpace = maxTapes * TAPE_BUFFER_OVERHEAD;
        if (tapeSpace + GetMemoryChunkSpace(state->memtuples) < state->allowedMem)
                USEMEM(state, tapeSpace);

        /*
         * Make sure that the temp file(s) underlying the tape set are created in
         * suitable temp tablespaces.
         */
        PrepareTempTablespaces();

        /*
         * Create the tape set and allocate the per-tape data arrays.
         */
        state->tapeset = LogicalTapeSetCreate(maxTapes);

        state->mergeactive = (bool *) palloc0(maxTapes * sizeof(bool));
        state->mergenext = (int *) palloc0(maxTapes * sizeof(int));
        state->mergelast = (int *) palloc0(maxTapes * sizeof(int));
        state->mergeavailslots = (int *) palloc0(maxTapes * sizeof(int));
        state->mergeavailmem = (long *) palloc0(maxTapes * sizeof(long));
        state->tp_fib = (int *) palloc0(maxTapes * sizeof(int));
        state->tp_runs = (int *) palloc0(maxTapes * sizeof(int));
        state->tp_dummy = (int *) palloc0(maxTapes * sizeof(int));
        state->tp_tapenum = (int *) palloc0(maxTapes * sizeof(int));

        /*
         * Convert the unsorted contents of memtuples[] into a heap. Each tuple is
         * marked as belonging to run number zero.
         *
         * NOTE: we pass false for checkIndex since there's no point in comparing
         * indexes in this step, even though we do intend the indexes to be part
         * of the sort key...
         */
        ntuples = state->memtupcount;
        state->memtupcount = 0;         /* make the heap empty */
        for (j = 0; j < ntuples; j++)
        {
                /* Must copy source tuple to avoid possible overwrite */
                SortTuple       stup = state->memtuples[j];

                tuplesort_heap_insert(state, &stup, 0, false);
        }
        Assert(state->memtupcount == ntuples);

        state->currentRun = 0;

        /*
         * Initialize variables of Algorithm D (step D1).
         */
        for (j = 0; j < maxTapes; j++)
        {
                state->tp_fib[j] = 1;
                state->tp_runs[j] = 0;
                state->tp_dummy[j] = 1;
                state->tp_tapenum[j] = j;
        }
        state->tp_fib[state->tapeRange] = 0;
        state->tp_dummy[state->tapeRange] = 0;

        state->Level = 1;
        state->destTape = 0;

        state->status = TSS_BUILDRUNS;
}

/*
 * selectnewtape -- select new tape for new initial run.
 *
 * This is called after finishing a run when we know another run
 * must be started.  This implements steps D3, D4 of Algorithm D.
 */
static void
selectnewtape(Tuplesortstate *state)
{
        int                     j;
        int                     a;

        /* Step D3: advance j (destTape) */
        if (state->tp_dummy[state->destTape] < state->tp_dummy[state->destTape + 1])
        {
                state->destTape++;
                return;
        }
        if (state->tp_dummy[state->destTape] != 0)
        {
                state->destTape = 0;
                return;
        }

        /* Step D4: increase level */
        state->Level++;
        a = state->tp_fib[0];
        for (j = 0; j < state->tapeRange; j++)
        {
                state->tp_dummy[j] = a + state->tp_fib[j + 1] - state->tp_fib[j];
                state->tp_fib[j] = a + state->tp_fib[j + 1];
        }
        state->destTape = 0;
}

/*
 * mergeruns -- merge all the completed initial runs.
 *
 * This implements steps D5, D6 of Algorithm D.  All input data has
 * already been written to initial runs on tape (see dumptuples).
 */
static void
mergeruns(Tuplesortstate *state)
{
        int                     tapenum,
                                svTape,
                                svRuns,
                                svDummy;

        Assert(state->status == TSS_BUILDRUNS);
        Assert(state->memtupcount == 0);

        /*
         * If we produced only one initial run (quite likely if the total data
         * volume is between 1X and 2X workMem), we can just use that tape as the
         * finished output, rather than doing a useless merge.  (This obvious
         * optimization is not in Knuth's algorithm.)
         */
        if (state->currentRun == 1)
        {
                state->result_tape = state->tp_tapenum[state->destTape];
                /* must freeze and rewind the finished output tape */
                LogicalTapeFreeze(state->tapeset, state->result_tape);
                state->status = TSS_SORTEDONTAPE;
                return;
        }

        /* End of step D2: rewind all output tapes to prepare for merging */
        for (tapenum = 0; tapenum < state->tapeRange; tapenum++)
                LogicalTapeRewind(state->tapeset, tapenum, false);

        for (;;)
        {
                /*
                 * At this point we know that tape[T] is empty.  If there's just one
                 * (real or dummy) run left on each input tape, then only one merge
                 * pass remains.  If we don't have to produce a materialized sorted
                 * tape, we can stop at this point and do the final merge on-the-fly.
                 */
                if (!state->randomAccess)
                {
                        bool            allOneRun = true;

                        Assert(state->tp_runs[state->tapeRange] == 0);
                        for (tapenum = 0; tapenum < state->tapeRange; tapenum++)
                        {
                                if (state->tp_runs[tapenum] + state->tp_dummy[tapenum] != 1)
                                {
                                        allOneRun = false;
                                        break;
                                }
                        }
                        if (allOneRun)
                        {
                                /* Tell logtape.c we won't be writing anymore */
                                LogicalTapeSetForgetFreeSpace(state->tapeset);
                                /* Initialize for the final merge pass */
                                beginmerge(state);
                                state->status = TSS_FINALMERGE;
                                return;
                        }
                }

                /* Step D5: merge runs onto tape[T] until tape[P] is empty */
                while (state->tp_runs[state->tapeRange - 1] ||
                           state->tp_dummy[state->tapeRange - 1])
                {
                        bool            allDummy = true;

                        for (tapenum = 0; tapenum < state->tapeRange; tapenum++)
                        {
                                if (state->tp_dummy[tapenum] == 0)
                                {
                                        allDummy = false;
                                        break;
                                }
                        }

                        if (allDummy)
                        {
                                state->tp_dummy[state->tapeRange]++;
                                for (tapenum = 0; tapenum < state->tapeRange; tapenum++)
                                        state->tp_dummy[tapenum]--;
                        }
                        else
                                mergeonerun(state);
                }

                /* Step D6: decrease level */
                if (--state->Level == 0)
                        break;
                /* rewind output tape T to use as new input */
                LogicalTapeRewind(state->tapeset, state->tp_tapenum[state->tapeRange],
                                                  false);
                /* rewind used-up input tape P, and prepare it for write pass */
                LogicalTapeRewind(state->tapeset, state->tp_tapenum[state->tapeRange - 1],
                                                  true);
                state->tp_runs[state->tapeRange - 1] = 0;

                /*
                 * reassign tape units per step D6; note we no longer care about A[]
                 */
                svTape = state->tp_tapenum[state->tapeRange];
                svDummy = state->tp_dummy[state->tapeRange];
                svRuns = state->tp_runs[state->tapeRange];
                for (tapenum = state->tapeRange; tapenum > 0; tapenum--)
                {
                        state->tp_tapenum[tapenum] = state->tp_tapenum[tapenum - 1];
                        state->tp_dummy[tapenum] = state->tp_dummy[tapenum - 1];
                        state->tp_runs[tapenum] = state->tp_runs[tapenum - 1];
                }
                state->tp_tapenum[0] = svTape;
                state->tp_dummy[0] = svDummy;
                state->tp_runs[0] = svRuns;
        }

        /*
         * Done.  Knuth says that the result is on TAPE[1], but since we exited
         * the loop without performing the last iteration of step D6, we have not
         * rearranged the tape unit assignment, and therefore the result is on
         * TAPE[T].  We need to do it this way so that we can freeze the final
         * output tape while rewinding it.      The last iteration of step D6 would be
         * a waste of cycles anyway...
         */
        state->result_tape = state->tp_tapenum[state->tapeRange];
        LogicalTapeFreeze(state->tapeset, state->result_tape);
        state->status = TSS_SORTEDONTAPE;
}

/*
 * Merge one run from each input tape, except ones with dummy runs.
 *
 * This is the inner loop of Algorithm D step D5.  We know that the
 * output tape is TAPE[T].
 */
static void
mergeonerun(Tuplesortstate *state)
{
        int                     destTape = state->tp_tapenum[state->tapeRange];
        int                     srcTape;
        int                     tupIndex;
        SortTuple  *tup;
        long            priorAvail,
                                spaceFreed;

        /*
         * Start the merge by loading one tuple from each active source tape into
         * the heap.  We can also decrease the input run/dummy run counts.
         */
        beginmerge(state);

        /*
         * Execute merge by repeatedly extracting lowest tuple in heap, writing it
         * out, and replacing it with next tuple from same tape (if there is
         * another one).
         */
        while (state->memtupcount > 0)
        {
                /* write the tuple to destTape */
                priorAvail = state->availMem;
                srcTape = state->memtuples[0].tupindex;
                WRITETUP(state, destTape, &state->memtuples[0]);
                /* writetup adjusted total free space, now fix per-tape space */
                spaceFreed = state->availMem - priorAvail;
                state->mergeavailmem[srcTape] += spaceFreed;
                /* compact the heap */
                tuplesort_heap_siftup(state, false);
                if ((tupIndex = state->mergenext[srcTape]) == 0)
                {
                        /* out of preloaded data on this tape, try to read more */
                        mergepreread(state);
                        /* if still no data, we've reached end of run on this tape */
                        if ((tupIndex = state->mergenext[srcTape]) == 0)
                                continue;
                }
                /* pull next preread tuple from list, insert in heap */
                tup = &state->memtuples[tupIndex];
                state->mergenext[srcTape] = tup->tupindex;
                if (state->mergenext[srcTape] == 0)
                        state->mergelast[srcTape] = 0;
                tuplesort_heap_insert(state, tup, srcTape, false);
                /* put the now-unused memtuples entry on the freelist */
                tup->tupindex = state->mergefreelist;
                state->mergefreelist = tupIndex;
                state->mergeavailslots[srcTape]++;
        }

        /*
         * When the heap empties, we're done.  Write an end-of-run marker on the
         * output tape, and increment its count of real runs.
         */
        markrunend(state, destTape);
        state->tp_runs[state->tapeRange]++;

#ifdef TRACE_SORT
        if (trace_sort)
                elog(LOG, "finished %d-way merge step: %s", state->activeTapes,
                         pg_rusage_show(&state->ru_start));
#endif
}

/*
 * beginmerge - initialize for a merge pass
 *
 * We decrease the counts of real and dummy runs for each tape, and mark
 * which tapes contain active input runs in mergeactive[].      Then, load
 * as many tuples as we can from each active input tape, and finally
 * fill the merge heap with the first tuple from each active tape.
 */
static void
beginmerge(Tuplesortstate *state)
{
        int                     activeTapes;
        int                     tapenum;
        int                     srcTape;
        int                     slotsPerTape;
        long            spacePerTape;

        /* Heap should be empty here */
        Assert(state->memtupcount == 0);

        /* Adjust run counts and mark the active tapes */
        memset(state->mergeactive, 0,
                   state->maxTapes * sizeof(*state->mergeactive));
        activeTapes = 0;
        for (tapenum = 0; tapenum < state->tapeRange; tapenum++)
        {
                if (state->tp_dummy[tapenum] > 0)
                        state->tp_dummy[tapenum]--;
                else
                {
                        Assert(state->tp_runs[tapenum] > 0);
                        state->tp_runs[tapenum]--;
                        srcTape = state->tp_tapenum[tapenum];
                        state->mergeactive[srcTape] = true;
                        activeTapes++;
                }
        }
        state->activeTapes = activeTapes;

        /* Clear merge-pass state variables */
        memset(state->mergenext, 0,
                   state->maxTapes * sizeof(*state->mergenext));
        memset(state->mergelast, 0,
                   state->maxTapes * sizeof(*state->mergelast));
        state->mergefreelist = 0;       /* nothing in the freelist */
        state->mergefirstfree = activeTapes;            /* 1st slot avail for preread */

        /*
         * Initialize space allocation to let each active input tape have an equal
         * share of preread space.
         */
        Assert(activeTapes > 0);
        slotsPerTape = (state->memtupsize - state->mergefirstfree) / activeTapes;
        Assert(slotsPerTape > 0);
        spacePerTape = state->availMem / activeTapes;
        for (srcTape = 0; srcTape < state->maxTapes; srcTape++)
        {
                if (state->mergeactive[srcTape])
                {
                        state->mergeavailslots[srcTape] = slotsPerTape;
                        state->mergeavailmem[srcTape] = spacePerTape;
                }
        }

        /*
         * Preread as many tuples as possible (and at least one) from each active
         * tape
         */
        mergepreread(state);

        /* Load the merge heap with the first tuple from each input tape */
        for (srcTape = 0; srcTape < state->maxTapes; srcTape++)
        {
                int                     tupIndex = state->mergenext[srcTape];
                SortTuple  *tup;

                if (tupIndex)
                {
                        tup = &state->memtuples[tupIndex];
                        state->mergenext[srcTape] = tup->tupindex;
                        if (state->mergenext[srcTape] == 0)
                                state->mergelast[srcTape] = 0;
                        tuplesort_heap_insert(state, tup, srcTape, false);
                        /* put the now-unused memtuples entry on the freelist */
                        tup->tupindex = state->mergefreelist;
                        state->mergefreelist = tupIndex;
                        state->mergeavailslots[srcTape]++;
                }
        }
}

/*
 * mergepreread - load tuples from merge input tapes
 *
 * This routine exists to improve sequentiality of reads during a merge pass,
 * as explained in the header comments of this file.  Load tuples from each
 * active source tape until the tape's run is exhausted or it has used up
 * its fair share of available memory.  In any case, we guarantee that there
 * is at least one preread tuple available from each unexhausted input tape.
 *
 * We invoke this routine at the start of a merge pass for initial load,
 * and then whenever any tape's preread data runs out.  Note that we load
 * as much data as possible from all tapes, not just the one that ran out.
 * This is because logtape.c works best with a usage pattern that alternates
 * between reading a lot of data and writing a lot of data, so whenever we
 * are forced to read, we should fill working memory completely.
 *
 * In FINALMERGE state, we *don't* use this routine, but instead just preread
 * from the single tape that ran dry.  There's no read/write alternation in
 * that state and so no point in scanning through all the tapes to fix one.
 * (Moreover, there may be quite a lot of inactive tapes in that state, since
 * we might have had many fewer runs than tapes.  In a regular tape-to-tape
 * merge we can expect most of the tapes to be active.)
 */
static void
mergepreread(Tuplesortstate *state)
{
        int                     srcTape;

        for (srcTape = 0; srcTape < state->maxTapes; srcTape++)
                mergeprereadone(state, srcTape);
}

/*
 * mergeprereadone - load tuples from one merge input tape
 *
 * Read tuples from the specified tape until it has used up its free memory
 * or array slots; but ensure that we have at least one tuple, if any are
 * to be had.
 */
static void
mergeprereadone(Tuplesortstate *state, int srcTape)
{
        unsigned int tuplen;
        SortTuple       stup;
        int                     tupIndex;
        long            priorAvail,
                                spaceUsed;

        if (!state->mergeactive[srcTape])
                return;                                 /* tape's run is already exhausted */
        priorAvail = state->availMem;
        state->availMem = state->mergeavailmem[srcTape];
        while ((state->mergeavailslots[srcTape] > 0 && !LACKMEM(state)) ||
                   state->mergenext[srcTape] == 0)
        {
                /* read next tuple, if any */
                if ((tuplen = getlen(state, srcTape, true)) == 0)
                {
                        state->mergeactive[srcTape] = false;
                        break;
                }
                READTUP(state, &stup, srcTape, tuplen);
                /* find a free slot in memtuples[] for it */
                tupIndex = state->mergefreelist;
                if (tupIndex)
                        state->mergefreelist = state->memtuples[tupIndex].tupindex;
                else
                {
                        tupIndex = state->mergefirstfree++;
                        Assert(tupIndex < state->memtupsize);
                }
                state->mergeavailslots[srcTape]--;
                /* store tuple, append to list for its tape */
                stup.tupindex = 0;
                state->memtuples[tupIndex] = stup;
                if (state->mergelast[srcTape])
                        state->memtuples[state->mergelast[srcTape]].tupindex = tupIndex;
                else
                        state->mergenext[srcTape] = tupIndex;
                state->mergelast[srcTape] = tupIndex;
        }
        /* update per-tape and global availmem counts */
        spaceUsed = state->mergeavailmem[srcTape] - state->availMem;
        state->mergeavailmem[srcTape] = state->availMem;
        state->availMem = priorAvail - spaceUsed;
}

/*
 * dumptuples - remove tuples from heap and write to tape
 *
 * This is used during initial-run building, but not during merging.
 *
 * When alltuples = false, dump only enough tuples to get under the
 * availMem limit (and leave at least one tuple in the heap in any case,
 * since puttuple assumes it always has a tuple to compare to).  We also
 * insist there be at least one free slot in the memtuples[] array.
 *
 * When alltuples = true, dump everything currently in memory.
 * (This case is only used at end of input data.)
 *
 * If we empty the heap, close out the current run and return (this should
 * only happen at end of input data).  If we see that the tuple run number
 * at the top of the heap has changed, start a new run.
 */
static void
dumptuples(Tuplesortstate *state, bool alltuples)
{
        while (alltuples ||
                   (LACKMEM(state) && state->memtupcount > 1) ||
                   state->memtupcount >= state->memtupsize)
        {
                /*
                 * Dump the heap's frontmost entry, and sift up to remove it from the
                 * heap.
                 */
                Assert(state->memtupcount > 0);
                WRITETUP(state, state->tp_tapenum[state->destTape],
                                 &state->memtuples[0]);
                tuplesort_heap_siftup(state, true);

                /*
                 * If the heap is empty *or* top run number has changed, we've
                 * finished the current run.
                 */
                if (state->memtupcount == 0 ||
                        state->currentRun != state->memtuples[0].tupindex)
                {
                        markrunend(state, state->tp_tapenum[state->destTape]);
                        state->currentRun++;
                        state->tp_runs[state->destTape]++;
                        state->tp_dummy[state->destTape]--; /* per Alg D step D2 */

#ifdef TRACE_SORT
                        if (trace_sort)
                                elog(LOG, "finished writing%s run %d to tape %d: %s",
                                         (state->memtupcount == 0) ? " final" : "",
                                         state->currentRun, state->destTape,
                                         pg_rusage_show(&state->ru_start));
#endif

                        /*
                         * Done if heap is empty, else prepare for new run.
                         */
                        if (state->memtupcount == 0)
                                break;
                        Assert(state->currentRun == state->memtuples[0].tupindex);
                        selectnewtape(state);
                }
        }
}

/*
 * tuplesort_rescan             - rewind and replay the scan
 */
void
tuplesort_rescan(Tuplesortstate *state)
{
        MemoryContext oldcontext = MemoryContextSwitchTo(state->sortcontext);

        Assert(state->randomAccess);

        switch (state->status)
        {
                case TSS_SORTEDINMEM:
                        state->current = 0;
                        state->eof_reached = false;
                        state->markpos_offset = 0;
                        state->markpos_eof = false;
                        break;
                case TSS_SORTEDONTAPE:
                        LogicalTapeRewind(state->tapeset,
                                                          state->result_tape,
                                                          false);
                        state->eof_reached = false;
                        state->markpos_block = 0L;
                        state->markpos_offset = 0;
                        state->markpos_eof = false;
                        break;
                default:
                        elog(ERROR, "invalid tuplesort state");
                        break;
        }

        MemoryContextSwitchTo(oldcontext);
}

/*
 * tuplesort_markpos    - saves current position in the merged sort file
 */
void
tuplesort_markpos(Tuplesortstate *state)
{
        MemoryContext oldcontext = MemoryContextSwitchTo(state->sortcontext);

        Assert(state->randomAccess);

        switch (state->status)
        {
                case TSS_SORTEDINMEM:
                        state->markpos_offset = state->current;
                        state->markpos_eof = state->eof_reached;
                        break;
                case TSS_SORTEDONTAPE:
                        LogicalTapeTell(state->tapeset,
                                                        state->result_tape,
                                                        &state->markpos_block,
                                                        &state->markpos_offset);
                        state->markpos_eof = state->eof_reached;
                        break;
                default:
                        elog(ERROR, "invalid tuplesort state");
                        break;
        }

        MemoryContextSwitchTo(oldcontext);
}

/*
 * tuplesort_restorepos - restores current position in merged sort file to
 *                                                last saved position
 */
void
tuplesort_restorepos(Tuplesortstate *state)
{
        MemoryContext oldcontext = MemoryContextSwitchTo(state->sortcontext);

        Assert(state->randomAccess);

        switch (state->status)
        {
                case TSS_SORTEDINMEM:
                        state->current = state->markpos_offset;
                        state->eof_reached = state->markpos_eof;
                        break;
                case TSS_SORTEDONTAPE:
                        if (!LogicalTapeSeek(state->tapeset,
                                                                 state->result_tape,
                                                                 state->markpos_block,
                                                                 state->markpos_offset))
                                elog(ERROR, "tuplesort_restorepos failed");
                        state->eof_reached = state->markpos_eof;
                        break;
                default:
                        elog(ERROR, "invalid tuplesort state");
                        break;
        }

        MemoryContextSwitchTo(oldcontext);
}

/*
 * tuplesort_explain - produce a line of information for EXPLAIN ANALYZE
 *
 * This can be called after tuplesort_performsort() finishes to obtain
 * printable summary information about how the sort was performed.
 *
 * The result is a palloc'd string.
 */
char *
tuplesort_explain(Tuplesortstate *state)
{
        char       *result = (char *) palloc(100);
        long            spaceUsed;

        /*
         * Note: it might seem we should print both memory and disk usage for a
         * disk-based sort.  However, the current code doesn't track memory space
         * accurately once we have begun to return tuples to the caller (since we
         * don't account for pfree's the caller is expected to do), so we cannot
         * rely on availMem in a disk sort.  This does not seem worth the overhead
         * to fix.      Is it worth creating an API for the memory context code to
         * tell us how much is actually used in sortcontext?
         */
        if (state->tapeset)
                spaceUsed = LogicalTapeSetBlocks(state->tapeset) * (BLCKSZ / 1024);
        else
                spaceUsed = (state->allowedMem - state->availMem + 1023) / 1024;

        switch (state->status)
        {
                case TSS_SORTEDINMEM:
                        if (state->boundUsed)
                                snprintf(result, 100,
                                                 "Sort Method:  top-N heapsort  Memory: %ldkB",
                                                 spaceUsed);
                        else
                                snprintf(result, 100,
                                                 "Sort Method:  quicksort  Memory: %ldkB",
                                                 spaceUsed);
                        break;
                case TSS_SORTEDONTAPE:
                        snprintf(result, 100,
                                         "Sort Method:  external sort  Disk: %ldkB",
                                         spaceUsed);
                        break;
                case TSS_FINALMERGE:
                        snprintf(result, 100,
                                         "Sort Method:  external merge  Disk: %ldkB",
                                         spaceUsed);
                        break;
                default:
                        snprintf(result, 100, "sort still in progress");
                        break;
        }

        return result;
}


/*
 * Heap manipulation routines, per Knuth's Algorithm 5.2.3H.
 *
 * Compare two SortTuples.      If checkIndex is true, use the tuple index
 * as the front of the sort key; otherwise, no.
 */

#define HEAPCOMPARE(tup1,tup2) \
        (checkIndex && ((tup1)->tupindex != (tup2)->tupindex) ? \
         ((tup1)->tupindex) - ((tup2)->tupindex) : \
         COMPARETUP(state, tup1, tup2))

/*
 * Convert the existing unordered array of SortTuples to a bounded heap,
 * discarding all but the smallest "state->bound" tuples.
 *
 * When working with a bounded heap, we want to keep the largest entry
 * at the root (array entry zero), instead of the smallest as in the normal
 * sort case.  This allows us to discard the largest entry cheaply.
 * Therefore, we temporarily reverse the sort direction.
 *
 * We assume that all entries in a bounded heap will always have tupindex
 * zero; it therefore doesn't matter that HEAPCOMPARE() doesn't reverse
 * the direction of comparison for tupindexes.
 */
static void
make_bounded_heap(Tuplesortstate *state)
{
        int                     tupcount = state->memtupcount;
        int                     i;

        Assert(state->status == TSS_INITIAL);
        Assert(state->bounded);
        Assert(tupcount >= state->bound);

        /* Reverse sort direction so largest entry will be at root */
        REVERSEDIRECTION(state);

        state->memtupcount = 0;         /* make the heap empty */
        for (i = 0; i < tupcount; i++)
        {
                if (state->memtupcount >= state->bound &&
                  COMPARETUP(state, &state->memtuples[i], &state->memtuples[0]) <= 0)
                {
                        /* New tuple would just get thrown out, so skip it */
                        free_sort_tuple(state, &state->memtuples[i]);
                }
                else
                {
                        /* Insert next tuple into heap */
                        /* Must copy source tuple to avoid possible overwrite */
                        SortTuple       stup = state->memtuples[i];

                        tuplesort_heap_insert(state, &stup, 0, false);

                        /* If heap too full, discard largest entry */
                        if (state->memtupcount > state->bound)
                        {
                                free_sort_tuple(state, &state->memtuples[0]);
                                tuplesort_heap_siftup(state, false);
                        }
                }
        }

        Assert(state->memtupcount == state->bound);
        state->status = TSS_BOUNDED;
}

/*
 * Convert the bounded heap to a properly-sorted array
 */
static void
sort_bounded_heap(Tuplesortstate *state)
{
        int                     tupcount = state->memtupcount;

        Assert(state->status == TSS_BOUNDED);
        Assert(state->bounded);
        Assert(tupcount == state->bound);

        /*
         * We can unheapify in place because each sift-up will remove the largest
         * entry, which we can promptly store in the newly freed slot at the end.
         * Once we're down to a single-entry heap, we're done.
         */
        while (state->memtupcount > 1)
        {
                SortTuple       stup = state->memtuples[0];

                /* this sifts-up the next-largest entry and decreases memtupcount */
                tuplesort_heap_siftup(state, false);
                state->memtuples[state->memtupcount] = stup;
        }
        state->memtupcount = tupcount;

        /*
         * Reverse sort direction back to the original state.  This is not
         * actually necessary but seems like a good idea for tidiness.
         */
        REVERSEDIRECTION(state);

        state->status = TSS_SORTEDINMEM;
        state->boundUsed = true;
}

/*
 * Insert a new tuple into an empty or existing heap, maintaining the
 * heap invariant.      Caller is responsible for ensuring there's room.
 *
 * Note: we assume *tuple is a temporary variable that can be scribbled on.
 * For some callers, tuple actually points to a memtuples[] entry above the
 * end of the heap.  This is safe as long as it's not immediately adjacent
 * to the end of the heap (ie, in the [memtupcount] array entry) --- if it
 * is, it might get overwritten before being moved into the heap!
 */
static void
tuplesort_heap_insert(Tuplesortstate *state, SortTuple *tuple,
                                          int tupleindex, bool checkIndex)
{
        SortTuple  *memtuples;
        int                     j;

        /*
         * Save the tupleindex --- see notes above about writing on *tuple. It's a
         * historical artifact that tupleindex is passed as a separate argument
         * and not in *tuple, but it's notationally convenient so let's leave it
         * that way.
         */
        tuple->tupindex = tupleindex;

        memtuples = state->memtuples;
        Assert(state->memtupcount < state->memtupsize);

        /*
         * Sift-up the new entry, per Knuth 5.2.3 exercise 16. Note that Knuth is
         * using 1-based array indexes, not 0-based.
         */
        j = state->memtupcount++;
        while (j > 0)
        {
                int                     i = (j - 1) >> 1;

                if (HEAPCOMPARE(tuple, &memtuples[i]) >= 0)
                        break;
                memtuples[j] = memtuples[i];
                j = i;
        }
        memtuples[j] = *tuple;
}

/*
 * The tuple at state->memtuples[0] has been removed from the heap.
 * Decrement memtupcount, and sift up to maintain the heap invariant.
 */
static void
tuplesort_heap_siftup(Tuplesortstate *state, bool checkIndex)
{
        SortTuple  *memtuples = state->memtuples;
        SortTuple  *tuple;
        int                     i,
                                n;

        if (--state->memtupcount <= 0)
                return;
        n = state->memtupcount;
        tuple = &memtuples[n];          /* tuple that must be reinserted */
        i = 0;                                          /* i is where the "hole" is */
        for (;;)
        {
                int                     j = 2 * i + 1;

                if (j >= n)
                        break;
                if (j + 1 < n &&
                        HEAPCOMPARE(&memtuples[j], &memtuples[j + 1]) > 0)
                        j++;
                if (HEAPCOMPARE(tuple, &memtuples[j]) <= 0)
                        break;
                memtuples[i] = memtuples[j];
                i = j;
        }
        memtuples[i] = *tuple;
}


/*
 * Tape interface routines
 */

static unsigned int
getlen(Tuplesortstate *state, int tapenum, bool eofOK)
{
        unsigned int len;

        if (LogicalTapeRead(state->tapeset, tapenum, (void *) &len,
                                                sizeof(len)) != sizeof(len))
                elog(ERROR, "unexpected end of tape");
        if (len == 0 && !eofOK)
                elog(ERROR, "unexpected end of data");
        return len;
}

static void
markrunend(Tuplesortstate *state, int tapenum)
{
        unsigned int len = 0;

        LogicalTapeWrite(state->tapeset, tapenum, (void *) &len, sizeof(len));
}


/*
 * Set up for an external caller of ApplySortFunction.  This function
 * basically just exists to localize knowledge of the encoding of sk_flags
 * used in this module.
 */
void
SelectSortFunction(Oid sortOperator,
                                   bool nulls_first,
                                   Oid *sortFunction,
                                   int *sortFlags)
{
        bool            reverse;

        if (!get_compare_function_for_ordering_op(sortOperator,
                                                                                          sortFunction, &reverse))
                elog(ERROR, "operator %u is not a valid ordering operator",
                         sortOperator);

        *sortFlags = reverse ? SK_BT_DESC : 0;
        if (nulls_first)
                *sortFlags |= SK_BT_NULLS_FIRST;
}

/*
 * Inline-able copy of FunctionCall2() to save some cycles in sorting.
 */
static inline Datum
myFunctionCall2(FmgrInfo *flinfo, Datum arg1, Datum arg2)
{
        FunctionCallInfoData fcinfo;
        Datum           result;

        InitFunctionCallInfoData(fcinfo, flinfo, 2, NULL, NULL);

        fcinfo.arg[0] = arg1;
        fcinfo.arg[1] = arg2;
        fcinfo.argnull[0] = false;
        fcinfo.argnull[1] = false;

        result = FunctionCallInvoke(&fcinfo);

        /* Check for null result, since caller is clearly not expecting one */
        if (fcinfo.isnull)
                elog(ERROR, "function %u returned NULL", fcinfo.flinfo->fn_oid);

        return result;
}

/*
 * Apply a sort function (by now converted to fmgr lookup form)
 * and return a 3-way comparison result.  This takes care of handling
 * reverse-sort and NULLs-ordering properly.  We assume that DESC and
 * NULLS_FIRST options are encoded in sk_flags the same way btree does it.
 */
static inline int32
inlineApplySortFunction(FmgrInfo *sortFunction, int sk_flags,
                                                Datum datum1, bool isNull1,
                                                Datum datum2, bool isNull2)
{
        int32           compare;

        if (isNull1)
        {
                if (isNull2)
                        compare = 0;            /* NULL "=" NULL */
                else if (sk_flags & SK_BT_NULLS_FIRST)
                        compare = -1;           /* NULL "<" NOT_NULL */
                else
                        compare = 1;            /* NULL ">" NOT_NULL */
        }
        else if (isNull2)
        {
                if (sk_flags & SK_BT_NULLS_FIRST)
                        compare = 1;            /* NOT_NULL ">" NULL */
                else
                        compare = -1;           /* NOT_NULL "<" NULL */
        }
        else
        {
                compare = DatumGetInt32(myFunctionCall2(sortFunction,
                                                                                                datum1, datum2));

                if (sk_flags & SK_BT_DESC)
                        compare = -compare;
        }

        return compare;
}

/*
 * Non-inline ApplySortFunction() --- this is needed only to conform to
 * C99's brain-dead notions about how to implement inline functions...
 */
int32
ApplySortFunction(FmgrInfo *sortFunction, int sortFlags,
                                  Datum datum1, bool isNull1,
                                  Datum datum2, bool isNull2)
{
        return inlineApplySortFunction(sortFunction, sortFlags,
                                                                   datum1, isNull1,
                                                                   datum2, isNull2);
}


/*
 * Routines specialized for HeapTuple (actually MinimalTuple) case
 */

static int
comparetup_heap(const SortTuple *a, const SortTuple *b, Tuplesortstate *state)
{
        ScanKey         scanKey = state->scanKeys;
        HeapTupleData ltup;
        HeapTupleData rtup;
        TupleDesc       tupDesc;
        int                     nkey;
        int32           compare;

        /* Allow interrupting long sorts */
        CHECK_FOR_INTERRUPTS();

        /* Compare the leading sort key */
        compare = inlineApplySortFunction(&scanKey->sk_func, scanKey->sk_flags,
                                                                          a->datum1, a->isnull1,
                                                                          b->datum1, b->isnull1);
        if (compare != 0)
                return compare;

        /* Compare additional sort keys */
        ltup.t_len = ((MinimalTuple) a->tuple)->t_len + MINIMAL_TUPLE_OFFSET;
        ltup.t_data = (HeapTupleHeader) ((char *) a->tuple - MINIMAL_TUPLE_OFFSET);
        rtup.t_len = ((MinimalTuple) b->tuple)->t_len + MINIMAL_TUPLE_OFFSET;
        rtup.t_data = (HeapTupleHeader) ((char *) b->tuple - MINIMAL_TUPLE_OFFSET);
        tupDesc = state->tupDesc;
        scanKey++;
        for (nkey = 1; nkey < state->nKeys; nkey++, scanKey++)
        {
                AttrNumber      attno = scanKey->sk_attno;
                Datum           datum1,
                                        datum2;
                bool            isnull1,
                                        isnull2;

                datum1 = heap_getattr(&ltup, attno, tupDesc, &isnull1);
                datum2 = heap_getattr(&rtup, attno, tupDesc, &isnull2);

                compare = inlineApplySortFunction(&scanKey->sk_func, scanKey->sk_flags,
                                                                                  datum1, isnull1,
                                                                                  datum2, isnull2);
                if (compare != 0)
                        return compare;
        }

        return 0;
}

static void
copytup_heap(Tuplesortstate *state, SortTuple *stup, void *tup)
{
        /*
         * We expect the passed "tup" to be a TupleTableSlot, and form a
         * MinimalTuple using the exported interface for that.
         */
        TupleTableSlot *slot = (TupleTableSlot *) tup;
        MinimalTuple tuple;
        HeapTupleData htup;

        /* copy the tuple into sort storage */
        tuple = ExecCopySlotMinimalTuple(slot);
        stup->tuple = (void *) tuple;
        USEMEM(state, GetMemoryChunkSpace(tuple));
        /* set up first-column key value */
        htup.t_len = tuple->t_len + MINIMAL_TUPLE_OFFSET;
        htup.t_data = (HeapTupleHeader) ((char *) tuple - MINIMAL_TUPLE_OFFSET);
        stup->datum1 = heap_getattr(&htup,
                                                                state->scanKeys[0].sk_attno,
                                                                state->tupDesc,
                                                                &stup->isnull1);
}

static void
writetup_heap(Tuplesortstate *state, int tapenum, SortTuple *stup)
{
        MinimalTuple tuple = (MinimalTuple) stup->tuple;
        /* the part of the MinimalTuple we'll write: */
        char       *tupbody = (char *) tuple + MINIMAL_TUPLE_DATA_OFFSET;
        unsigned int tupbodylen = tuple->t_len - MINIMAL_TUPLE_DATA_OFFSET;
        /* total on-disk footprint: */
        unsigned int tuplen = tupbodylen + sizeof(int);

        LogicalTapeWrite(state->tapeset, tapenum,
                                         (void *) &tuplen, sizeof(tuplen));
        LogicalTapeWrite(state->tapeset, tapenum,
                                         (void *) tupbody, tupbodylen);
        if (state->randomAccess)        /* need trailing length word? */
                LogicalTapeWrite(state->tapeset, tapenum,
                                                 (void *) &tuplen, sizeof(tuplen));

        FREEMEM(state, GetMemoryChunkSpace(tuple));
        heap_free_minimal_tuple(tuple);
}

static void
readtup_heap(Tuplesortstate *state, SortTuple *stup,
                         int tapenum, unsigned int len)
{
        unsigned int tupbodylen = len - sizeof(int);
        unsigned int tuplen = tupbodylen + MINIMAL_TUPLE_DATA_OFFSET;
        MinimalTuple tuple = (MinimalTuple) palloc(tuplen);
        char       *tupbody = (char *) tuple + MINIMAL_TUPLE_DATA_OFFSET;
        HeapTupleData htup;

        USEMEM(state, GetMemoryChunkSpace(tuple));
        /* read in the tuple proper */
        tuple->t_len = tuplen;
        if (LogicalTapeRead(state->tapeset, tapenum,
                                                (void *) tupbody,
                                                tupbodylen) != (size_t) tupbodylen)
                elog(ERROR, "unexpected end of data");
        if (state->randomAccess)        /* need trailing length word? */
                if (LogicalTapeRead(state->tapeset, tapenum, (void *) &tuplen,
                                                        sizeof(tuplen)) != sizeof(tuplen))
                        elog(ERROR, "unexpected end of data");
        stup->tuple = (void *) tuple;
        /* set up first-column key value */
        htup.t_len = tuple->t_len + MINIMAL_TUPLE_OFFSET;
        htup.t_data = (HeapTupleHeader) ((char *) tuple - MINIMAL_TUPLE_OFFSET);
        stup->datum1 = heap_getattr(&htup,
                                                                state->scanKeys[0].sk_attno,
                                                                state->tupDesc,
                                                                &stup->isnull1);
}

static void
reversedirection_heap(Tuplesortstate *state)
{
        ScanKey         scanKey = state->scanKeys;
        int                     nkey;

        for (nkey = 0; nkey < state->nKeys; nkey++, scanKey++)
        {
                scanKey->sk_flags ^= (SK_BT_DESC | SK_BT_NULLS_FIRST);
        }
}


/*
 * Routines specialized for IndexTuple case
 *
 * The btree and hash cases require separate comparison functions, but the
 * IndexTuple representation is the same so the copy/write/read support
 * functions can be shared.
 */

static int
comparetup_index_btree(const SortTuple *a, const SortTuple *b,
                                           Tuplesortstate *state)
{
        /*
         * This is similar to _bt_tuplecompare(), but we have already done the
         * index_getattr calls for the first column, and we need to keep track of
         * whether any null fields are present.  Also see the special treatment
         * for equal keys at the end.
         */
        ScanKey         scanKey = state->indexScanKey;
        IndexTuple      tuple1;
        IndexTuple      tuple2;
        int                     keysz;
        TupleDesc       tupDes;
        bool            equal_hasnull = false;
        int                     nkey;
        int32           compare;

        /* Allow interrupting long sorts */
        CHECK_FOR_INTERRUPTS();

        /* Compare the leading sort key */
        compare = inlineApplySortFunction(&scanKey->sk_func, scanKey->sk_flags,
                                                                          a->datum1, a->isnull1,
                                                                          b->datum1, b->isnull1);
        if (compare != 0)
                return compare;

        /* they are equal, so we only need to examine one null flag */
        if (a->isnull1)
                equal_hasnull = true;

        /* Compare additional sort keys */
        tuple1 = (IndexTuple) a->tuple;
        tuple2 = (IndexTuple) b->tuple;
        keysz = state->nKeys;
        tupDes = RelationGetDescr(state->indexRel);
        scanKey++;
        for (nkey = 2; nkey <= keysz; nkey++, scanKey++)
        {
                Datum           datum1,
                                        datum2;
                bool            isnull1,
                                        isnull2;

                datum1 = index_getattr(tuple1, nkey, tupDes, &isnull1);
                datum2 = index_getattr(tuple2, nkey, tupDes, &isnull2);

                compare = inlineApplySortFunction(&scanKey->sk_func, scanKey->sk_flags,
                                                                                  datum1, isnull1,
                                                                                  datum2, isnull2);
                if (compare != 0)
                        return compare;         /* done when we find unequal attributes */

                /* they are equal, so we only need to examine one null flag */
                if (isnull1)
                        equal_hasnull = true;
        }

        /*
         * If btree has asked us to enforce uniqueness, complain if two equal
         * tuples are detected (unless there was at least one NULL field).
         *
         * It is sufficient to make the test here, because if two tuples are equal
         * they *must* get compared at some stage of the sort --- otherwise the
         * sort algorithm wouldn't have checked whether one must appear before the
         * other.
         *
         * Some rather brain-dead implementations of qsort will sometimes call the
         * comparison routine to compare a value to itself.  (At this writing only
         * QNX 4 is known to do such silly things; we don't support QNX anymore,
         * but perhaps the behavior still exists elsewhere.)  Don't raise a bogus
         * error in that case.
         */
        if (state->enforceUnique && !equal_hasnull && tuple1 != tuple2)
                ereport(ERROR,
                                (errcode(ERRCODE_UNIQUE_VIOLATION),
                                 errmsg("could not create unique index \"%s\"",
                                                RelationGetRelationName(state->indexRel)),
                                 errdetail("Table contains duplicated values.")));

        /*
         * If key values are equal, we sort on ItemPointer.  This does not affect
         * validity of the finished index, but it offers cheap insurance against
         * performance problems with bad qsort implementations that have trouble
         * with large numbers of equal keys.
         */
        {
                BlockNumber blk1 = ItemPointerGetBlockNumber(&tuple1->t_tid);
                BlockNumber blk2 = ItemPointerGetBlockNumber(&tuple2->t_tid);

                if (blk1 != blk2)
                        return (blk1 < blk2) ? -1 : 1;
        }
        {
                OffsetNumber pos1 = ItemPointerGetOffsetNumber(&tuple1->t_tid);
                OffsetNumber pos2 = ItemPointerGetOffsetNumber(&tuple2->t_tid);

                if (pos1 != pos2)
                        return (pos1 < pos2) ? -1 : 1;
        }

        return 0;
}

static int
comparetup_index_hash(const SortTuple *a, const SortTuple *b,
                                          Tuplesortstate *state)
{
        uint32          hash1;
        uint32          hash2;
        IndexTuple      tuple1;
        IndexTuple      tuple2;

        /* Allow interrupting long sorts */
        CHECK_FOR_INTERRUPTS();

        /*
         * Fetch hash keys and mask off bits we don't want to sort by.
         * We know that the first column of the index tuple is the hash key.
         */
        Assert(!a->isnull1);
        hash1 = DatumGetUInt32(a->datum1) & state->hash_mask;
        Assert(!b->isnull1);
        hash2 = DatumGetUInt32(b->datum1) & state->hash_mask;

        if (hash1 > hash2)
                return 1;
        else if (hash1 < hash2)
                return -1;

        /*
         * If hash values are equal, we sort on ItemPointer.  This does not affect
         * validity of the finished index, but it offers cheap insurance against
         * performance problems with bad qsort implementations that have trouble
         * with large numbers of equal keys.
         */
        tuple1 = (IndexTuple) a->tuple;
        tuple2 = (IndexTuple) b->tuple;

        {
                BlockNumber blk1 = ItemPointerGetBlockNumber(&tuple1->t_tid);
                BlockNumber blk2 = ItemPointerGetBlockNumber(&tuple2->t_tid);

                if (blk1 != blk2)
                        return (blk1 < blk2) ? -1 : 1;
        }
        {
                OffsetNumber pos1 = ItemPointerGetOffsetNumber(&tuple1->t_tid);
                OffsetNumber pos2 = ItemPointerGetOffsetNumber(&tuple2->t_tid);

                if (pos1 != pos2)
                        return (pos1 < pos2) ? -1 : 1;
        }

        return 0;
}

static void
copytup_index(Tuplesortstate *state, SortTuple *stup, void *tup)
{
        IndexTuple      tuple = (IndexTuple) tup;
        unsigned int tuplen = IndexTupleSize(tuple);
        IndexTuple      newtuple;

        /* copy the tuple into sort storage */
        newtuple = (IndexTuple) palloc(tuplen);
        memcpy(newtuple, tuple, tuplen);
        USEMEM(state, GetMemoryChunkSpace(newtuple));
        stup->tuple = (void *) newtuple;
        /* set up first-column key value */
        stup->datum1 = index_getattr(newtuple,
                                                                 1,
                                                                 RelationGetDescr(state->indexRel),
                                                                 &stup->isnull1);
}

static void
writetup_index(Tuplesortstate *state, int tapenum, SortTuple *stup)
{
        IndexTuple      tuple = (IndexTuple) stup->tuple;
        unsigned int tuplen;

        tuplen = IndexTupleSize(tuple) + sizeof(tuplen);
        LogicalTapeWrite(state->tapeset, tapenum,
                                         (void *) &tuplen, sizeof(tuplen));
        LogicalTapeWrite(state->tapeset, tapenum,
                                         (void *) tuple, IndexTupleSize(tuple));
        if (state->randomAccess)        /* need trailing length word? */
                LogicalTapeWrite(state->tapeset, tapenum,
                                                 (void *) &tuplen, sizeof(tuplen));

        FREEMEM(state, GetMemoryChunkSpace(tuple));
        pfree(tuple);
}

static void
readtup_index(Tuplesortstate *state, SortTuple *stup,
                          int tapenum, unsigned int len)
{
        unsigned int tuplen = len - sizeof(unsigned int);
        IndexTuple      tuple = (IndexTuple) palloc(tuplen);

        USEMEM(state, GetMemoryChunkSpace(tuple));
        if (LogicalTapeRead(state->tapeset, tapenum, (void *) tuple,
                                                tuplen) != tuplen)
                elog(ERROR, "unexpected end of data");
        if (state->randomAccess)        /* need trailing length word? */
                if (LogicalTapeRead(state->tapeset, tapenum, (void *) &tuplen,
                                                        sizeof(tuplen)) != sizeof(tuplen))
                        elog(ERROR, "unexpected end of data");
        stup->tuple = (void *) tuple;
        /* set up first-column key value */
        stup->datum1 = index_getattr(tuple,
                                                                 1,
                                                                 RelationGetDescr(state->indexRel),
                                                                 &stup->isnull1);
}

static void
reversedirection_index_btree(Tuplesortstate *state)
{
        ScanKey         scanKey = state->indexScanKey;
        int                     nkey;

        for (nkey = 0; nkey < state->nKeys; nkey++, scanKey++)
        {
                scanKey->sk_flags ^= (SK_BT_DESC | SK_BT_NULLS_FIRST);
        }
}

static void
reversedirection_index_hash(Tuplesortstate *state)
{
        /* We don't support reversing direction in a hash index sort */
        elog(ERROR, "reversedirection_index_hash is not implemented");
}


/*
 * Routines specialized for DatumTuple case
 */

static int
comparetup_datum(const SortTuple *a, const SortTuple *b, Tuplesortstate *state)
{
        /* Allow interrupting long sorts */
        CHECK_FOR_INTERRUPTS();

        return inlineApplySortFunction(&state->sortOpFn, state->sortFnFlags,
                                                                   a->datum1, a->isnull1,
                                                                   b->datum1, b->isnull1);
}

static void
copytup_datum(Tuplesortstate *state, SortTuple *stup, void *tup)
{
        /* Not currently needed */
        elog(ERROR, "copytup_datum() should not be called");
}

static void
writetup_datum(Tuplesortstate *state, int tapenum, SortTuple *stup)
{
        void       *waddr;
        unsigned int tuplen;
        unsigned int writtenlen;

        if (stup->isnull1)
        {
                waddr = NULL;
                tuplen = 0;
        }
        else if (state->datumTypeByVal)
        {
                waddr = &stup->datum1;
                tuplen = sizeof(Datum);
        }
        else
        {
                waddr = DatumGetPointer(stup->datum1);
                tuplen = datumGetSize(stup->datum1, false, state->datumTypeLen);
                Assert(tuplen != 0);
        }

        writtenlen = tuplen + sizeof(unsigned int);

        LogicalTapeWrite(state->tapeset, tapenum,
                                         (void *) &writtenlen, sizeof(writtenlen));
        LogicalTapeWrite(state->tapeset, tapenum,
                                         waddr, tuplen);
        if (state->randomAccess)        /* need trailing length word? */
                LogicalTapeWrite(state->tapeset, tapenum,
                                                 (void *) &writtenlen, sizeof(writtenlen));

        if (stup->tuple)
        {
                FREEMEM(state, GetMemoryChunkSpace(stup->tuple));
                pfree(stup->tuple);
        }
}

static void
readtup_datum(Tuplesortstate *state, SortTuple *stup,
                          int tapenum, unsigned int len)
{
        unsigned int tuplen = len - sizeof(unsigned int);

        if (tuplen == 0)
        {
                /* it's NULL */
                stup->datum1 = (Datum) 0;
                stup->isnull1 = true;
                stup->tuple = NULL;
        }
        else if (state->datumTypeByVal)
        {
                Assert(tuplen == sizeof(Datum));
                if (LogicalTapeRead(state->tapeset, tapenum, (void *) &stup->datum1,
                                                        tuplen) != tuplen)
                        elog(ERROR, "unexpected end of data");
                stup->isnull1 = false;
                stup->tuple = NULL;
        }
        else
        {
                void       *raddr = palloc(tuplen);

                if (LogicalTapeRead(state->tapeset, tapenum, raddr,
                                                        tuplen) != tuplen)
                        elog(ERROR, "unexpected end of data");
                stup->datum1 = PointerGetDatum(raddr);
                stup->isnull1 = false;
                stup->tuple = raddr;
                USEMEM(state, GetMemoryChunkSpace(raddr));
        }

        if (state->randomAccess)        /* need trailing length word? */
                if (LogicalTapeRead(state->tapeset, tapenum, (void *) &tuplen,
                                                        sizeof(tuplen)) != sizeof(tuplen))
                        elog(ERROR, "unexpected end of data");
}

static void
reversedirection_datum(Tuplesortstate *state)
{
        state->sortFnFlags ^= (SK_BT_DESC | SK_BT_NULLS_FIRST);
}

/*
 * Convenience routine to free a tuple previously loaded into sort memory
 */
static void
free_sort_tuple(Tuplesortstate *state, SortTuple *stup)
{
        FREEMEM(state, GetMemoryChunkSpace(stup->tuple));
        pfree(stup->tuple);
}