Force a checkpoint in CREATE DATABASE before starting to copy the files,

[PostgreSQL.git] / src / backend / access / heap / tuptoaster.c

/*-------------------------------------------------------------------------
 *
 * tuptoaster.c
 *        Support routines for external and compressed storage of
 *        variable size attributes.
 *
 * Copyright (c) 2000-2008, PostgreSQL Global Development Group
 *
 *
 * IDENTIFICATION
 *        $PostgreSQL$
 *
 *
 * INTERFACE ROUTINES
 *              toast_insert_or_update -
 *                      Try to make a given tuple fit into one page by compressing
 *                      or moving off attributes
 *
 *              toast_delete -
 *                      Reclaim toast storage when a tuple is deleted
 *
 *              heap_tuple_untoast_attr -
 *                      Fetch back a given value from the "secondary" relation
 *
 *-------------------------------------------------------------------------
 */

#include "postgres.h"

#include <unistd.h>
#include <fcntl.h>

#include "access/genam.h"
#include "access/heapam.h"
#include "access/tuptoaster.h"
#include "access/xact.h"
#include "catalog/catalog.h"
#include "utils/fmgroids.h"
#include "utils/pg_lzcompress.h"
#include "utils/rel.h"
#include "utils/typcache.h"
#include "utils/tqual.h"


#undef TOAST_DEBUG

/* Size of an EXTERNAL datum that contains a standard TOAST pointer */
#define TOAST_POINTER_SIZE (VARHDRSZ_EXTERNAL + sizeof(struct varatt_external))

/*
 * Testing whether an externally-stored value is compressed now requires
 * comparing extsize (the actual length of the external data) to rawsize
 * (the original uncompressed datum's size).  The latter includes VARHDRSZ
 * overhead, the former doesn't.  We never use compression unless it actually
 * saves space, so we expect either equality or less-than.
 */
#define VARATT_EXTERNAL_IS_COMPRESSED(toast_pointer) \
        ((toast_pointer).va_extsize < (toast_pointer).va_rawsize - VARHDRSZ)

/*
 * Macro to fetch the possibly-unaligned contents of an EXTERNAL datum
 * into a local "struct varatt_external" toast pointer.  This should be
 * just a memcpy, but some versions of gcc seem to produce broken code
 * that assumes the datum contents are aligned.  Introducing an explicit
 * intermediate "varattrib_1b_e *" variable seems to fix it.
 */
#define VARATT_EXTERNAL_GET_POINTER(toast_pointer, attr) \
do { \
        varattrib_1b_e *attre = (varattrib_1b_e *) (attr); \
        Assert(VARATT_IS_EXTERNAL(attre)); \
        Assert(VARSIZE_EXTERNAL(attre) == sizeof(toast_pointer) + VARHDRSZ_EXTERNAL); \
        memcpy(&(toast_pointer), VARDATA_EXTERNAL(attre), sizeof(toast_pointer)); \
} while (0)


static void toast_delete_datum(Relation rel, Datum value);
static Datum toast_save_datum(Relation rel, Datum value,
                                 bool use_wal, bool use_fsm);
static struct varlena *toast_fetch_datum(struct varlena * attr);
static struct varlena *toast_fetch_datum_slice(struct varlena * attr,
                                                int32 sliceoffset, int32 length);


/* ----------
 * heap_tuple_fetch_attr -
 *
 *      Public entry point to get back a toasted value from
 *      external storage (possibly still in compressed format).
 *
 * This will return a datum that contains all the data internally, ie, not
 * relying on external storage, but it can still be compressed or have a short
 * header.
 ----------
 */
struct varlena *
heap_tuple_fetch_attr(struct varlena * attr)
{
        struct varlena *result;

        if (VARATT_IS_EXTERNAL(attr))
        {
                /*
                 * This is an external stored plain value
                 */
                result = toast_fetch_datum(attr);
        }
        else
        {
                /*
                 * This is a plain value inside of the main tuple - why am I called?
                 */
                result = attr;
        }

        return result;
}


/* ----------
 * heap_tuple_untoast_attr -
 *
 *      Public entry point to get back a toasted value from compression
 *      or external storage.
 * ----------
 */
struct varlena *
heap_tuple_untoast_attr(struct varlena * attr)
{
        if (VARATT_IS_EXTERNAL(attr))
        {
                /*
                 * This is an externally stored datum --- fetch it back from there
                 */
                attr = toast_fetch_datum(attr);
                /* If it's compressed, decompress it */
                if (VARATT_IS_COMPRESSED(attr))
                {
                        PGLZ_Header *tmp = (PGLZ_Header *) attr;

                        attr = (struct varlena *) palloc(PGLZ_RAW_SIZE(tmp) + VARHDRSZ);
                        SET_VARSIZE(attr, PGLZ_RAW_SIZE(tmp) + VARHDRSZ);
                        pglz_decompress(tmp, VARDATA(attr));
                        pfree(tmp);
                }
        }
        else if (VARATT_IS_COMPRESSED(attr))
        {
                /*
                 * This is a compressed value inside of the main tuple
                 */
                PGLZ_Header *tmp = (PGLZ_Header *) attr;

                attr = (struct varlena *) palloc(PGLZ_RAW_SIZE(tmp) + VARHDRSZ);
                SET_VARSIZE(attr, PGLZ_RAW_SIZE(tmp) + VARHDRSZ);
                pglz_decompress(tmp, VARDATA(attr));
        }
        else if (VARATT_IS_SHORT(attr))
        {
                /*
                 * This is a short-header varlena --- convert to 4-byte header format
                 */
                Size            data_size = VARSIZE_SHORT(attr) - VARHDRSZ_SHORT;
                Size            new_size = data_size + VARHDRSZ;
                struct varlena *new_attr;

                new_attr = (struct varlena *) palloc(new_size);
                SET_VARSIZE(new_attr, new_size);
                memcpy(VARDATA(new_attr), VARDATA_SHORT(attr), data_size);
                attr = new_attr;
        }

        return attr;
}


/* ----------
 * heap_tuple_untoast_attr_slice -
 *
 *              Public entry point to get back part of a toasted value
 *              from compression or external storage.
 * ----------
 */
struct varlena *
heap_tuple_untoast_attr_slice(struct varlena * attr,
                                                          int32 sliceoffset, int32 slicelength)
{
        struct varlena *preslice;
        struct varlena *result;
        char       *attrdata;
        int32           attrsize;

        if (VARATT_IS_EXTERNAL(attr))
        {
                struct varatt_external toast_pointer;

                VARATT_EXTERNAL_GET_POINTER(toast_pointer, attr);

                /* fast path for non-compressed external datums */
                if (!VARATT_EXTERNAL_IS_COMPRESSED(toast_pointer))
                        return toast_fetch_datum_slice(attr, sliceoffset, slicelength);

                /* fetch it back (compressed marker will get set automatically) */
                preslice = toast_fetch_datum(attr);
        }
        else
                preslice = attr;

        if (VARATT_IS_COMPRESSED(preslice))
        {
                PGLZ_Header *tmp = (PGLZ_Header *) preslice;
                Size            size = PGLZ_RAW_SIZE(tmp) + VARHDRSZ;

                preslice = (struct varlena *) palloc(size);
                SET_VARSIZE(preslice, size);
                pglz_decompress(tmp, VARDATA(preslice));

                if (tmp != (PGLZ_Header *) attr)
                        pfree(tmp);
        }

        if (VARATT_IS_SHORT(preslice))
        {
                attrdata = VARDATA_SHORT(preslice);
                attrsize = VARSIZE_SHORT(preslice) - VARHDRSZ_SHORT;
        }
        else
        {
                attrdata = VARDATA(preslice);
                attrsize = VARSIZE(preslice) - VARHDRSZ;
        }

        /* slicing of datum for compressed cases and plain value */

        if (sliceoffset >= attrsize)
        {
                sliceoffset = 0;
                slicelength = 0;
        }

        if (((sliceoffset + slicelength) > attrsize) || slicelength < 0)
                slicelength = attrsize - sliceoffset;

        result = (struct varlena *) palloc(slicelength + VARHDRSZ);
        SET_VARSIZE(result, slicelength + VARHDRSZ);

        memcpy(VARDATA(result), attrdata + sliceoffset, slicelength);

        if (preslice != attr)
                pfree(preslice);

        return result;
}


/* ----------
 * toast_raw_datum_size -
 *
 *      Return the raw (detoasted) size of a varlena datum
 *      (including the VARHDRSZ header)
 * ----------
 */
Size
toast_raw_datum_size(Datum value)
{
        struct varlena *attr = (struct varlena *) DatumGetPointer(value);
        Size            result;

        if (VARATT_IS_EXTERNAL(attr))
        {
                /* va_rawsize is the size of the original datum -- including header */
                struct varatt_external toast_pointer;

                VARATT_EXTERNAL_GET_POINTER(toast_pointer, attr);
                result = toast_pointer.va_rawsize;
        }
        else if (VARATT_IS_COMPRESSED(attr))
        {
                /* here, va_rawsize is just the payload size */
                result = VARRAWSIZE_4B_C(attr) + VARHDRSZ;
        }
        else if (VARATT_IS_SHORT(attr))
        {
                /*
                 * we have to normalize the header length to VARHDRSZ or else the
                 * callers of this function will be confused.
                 */
                result = VARSIZE_SHORT(attr) - VARHDRSZ_SHORT + VARHDRSZ;
        }
        else
        {
                /* plain untoasted datum */
                result = VARSIZE(attr);
        }
        return result;
}

/* ----------
 * toast_datum_size
 *
 *      Return the physical storage size (possibly compressed) of a varlena datum
 * ----------
 */
Size
toast_datum_size(Datum value)
{
        struct varlena *attr = (struct varlena *) DatumGetPointer(value);
        Size            result;

        if (VARATT_IS_EXTERNAL(attr))
        {
                /*
                 * Attribute is stored externally - return the extsize whether
                 * compressed or not.  We do not count the size of the toast pointer
                 * ... should we?
                 */
                struct varatt_external toast_pointer;

                VARATT_EXTERNAL_GET_POINTER(toast_pointer, attr);
                result = toast_pointer.va_extsize;
        }
        else if (VARATT_IS_SHORT(attr))
        {
                result = VARSIZE_SHORT(attr);
        }
        else
        {
                /*
                 * Attribute is stored inline either compressed or not, just calculate
                 * the size of the datum in either case.
                 */
                result = VARSIZE(attr);
        }
        return result;
}


/* ----------
 * toast_delete -
 *
 *      Cascaded delete toast-entries on DELETE
 * ----------
 */
void
toast_delete(Relation rel, HeapTuple oldtup)
{
        TupleDesc       tupleDesc;
        Form_pg_attribute *att;
        int                     numAttrs;
        int                     i;
        Datum           toast_values[MaxHeapAttributeNumber];
        bool            toast_isnull[MaxHeapAttributeNumber];

        /*
         * We should only ever be called for tuples of plain relations ---
         * recursing on a toast rel is bad news.
         */
        Assert(rel->rd_rel->relkind == RELKIND_RELATION);

        /*
         * Get the tuple descriptor and break down the tuple into fields.
         *
         * NOTE: it's debatable whether to use heap_deformtuple() here or just
         * heap_getattr() only the varlena columns.  The latter could win if there
         * are few varlena columns and many non-varlena ones. However,
         * heap_deformtuple costs only O(N) while the heap_getattr way would cost
         * O(N^2) if there are many varlena columns, so it seems better to err on
         * the side of linear cost.  (We won't even be here unless there's at
         * least one varlena column, by the way.)
         */
        tupleDesc = rel->rd_att;
        att = tupleDesc->attrs;
        numAttrs = tupleDesc->natts;

        Assert(numAttrs <= MaxHeapAttributeNumber);
        heap_deform_tuple(oldtup, tupleDesc, toast_values, toast_isnull);

        /*
         * Check for external stored attributes and delete them from the secondary
         * relation.
         */
        for (i = 0; i < numAttrs; i++)
        {
                if (att[i]->attlen == -1)
                {
                        Datum           value = toast_values[i];

                        if (!toast_isnull[i] && VARATT_IS_EXTERNAL(PointerGetDatum(value)))
                                toast_delete_datum(rel, value);
                }
        }
}


/* ----------
 * toast_insert_or_update -
 *
 *      Delete no-longer-used toast-entries and create new ones to
 *      make the new tuple fit on INSERT or UPDATE
 *
 * Inputs:
 *      newtup: the candidate new tuple to be inserted
 *      oldtup: the old row version for UPDATE, or NULL for INSERT
 *      use_wal, use_fsm: flags to be passed to heap_insert() for toast rows
 * Result:
 *      either newtup if no toasting is needed, or a palloc'd modified tuple
 *      that is what should actually get stored
 *
 * NOTE: neither newtup nor oldtup will be modified.  This is a change
 * from the pre-8.1 API of this routine.
 * ----------
 */
HeapTuple
toast_insert_or_update(Relation rel, HeapTuple newtup, HeapTuple oldtup,
                                           bool use_wal, bool use_fsm)
{
        HeapTuple       result_tuple;
        TupleDesc       tupleDesc;
        Form_pg_attribute *att;
        int                     numAttrs;
        int                     i;

        bool            need_change = false;
        bool            need_free = false;
        bool            need_delold = false;
        bool            has_nulls = false;

        Size            maxDataLen;
        Size            hoff;

        char            toast_action[MaxHeapAttributeNumber];
        bool            toast_isnull[MaxHeapAttributeNumber];
        bool            toast_oldisnull[MaxHeapAttributeNumber];
        Datum           toast_values[MaxHeapAttributeNumber];
        Datum           toast_oldvalues[MaxHeapAttributeNumber];
        int32           toast_sizes[MaxHeapAttributeNumber];
        bool            toast_free[MaxHeapAttributeNumber];
        bool            toast_delold[MaxHeapAttributeNumber];

        /*
         * We should only ever be called for tuples of plain relations ---
         * recursing on a toast rel is bad news.
         */
        Assert(rel->rd_rel->relkind == RELKIND_RELATION);

        /*
         * Get the tuple descriptor and break down the tuple(s) into fields.
         */
        tupleDesc = rel->rd_att;
        att = tupleDesc->attrs;
        numAttrs = tupleDesc->natts;

        Assert(numAttrs <= MaxHeapAttributeNumber);
        heap_deform_tuple(newtup, tupleDesc, toast_values, toast_isnull);
        if (oldtup != NULL)
                heap_deform_tuple(oldtup, tupleDesc, toast_oldvalues, toast_oldisnull);

        /* ----------
         * Then collect information about the values given
         *
         * NOTE: toast_action[i] can have these values:
         *              ' '             default handling
         *              'p'             already processed --- don't touch it
         *              'x'             incompressible, but OK to move off
         *
         * NOTE: toast_sizes[i] is only made valid for varlena attributes with
         *              toast_action[i] different from 'p'.
         * ----------
         */
        memset(toast_action, ' ', numAttrs * sizeof(char));
        memset(toast_free, 0, numAttrs * sizeof(bool));
        memset(toast_delold, 0, numAttrs * sizeof(bool));

        for (i = 0; i < numAttrs; i++)
        {
                struct varlena *old_value;
                struct varlena *new_value;

                if (oldtup != NULL)
                {
                        /*
                         * For UPDATE get the old and new values of this attribute
                         */
                        old_value = (struct varlena *) DatumGetPointer(toast_oldvalues[i]);
                        new_value = (struct varlena *) DatumGetPointer(toast_values[i]);

                        /*
                         * If the old value is an external stored one, check if it has
                         * changed so we have to delete it later.
                         */
                        if (att[i]->attlen == -1 && !toast_oldisnull[i] &&
                                VARATT_IS_EXTERNAL(old_value))
                        {
                                if (toast_isnull[i] || !VARATT_IS_EXTERNAL(new_value) ||
                                        memcmp((char *) old_value, (char *) new_value,
                                                   VARSIZE_EXTERNAL(old_value)) != 0)
                                {
                                        /*
                                         * The old external stored value isn't needed any more
                                         * after the update
                                         */
                                        toast_delold[i] = true;
                                        need_delold = true;
                                }
                                else
                                {
                                        /*
                                         * This attribute isn't changed by this update so we reuse
                                         * the original reference to the old value in the new
                                         * tuple.
                                         */
                                        toast_action[i] = 'p';
                                        continue;
                                }
                        }
                }
                else
                {
                        /*
                         * For INSERT simply get the new value
                         */
                        new_value = (struct varlena *) DatumGetPointer(toast_values[i]);
                }

                /*
                 * Handle NULL attributes
                 */
                if (toast_isnull[i])
                {
                        toast_action[i] = 'p';
                        has_nulls = true;
                        continue;
                }

                /*
                 * Now look at varlena attributes
                 */
                if (att[i]->attlen == -1)
                {
                        /*
                         * If the table's attribute says PLAIN always, force it so.
                         */
                        if (att[i]->attstorage == 'p')
                                toast_action[i] = 'p';

                        /*
                         * We took care of UPDATE above, so any external value we find
                         * still in the tuple must be someone else's we cannot reuse.
                         * Fetch it back (without decompression, unless we are forcing
                         * PLAIN storage).      If necessary, we'll push it out as a new
                         * external value below.
                         */
                        if (VARATT_IS_EXTERNAL(new_value))
                        {
                                if (att[i]->attstorage == 'p')
                                        new_value = heap_tuple_untoast_attr(new_value);
                                else
                                        new_value = heap_tuple_fetch_attr(new_value);
                                toast_values[i] = PointerGetDatum(new_value);
                                toast_free[i] = true;
                                need_change = true;
                                need_free = true;
                        }

                        /*
                         * Remember the size of this attribute
                         */
                        toast_sizes[i] = VARSIZE_ANY(new_value);
                }
                else
                {
                        /*
                         * Not a varlena attribute, plain storage always
                         */
                        toast_action[i] = 'p';
                }
        }

        /* ----------
         * Compress and/or save external until data fits into target length
         *
         *      1: Inline compress attributes with attstorage 'x', and store very
         *         large attributes with attstorage 'x' or 'e' external immediately
         *      2: Store attributes with attstorage 'x' or 'e' external
         *      3: Inline compress attributes with attstorage 'm'
         *      4: Store attributes with attstorage 'm' external
         * ----------
         */

        /* compute header overhead --- this should match heap_form_tuple() */
        hoff = offsetof(HeapTupleHeaderData, t_bits);
        if (has_nulls)
                hoff += BITMAPLEN(numAttrs);
        if (newtup->t_data->t_infomask & HEAP_HASOID)
                hoff += sizeof(Oid);
        hoff = MAXALIGN(hoff);
        Assert(hoff == newtup->t_data->t_hoff);
        /* now convert to a limit on the tuple data size */
        maxDataLen = TOAST_TUPLE_TARGET - hoff;

        /*
         * Look for attributes with attstorage 'x' to compress.  Also find large
         * attributes with attstorage 'x' or 'e', and store them external.
         */
        while (heap_compute_data_size(tupleDesc,
                                                                  toast_values, toast_isnull) > maxDataLen)
        {
                int                     biggest_attno = -1;
                int32           biggest_size = MAXALIGN(TOAST_POINTER_SIZE);
                Datum           old_value;
                Datum           new_value;

                /*
                 * Search for the biggest yet unprocessed internal attribute
                 */
                for (i = 0; i < numAttrs; i++)
                {
                        if (toast_action[i] != ' ')
                                continue;
                        if (VARATT_IS_EXTERNAL(DatumGetPointer(toast_values[i])))
                                continue;               /* can't happen, toast_action would be 'p' */
                        if (VARATT_IS_COMPRESSED(DatumGetPointer(toast_values[i])))
                                continue;
                        if (att[i]->attstorage != 'x' && att[i]->attstorage != 'e')
                                continue;
                        if (toast_sizes[i] > biggest_size)
                        {
                                biggest_attno = i;
                                biggest_size = toast_sizes[i];
                        }
                }

                if (biggest_attno < 0)
                        break;

                /*
                 * Attempt to compress it inline, if it has attstorage 'x'
                 */
                i = biggest_attno;
                if (att[i]->attstorage == 'x')
                {
                        old_value = toast_values[i];
                        new_value = toast_compress_datum(old_value);

                        if (DatumGetPointer(new_value) != NULL)
                        {
                                /* successful compression */
                                if (toast_free[i])
                                        pfree(DatumGetPointer(old_value));
                                toast_values[i] = new_value;
                                toast_free[i] = true;
                                toast_sizes[i] = VARSIZE(DatumGetPointer(toast_values[i]));
                                need_change = true;
                                need_free = true;
                        }
                        else
                        {
                                /* incompressible, ignore on subsequent compression passes */
                                toast_action[i] = 'x';
                        }
                }
                else
                {
                        /* has attstorage 'e', ignore on subsequent compression passes */
                        toast_action[i] = 'x';
                }

                /*
                 * If this value is by itself more than maxDataLen (after compression
                 * if any), push it out to the toast table immediately, if possible.
                 * This avoids uselessly compressing other fields in the common case
                 * where we have one long field and several short ones.
                 *
                 * XXX maybe the threshold should be less than maxDataLen?
                 */
                if (toast_sizes[i] > maxDataLen &&
                        rel->rd_rel->reltoastrelid != InvalidOid)
                {
                        old_value = toast_values[i];
                        toast_action[i] = 'p';
                        toast_values[i] = toast_save_datum(rel, toast_values[i],
                                                                                           use_wal, use_fsm);
                        if (toast_free[i])
                                pfree(DatumGetPointer(old_value));
                        toast_free[i] = true;
                        need_change = true;
                        need_free = true;
                }
        }

        /*
         * Second we look for attributes of attstorage 'x' or 'e' that are still
         * inline.      But skip this if there's no toast table to push them to.
         */
        while (heap_compute_data_size(tupleDesc,
                                                                  toast_values, toast_isnull) > maxDataLen &&
                   rel->rd_rel->reltoastrelid != InvalidOid)
        {
                int                     biggest_attno = -1;
                int32           biggest_size = MAXALIGN(TOAST_POINTER_SIZE);
                Datum           old_value;

                /*------
                 * Search for the biggest yet inlined attribute with
                 * attstorage equals 'x' or 'e'
                 *------
                 */
                for (i = 0; i < numAttrs; i++)
                {
                        if (toast_action[i] == 'p')
                                continue;
                        if (VARATT_IS_EXTERNAL(DatumGetPointer(toast_values[i])))
                                continue;               /* can't happen, toast_action would be 'p' */
                        if (att[i]->attstorage != 'x' && att[i]->attstorage != 'e')
                                continue;
                        if (toast_sizes[i] > biggest_size)
                        {
                                biggest_attno = i;
                                biggest_size = toast_sizes[i];
                        }
                }

                if (biggest_attno < 0)
                        break;

                /*
                 * Store this external
                 */
                i = biggest_attno;
                old_value = toast_values[i];
                toast_action[i] = 'p';
                toast_values[i] = toast_save_datum(rel, toast_values[i],
                                                                                   use_wal, use_fsm);
                if (toast_free[i])
                        pfree(DatumGetPointer(old_value));
                toast_free[i] = true;

                need_change = true;
                need_free = true;
        }

        /*
         * Round 3 - this time we take attributes with storage 'm' into
         * compression
         */
        while (heap_compute_data_size(tupleDesc,
                                                                  toast_values, toast_isnull) > maxDataLen)
        {
                int                     biggest_attno = -1;
                int32           biggest_size = MAXALIGN(TOAST_POINTER_SIZE);
                Datum           old_value;
                Datum           new_value;

                /*
                 * Search for the biggest yet uncompressed internal attribute
                 */
                for (i = 0; i < numAttrs; i++)
                {
                        if (toast_action[i] != ' ')
                                continue;
                        if (VARATT_IS_EXTERNAL(DatumGetPointer(toast_values[i])))
                                continue;               /* can't happen, toast_action would be 'p' */
                        if (VARATT_IS_COMPRESSED(DatumGetPointer(toast_values[i])))
                                continue;
                        if (att[i]->attstorage != 'm')
                                continue;
                        if (toast_sizes[i] > biggest_size)
                        {
                                biggest_attno = i;
                                biggest_size = toast_sizes[i];
                        }
                }

                if (biggest_attno < 0)
                        break;

                /*
                 * Attempt to compress it inline
                 */
                i = biggest_attno;
                old_value = toast_values[i];
                new_value = toast_compress_datum(old_value);

                if (DatumGetPointer(new_value) != NULL)
                {
                        /* successful compression */
                        if (toast_free[i])
                                pfree(DatumGetPointer(old_value));
                        toast_values[i] = new_value;
                        toast_free[i] = true;
                        toast_sizes[i] = VARSIZE(DatumGetPointer(toast_values[i]));
                        need_change = true;
                        need_free = true;
                }
                else
                {
                        /* incompressible, ignore on subsequent compression passes */
                        toast_action[i] = 'x';
                }
        }

        /*
         * Finally we store attributes of type 'm' external, if possible.
         */
        while (heap_compute_data_size(tupleDesc,
                                                                  toast_values, toast_isnull) > maxDataLen &&
                   rel->rd_rel->reltoastrelid != InvalidOid)
        {
                int                     biggest_attno = -1;
                int32           biggest_size = MAXALIGN(TOAST_POINTER_SIZE);
                Datum           old_value;

                /*--------
                 * Search for the biggest yet inlined attribute with
                 * attstorage = 'm'
                 *--------
                 */
                for (i = 0; i < numAttrs; i++)
                {
                        if (toast_action[i] == 'p')
                                continue;
                        if (VARATT_IS_EXTERNAL(DatumGetPointer(toast_values[i])))
                                continue;               /* can't happen, toast_action would be 'p' */
                        if (att[i]->attstorage != 'm')
                                continue;
                        if (toast_sizes[i] > biggest_size)
                        {
                                biggest_attno = i;
                                biggest_size = toast_sizes[i];
                        }
                }

                if (biggest_attno < 0)
                        break;

                /*
                 * Store this external
                 */
                i = biggest_attno;
                old_value = toast_values[i];
                toast_action[i] = 'p';
                toast_values[i] = toast_save_datum(rel, toast_values[i],
                                                                                   use_wal, use_fsm);
                if (toast_free[i])
                        pfree(DatumGetPointer(old_value));
                toast_free[i] = true;

                need_change = true;
                need_free = true;
        }

        /*
         * In the case we toasted any values, we need to build a new heap tuple
         * with the changed values.
         */
        if (need_change)
        {
                HeapTupleHeader olddata = newtup->t_data;
                HeapTupleHeader new_data;
                int32           new_len;
                int32           new_data_len;

                /*
                 * Calculate the new size of the tuple.  Header size should not
                 * change, but data size might.
                 */
                new_len = offsetof(HeapTupleHeaderData, t_bits);
                if (has_nulls)
                        new_len += BITMAPLEN(numAttrs);
                if (olddata->t_infomask & HEAP_HASOID)
                        new_len += sizeof(Oid);
                new_len = MAXALIGN(new_len);
                Assert(new_len == olddata->t_hoff);
                new_data_len = heap_compute_data_size(tupleDesc,
                                                                                          toast_values, toast_isnull);
                new_len += new_data_len;

                /*
                 * Allocate and zero the space needed, and fill HeapTupleData fields.
                 */
                result_tuple = (HeapTuple) palloc0(HEAPTUPLESIZE + new_len);
                result_tuple->t_len = new_len;
                result_tuple->t_self = newtup->t_self;
                result_tuple->t_tableOid = newtup->t_tableOid;
                new_data = (HeapTupleHeader) ((char *) result_tuple + HEAPTUPLESIZE);
                result_tuple->t_data = new_data;

                /*
                 * Put the existing tuple header and the changed values into place
                 */
                memcpy(new_data, olddata, olddata->t_hoff);

                heap_fill_tuple(tupleDesc,
                                                toast_values,
                                                toast_isnull,
                                                (char *) new_data + olddata->t_hoff,
                                                new_data_len,
                                                &(new_data->t_infomask),
                                                has_nulls ? new_data->t_bits : NULL);
        }
        else
                result_tuple = newtup;

        /*
         * Free allocated temp values
         */
        if (need_free)
                for (i = 0; i < numAttrs; i++)
                        if (toast_free[i])
                                pfree(DatumGetPointer(toast_values[i]));

        /*
         * Delete external values from the old tuple
         */
        if (need_delold)
                for (i = 0; i < numAttrs; i++)
                        if (toast_delold[i])
                                toast_delete_datum(rel, toast_oldvalues[i]);

        return result_tuple;
}


/* ----------
 * toast_flatten_tuple_attribute -
 *
 *      If a Datum is of composite type, "flatten" it to contain no toasted fields.
 *      This must be invoked on any potentially-composite field that is to be
 *      inserted into a tuple.  Doing this preserves the invariant that toasting
 *      goes only one level deep in a tuple.
 *
 *      Note that flattening does not mean expansion of short-header varlenas,
 *      so in one sense toasting is allowed within composite datums.
 * ----------
 */
Datum
toast_flatten_tuple_attribute(Datum value,
                                                          Oid typeId, int32 typeMod)
{
        TupleDesc       tupleDesc;
        HeapTupleHeader olddata;
        HeapTupleHeader new_data;
        int32           new_len;
        int32           new_data_len;
        HeapTupleData tmptup;
        Form_pg_attribute *att;
        int                     numAttrs;
        int                     i;
        bool            need_change = false;
        bool            has_nulls = false;
        Datum           toast_values[MaxTupleAttributeNumber];
        bool            toast_isnull[MaxTupleAttributeNumber];
        bool            toast_free[MaxTupleAttributeNumber];

        /*
         * See if it's a composite type, and get the tupdesc if so.
         */
        tupleDesc = lookup_rowtype_tupdesc_noerror(typeId, typeMod, true);
        if (tupleDesc == NULL)
                return value;                   /* not a composite type */

        att = tupleDesc->attrs;
        numAttrs = tupleDesc->natts;

        /*
         * Break down the tuple into fields.
         */
        olddata = DatumGetHeapTupleHeader(value);
        Assert(typeId == HeapTupleHeaderGetTypeId(olddata));
        Assert(typeMod == HeapTupleHeaderGetTypMod(olddata));
        /* Build a temporary HeapTuple control structure */
        tmptup.t_len = HeapTupleHeaderGetDatumLength(olddata);
        ItemPointerSetInvalid(&(tmptup.t_self));
        tmptup.t_tableOid = InvalidOid;
        tmptup.t_data = olddata;

        Assert(numAttrs <= MaxTupleAttributeNumber);
        heap_deform_tuple(&tmptup, tupleDesc, toast_values, toast_isnull);

        memset(toast_free, 0, numAttrs * sizeof(bool));

        for (i = 0; i < numAttrs; i++)
        {
                /*
                 * Look at non-null varlena attributes
                 */
                if (toast_isnull[i])
                        has_nulls = true;
                else if (att[i]->attlen == -1)
                {
                        struct varlena *new_value;

                        new_value = (struct varlena *) DatumGetPointer(toast_values[i]);
                        if (VARATT_IS_EXTERNAL(new_value) ||
                                VARATT_IS_COMPRESSED(new_value))
                        {
                                new_value = heap_tuple_untoast_attr(new_value);
                                toast_values[i] = PointerGetDatum(new_value);
                                toast_free[i] = true;
                                need_change = true;
                        }
                }
        }

        /*
         * If nothing to untoast, just return the original tuple.
         */
        if (!need_change)
        {
                ReleaseTupleDesc(tupleDesc);
                return value;
        }

        /*
         * Calculate the new size of the tuple.  Header size should not change,
         * but data size might.
         */
        new_len = offsetof(HeapTupleHeaderData, t_bits);
        if (has_nulls)
                new_len += BITMAPLEN(numAttrs);
        if (olddata->t_infomask & HEAP_HASOID)
                new_len += sizeof(Oid);
        new_len = MAXALIGN(new_len);
        Assert(new_len == olddata->t_hoff);
        new_data_len = heap_compute_data_size(tupleDesc,
                                                                                  toast_values, toast_isnull);
        new_len += new_data_len;

        new_data = (HeapTupleHeader) palloc0(new_len);

        /*
         * Put the tuple header and the changed values into place
         */
        memcpy(new_data, olddata, olddata->t_hoff);

        HeapTupleHeaderSetDatumLength(new_data, new_len);

        heap_fill_tuple(tupleDesc,
                                        toast_values,
                                        toast_isnull,
                                        (char *) new_data + olddata->t_hoff,
                                        new_data_len,
                                        &(new_data->t_infomask),
                                        has_nulls ? new_data->t_bits : NULL);

        /*
         * Free allocated temp values
         */
        for (i = 0; i < numAttrs; i++)
                if (toast_free[i])
                        pfree(DatumGetPointer(toast_values[i]));
        ReleaseTupleDesc(tupleDesc);

        return PointerGetDatum(new_data);
}


/* ----------
 * toast_compress_datum -
 *
 *      Create a compressed version of a varlena datum
 *
 *      If we fail (ie, compressed result is actually bigger than original)
 *      then return NULL.  We must not use compressed data if it'd expand
 *      the tuple!
 *
 *      We use VAR{SIZE,DATA}_ANY so we can handle short varlenas here without
 *      copying them.  But we can't handle external or compressed datums.
 * ----------
 */
Datum
toast_compress_datum(Datum value)
{
        struct varlena *tmp;
        int32           valsize = VARSIZE_ANY_EXHDR(DatumGetPointer(value));

        Assert(!VARATT_IS_EXTERNAL(DatumGetPointer(value)));
        Assert(!VARATT_IS_COMPRESSED(DatumGetPointer(value)));

        /*
         * No point in wasting a palloc cycle if value size is out of the
         * allowed range for compression
         */
        if (valsize < PGLZ_strategy_default->min_input_size ||
                valsize > PGLZ_strategy_default->max_input_size)
                return PointerGetDatum(NULL);

        tmp = (struct varlena *) palloc(PGLZ_MAX_OUTPUT(valsize));

        /*
         * We recheck the actual size even if pglz_compress() reports success,
         * because it might be satisfied with having saved as little as one byte
         * in the compressed data --- which could turn into a net loss once you
         * consider header and alignment padding.  Worst case, the compressed
         * format might require three padding bytes (plus header, which is included
         * in VARSIZE(tmp)), whereas the uncompressed format would take only one
         * header byte and no padding if the value is short enough.  So we insist
         * on a savings of more than 2 bytes to ensure we have a gain.
         */
        if (pglz_compress(VARDATA_ANY(DatumGetPointer(value)), valsize,
                                          (PGLZ_Header *) tmp, PGLZ_strategy_default) &&
                VARSIZE(tmp) < valsize - 2)
        {
                /* successful compression */
                return PointerGetDatum(tmp);
        }
        else
        {
                /* incompressible data */
                pfree(tmp);
                return PointerGetDatum(NULL);
        }
}


/* ----------
 * toast_save_datum -
 *
 *      Save one single datum into the secondary relation and return
 *      a Datum reference for it.
 * ----------
 */
static Datum
toast_save_datum(Relation rel, Datum value,
                                 bool use_wal, bool use_fsm)
{
        Relation        toastrel;
        Relation        toastidx;
        HeapTuple       toasttup;
        TupleDesc       toasttupDesc;
        Datum           t_values[3];
        bool            t_isnull[3];
        CommandId       mycid = GetCurrentCommandId(true);
        struct varlena *result;
        struct varatt_external toast_pointer;
        struct
        {
                struct varlena hdr;
                char            data[TOAST_MAX_CHUNK_SIZE];     /* make struct big enough */
                int32           align_it;       /* ensure struct is aligned well enough */
        }                       chunk_data;
        int32           chunk_size;
        int32           chunk_seq = 0;
        char       *data_p;
        int32           data_todo;
        Pointer         dval = DatumGetPointer(value);

        /*
         * Open the toast relation and its index.  We can use the index to check
         * uniqueness of the OID we assign to the toasted item, even though it has
         * additional columns besides OID.
         */
        toastrel = heap_open(rel->rd_rel->reltoastrelid, RowExclusiveLock);
        toasttupDesc = toastrel->rd_att;
        toastidx = index_open(toastrel->rd_rel->reltoastidxid, RowExclusiveLock);

        /*
         * Get the data pointer and length, and compute va_rawsize and va_extsize.
         *
         * va_rawsize is the size of the equivalent fully uncompressed datum, so
         * we have to adjust for short headers.
         *
         * va_extsize is the actual size of the data payload in the toast records.
         */
        if (VARATT_IS_SHORT(dval))
        {
                data_p = VARDATA_SHORT(dval);
                data_todo = VARSIZE_SHORT(dval) - VARHDRSZ_SHORT;
                toast_pointer.va_rawsize = data_todo + VARHDRSZ;                /* as if not short */
                toast_pointer.va_extsize = data_todo;
        }
        else if (VARATT_IS_COMPRESSED(dval))
        {
                data_p = VARDATA(dval);
                data_todo = VARSIZE(dval) - VARHDRSZ;
                /* rawsize in a compressed datum is just the size of the payload */
                toast_pointer.va_rawsize = VARRAWSIZE_4B_C(dval) + VARHDRSZ;
                toast_pointer.va_extsize = data_todo;
                /* Assert that the numbers look like it's compressed */
                Assert(VARATT_EXTERNAL_IS_COMPRESSED(toast_pointer));
        }
        else
        {
                data_p = VARDATA(dval);
                data_todo = VARSIZE(dval) - VARHDRSZ;
                toast_pointer.va_rawsize = VARSIZE(dval);
                toast_pointer.va_extsize = data_todo;
        }

        toast_pointer.va_valueid = GetNewOidWithIndex(toastrel,
                                                                                                  RelationGetRelid(toastidx),
                                                                                                  (AttrNumber) 1);
        toast_pointer.va_toastrelid = rel->rd_rel->reltoastrelid;

        /*
         * Initialize constant parts of the tuple data
         */
        t_values[0] = ObjectIdGetDatum(toast_pointer.va_valueid);
        t_values[2] = PointerGetDatum(&chunk_data);
        t_isnull[0] = false;
        t_isnull[1] = false;
        t_isnull[2] = false;

        /*
         * Split up the item into chunks
         */
        while (data_todo > 0)
        {
                /*
                 * Calculate the size of this chunk
                 */
                chunk_size = Min(TOAST_MAX_CHUNK_SIZE, data_todo);

                /*
                 * Build a tuple and store it
                 */
                t_values[1] = Int32GetDatum(chunk_seq++);
                SET_VARSIZE(&chunk_data, chunk_size + VARHDRSZ);
                memcpy(VARDATA(&chunk_data), data_p, chunk_size);
                toasttup = heap_form_tuple(toasttupDesc, t_values, t_isnull);

                heap_insert(toastrel, toasttup, mycid, use_wal, use_fsm);

                /*
                 * Create the index entry.      We cheat a little here by not using
                 * FormIndexDatum: this relies on the knowledge that the index columns
                 * are the same as the initial columns of the table.
                 *
                 * Note also that there had better not be any user-created index on
                 * the TOAST table, since we don't bother to update anything else.
                 */
                index_insert(toastidx, t_values, t_isnull,
                                         &(toasttup->t_self),
                                         toastrel, toastidx->rd_index->indisunique);

                /*
                 * Free memory
                 */
                heap_freetuple(toasttup);

                /*
                 * Move on to next chunk
                 */
                data_todo -= chunk_size;
                data_p += chunk_size;
        }

        /*
         * Done - close toast relation
         */
        index_close(toastidx, RowExclusiveLock);
        heap_close(toastrel, RowExclusiveLock);

        /*
         * Create the TOAST pointer value that we'll return
         */
        result = (struct varlena *) palloc(TOAST_POINTER_SIZE);
        SET_VARSIZE_EXTERNAL(result, TOAST_POINTER_SIZE);
        memcpy(VARDATA_EXTERNAL(result), &toast_pointer, sizeof(toast_pointer));

        return PointerGetDatum(result);
}


/* ----------
 * toast_delete_datum -
 *
 *      Delete a single external stored value.
 * ----------
 */
static void
toast_delete_datum(Relation rel, Datum value)
{
        struct varlena *attr = (struct varlena *) DatumGetPointer(value);
        struct varatt_external toast_pointer;
        Relation        toastrel;
        Relation        toastidx;
        ScanKeyData toastkey;
        SysScanDesc toastscan;
        HeapTuple       toasttup;

        if (!VARATT_IS_EXTERNAL(attr))
                return;

        /* Must copy to access aligned fields */
        VARATT_EXTERNAL_GET_POINTER(toast_pointer, attr);

        /*
         * Open the toast relation and its index
         */
        toastrel = heap_open(toast_pointer.va_toastrelid, RowExclusiveLock);
        toastidx = index_open(toastrel->rd_rel->reltoastidxid, RowExclusiveLock);

        /*
         * Setup a scan key to find chunks with matching va_valueid
         */
        ScanKeyInit(&toastkey,
                                (AttrNumber) 1,
                                BTEqualStrategyNumber, F_OIDEQ,
                                ObjectIdGetDatum(toast_pointer.va_valueid));

        /*
         * Find all the chunks.  (We don't actually care whether we see them in
         * sequence or not, but since we've already locked the index we might
         * as well use systable_beginscan_ordered.)
         */
        toastscan = systable_beginscan_ordered(toastrel, toastidx,
                                                                                   SnapshotToast, 1, &toastkey);
        while ((toasttup = systable_getnext_ordered(toastscan, ForwardScanDirection)) != NULL)
        {
                /*
                 * Have a chunk, delete it
                 */
                simple_heap_delete(toastrel, &toasttup->t_self);
        }

        /*
         * End scan and close relations
         */
        systable_endscan_ordered(toastscan);
        index_close(toastidx, RowExclusiveLock);
        heap_close(toastrel, RowExclusiveLock);
}


/* ----------
 * toast_fetch_datum -
 *
 *      Reconstruct an in memory Datum from the chunks saved
 *      in the toast relation
 * ----------
 */
static struct varlena *
toast_fetch_datum(struct varlena * attr)
{
        Relation        toastrel;
        Relation        toastidx;
        ScanKeyData toastkey;
        SysScanDesc toastscan;
        HeapTuple       ttup;
        TupleDesc       toasttupDesc;
        struct varlena *result;
        struct varatt_external toast_pointer;
        int32           ressize;
        int32           residx,
                                nextidx;
        int32           numchunks;
        Pointer         chunk;
        bool            isnull;
        char       *chunkdata;
        int32           chunksize;

        /* Must copy to access aligned fields */
        VARATT_EXTERNAL_GET_POINTER(toast_pointer, attr);

        ressize = toast_pointer.va_extsize;
        numchunks = ((ressize - 1) / TOAST_MAX_CHUNK_SIZE) + 1;

        result = (struct varlena *) palloc(ressize + VARHDRSZ);

        if (VARATT_EXTERNAL_IS_COMPRESSED(toast_pointer))
                SET_VARSIZE_COMPRESSED(result, ressize + VARHDRSZ);
        else
                SET_VARSIZE(result, ressize + VARHDRSZ);

        /*
         * Open the toast relation and its index
         */
        toastrel = heap_open(toast_pointer.va_toastrelid, AccessShareLock);
        toasttupDesc = toastrel->rd_att;
        toastidx = index_open(toastrel->rd_rel->reltoastidxid, AccessShareLock);

        /*
         * Setup a scan key to fetch from the index by va_valueid
         */
        ScanKeyInit(&toastkey,
                                (AttrNumber) 1,
                                BTEqualStrategyNumber, F_OIDEQ,
                                ObjectIdGetDatum(toast_pointer.va_valueid));

        /*
         * Read the chunks by index
         *
         * Note that because the index is actually on (valueid, chunkidx) we will
         * see the chunks in chunkidx order, even though we didn't explicitly ask
         * for it.
         */
        nextidx = 0;

        toastscan = systable_beginscan_ordered(toastrel, toastidx,
                                                                                   SnapshotToast, 1, &toastkey);
        while ((ttup = systable_getnext_ordered(toastscan, ForwardScanDirection)) != NULL)
        {
                /*
                 * Have a chunk, extract the sequence number and the data
                 */
                residx = DatumGetInt32(fastgetattr(ttup, 2, toasttupDesc, &isnull));
                Assert(!isnull);
                chunk = DatumGetPointer(fastgetattr(ttup, 3, toasttupDesc, &isnull));
                Assert(!isnull);
                if (!VARATT_IS_EXTENDED(chunk))
                {
                        chunksize = VARSIZE(chunk) - VARHDRSZ;
                        chunkdata = VARDATA(chunk);
                }
                else if (VARATT_IS_SHORT(chunk))
                {
                        /* could happen due to heap_form_tuple doing its thing */
                        chunksize = VARSIZE_SHORT(chunk) - VARHDRSZ_SHORT;
                        chunkdata = VARDATA_SHORT(chunk);
                }
                else
                {
                        /* should never happen */
                        elog(ERROR, "found toasted toast chunk for toast value %u in %s",
                                 toast_pointer.va_valueid,
                                 RelationGetRelationName(toastrel));
                        chunksize = 0;          /* keep compiler quiet */
                        chunkdata = NULL;
                }

                /*
                 * Some checks on the data we've found
                 */
                if (residx != nextidx)
                        elog(ERROR, "unexpected chunk number %d (expected %d) for toast value %u in %s",
                                 residx, nextidx,
                                 toast_pointer.va_valueid,
                                 RelationGetRelationName(toastrel));
                if (residx < numchunks - 1)
                {
                        if (chunksize != TOAST_MAX_CHUNK_SIZE)
                                elog(ERROR, "unexpected chunk size %d (expected %d) in chunk %d of %d for toast value %u in %s",
                                         chunksize, (int) TOAST_MAX_CHUNK_SIZE,
                                         residx, numchunks,
                                         toast_pointer.va_valueid,
                                         RelationGetRelationName(toastrel));
                }
                else if (residx == numchunks - 1)
                {
                        if ((residx * TOAST_MAX_CHUNK_SIZE + chunksize) != ressize)
                                elog(ERROR, "unexpected chunk size %d (expected %d) in final chunk %d for toast value %u in %s",
                                         chunksize,
                                         (int) (ressize - residx * TOAST_MAX_CHUNK_SIZE),
                                         residx,
                                         toast_pointer.va_valueid,
                                         RelationGetRelationName(toastrel));
                }
                else
                        elog(ERROR, "unexpected chunk number %d (out of range %d..%d) for toast value %u in %s",
                                 residx,
                                 0, numchunks - 1,
                                 toast_pointer.va_valueid,
                                 RelationGetRelationName(toastrel));

                /*
                 * Copy the data into proper place in our result
                 */
                memcpy(VARDATA(result) + residx * TOAST_MAX_CHUNK_SIZE,
                           chunkdata,
                           chunksize);

                nextidx++;
        }

        /*
         * Final checks that we successfully fetched the datum
         */
        if (nextidx != numchunks)
                elog(ERROR, "missing chunk number %d for toast value %u in %s",
                         nextidx,
                         toast_pointer.va_valueid,
                         RelationGetRelationName(toastrel));

        /*
         * End scan and close relations
         */
        systable_endscan_ordered(toastscan);
        index_close(toastidx, AccessShareLock);
        heap_close(toastrel, AccessShareLock);

        return result;
}

/* ----------
 * toast_fetch_datum_slice -
 *
 *      Reconstruct a segment of a Datum from the chunks saved
 *      in the toast relation
 * ----------
 */
static struct varlena *
toast_fetch_datum_slice(struct varlena * attr, int32 sliceoffset, int32 length)
{
        Relation        toastrel;
        Relation        toastidx;
        ScanKeyData toastkey[3];
        int                     nscankeys;
        SysScanDesc toastscan;
        HeapTuple       ttup;
        TupleDesc       toasttupDesc;
        struct varlena *result;
        struct varatt_external toast_pointer;
        int32           attrsize;
        int32           residx;
        int32           nextidx;
        int                     numchunks;
        int                     startchunk;
        int                     endchunk;
        int32           startoffset;
        int32           endoffset;
        int                     totalchunks;
        Pointer         chunk;
        bool            isnull;
        char       *chunkdata;
        int32           chunksize;
        int32           chcpystrt;
        int32           chcpyend;

        Assert(VARATT_IS_EXTERNAL(attr));

        /* Must copy to access aligned fields */
        VARATT_EXTERNAL_GET_POINTER(toast_pointer, attr);

        /*
         * It's nonsense to fetch slices of a compressed datum -- this isn't lo_*
         * we can't return a compressed datum which is meaningful to toast later
         */
        Assert(!VARATT_EXTERNAL_IS_COMPRESSED(toast_pointer));

        attrsize = toast_pointer.va_extsize;
        totalchunks = ((attrsize - 1) / TOAST_MAX_CHUNK_SIZE) + 1;

        if (sliceoffset >= attrsize)
        {
                sliceoffset = 0;
                length = 0;
        }

        if (((sliceoffset + length) > attrsize) || length < 0)
                length = attrsize - sliceoffset;

        result = (struct varlena *) palloc(length + VARHDRSZ);

        if (VARATT_EXTERNAL_IS_COMPRESSED(toast_pointer))
                SET_VARSIZE_COMPRESSED(result, length + VARHDRSZ);
        else
                SET_VARSIZE(result, length + VARHDRSZ);

        if (length == 0)
                return result;                  /* Can save a lot of work at this point! */

        startchunk = sliceoffset / TOAST_MAX_CHUNK_SIZE;
        endchunk = (sliceoffset + length - 1) / TOAST_MAX_CHUNK_SIZE;
        numchunks = (endchunk - startchunk) + 1;

        startoffset = sliceoffset % TOAST_MAX_CHUNK_SIZE;
        endoffset = (sliceoffset + length - 1) % TOAST_MAX_CHUNK_SIZE;

        /*
         * Open the toast relation and its index
         */
        toastrel = heap_open(toast_pointer.va_toastrelid, AccessShareLock);
        toasttupDesc = toastrel->rd_att;
        toastidx = index_open(toastrel->rd_rel->reltoastidxid, AccessShareLock);

        /*
         * Setup a scan key to fetch from the index. This is either two keys or
         * three depending on the number of chunks.
         */
        ScanKeyInit(&toastkey[0],
                                (AttrNumber) 1,
                                BTEqualStrategyNumber, F_OIDEQ,
                                ObjectIdGetDatum(toast_pointer.va_valueid));

        /*
         * Use equality condition for one chunk, a range condition otherwise:
         */
        if (numchunks == 1)
        {
                ScanKeyInit(&toastkey[1],
                                        (AttrNumber) 2,
                                        BTEqualStrategyNumber, F_INT4EQ,
                                        Int32GetDatum(startchunk));
                nscankeys = 2;
        }
        else
        {
                ScanKeyInit(&toastkey[1],
                                        (AttrNumber) 2,
                                        BTGreaterEqualStrategyNumber, F_INT4GE,
                                        Int32GetDatum(startchunk));
                ScanKeyInit(&toastkey[2],
                                        (AttrNumber) 2,
                                        BTLessEqualStrategyNumber, F_INT4LE,
                                        Int32GetDatum(endchunk));
                nscankeys = 3;
        }

        /*
         * Read the chunks by index
         *
         * The index is on (valueid, chunkidx) so they will come in order
         */
        nextidx = startchunk;
        toastscan = systable_beginscan_ordered(toastrel, toastidx,
                                                                                   SnapshotToast, nscankeys, toastkey);
        while ((ttup = systable_getnext_ordered(toastscan, ForwardScanDirection)) != NULL)
        {
                /*
                 * Have a chunk, extract the sequence number and the data
                 */
                residx = DatumGetInt32(fastgetattr(ttup, 2, toasttupDesc, &isnull));
                Assert(!isnull);
                chunk = DatumGetPointer(fastgetattr(ttup, 3, toasttupDesc, &isnull));
                Assert(!isnull);
                if (!VARATT_IS_EXTENDED(chunk))
                {
                        chunksize = VARSIZE(chunk) - VARHDRSZ;
                        chunkdata = VARDATA(chunk);
                }
                else if (VARATT_IS_SHORT(chunk))
                {
                        /* could happen due to heap_form_tuple doing its thing */
                        chunksize = VARSIZE_SHORT(chunk) - VARHDRSZ_SHORT;
                        chunkdata = VARDATA_SHORT(chunk);
                }
                else
                {
                        /* should never happen */
                        elog(ERROR, "found toasted toast chunk for toast value %u in %s",
                                 toast_pointer.va_valueid,
                                 RelationGetRelationName(toastrel));
                        chunksize = 0;          /* keep compiler quiet */
                        chunkdata = NULL;
                }

                /*
                 * Some checks on the data we've found
                 */
                if ((residx != nextidx) || (residx > endchunk) || (residx < startchunk))
                        elog(ERROR, "unexpected chunk number %d (expected %d) for toast value %u in %s",
                                 residx, nextidx,
                                 toast_pointer.va_valueid,
                                 RelationGetRelationName(toastrel));
                if (residx < totalchunks - 1)
                {
                        if (chunksize != TOAST_MAX_CHUNK_SIZE)
                                elog(ERROR, "unexpected chunk size %d (expected %d) in chunk %d of %d for toast value %u in %s when fetching slice",
                                         chunksize, (int) TOAST_MAX_CHUNK_SIZE,
                                         residx, totalchunks,
                                         toast_pointer.va_valueid,
                                         RelationGetRelationName(toastrel));
                }
                else if (residx == totalchunks - 1)
                {
                        if ((residx * TOAST_MAX_CHUNK_SIZE + chunksize) != attrsize)
                                elog(ERROR, "unexpected chunk size %d (expected %d) in final chunk %d for toast value %u in %s when fetching slice",
                                         chunksize,
                                         (int) (attrsize - residx * TOAST_MAX_CHUNK_SIZE),
                                         residx,
                                         toast_pointer.va_valueid,
                                         RelationGetRelationName(toastrel));
                }
                else
                        elog(ERROR, "unexpected chunk number %d (out of range %d..%d) for toast value %u in %s",
                                 residx,
                                 0, totalchunks - 1,
                                 toast_pointer.va_valueid,
                                 RelationGetRelationName(toastrel));

                /*
                 * Copy the data into proper place in our result
                 */
                chcpystrt = 0;
                chcpyend = chunksize - 1;
                if (residx == startchunk)
                        chcpystrt = startoffset;
                if (residx == endchunk)
                        chcpyend = endoffset;

                memcpy(VARDATA(result) +
                           (residx * TOAST_MAX_CHUNK_SIZE - sliceoffset) + chcpystrt,
                           chunkdata + chcpystrt,
                           (chcpyend - chcpystrt) + 1);

                nextidx++;
        }

        /*
         * Final checks that we successfully fetched the datum
         */
        if (nextidx != (endchunk + 1))
                elog(ERROR, "missing chunk number %d for toast value %u in %s",
                         nextidx,
                         toast_pointer.va_valueid,
                         RelationGetRelationName(toastrel));

        /*
         * End scan and close relations
         */
        systable_endscan_ordered(toastscan);
        index_close(toastidx, AccessShareLock);
        heap_close(toastrel, AccessShareLock);

        return result;
}