Defer remove_useless_groupby_columns() work until query_planner()

[pgsql.git] / src / backend / storage / freespace / freespace.c

/*-------------------------------------------------------------------------
 *
 * freespace.c
 *        POSTGRES free space map for quickly finding free space in relations
 *
 *
 * Portions Copyright (c) 1996-2024, PostgreSQL Global Development Group
 * Portions Copyright (c) 1994, Regents of the University of California
 *
 * IDENTIFICATION
 *        src/backend/storage/freespace/freespace.c
 *
 *
 * NOTES:
 *
 *      Free Space Map keeps track of the amount of free space on pages, and
 *      allows quickly searching for a page with enough free space. The FSM is
 *      stored in a dedicated relation fork of all heap relations, and those
 *      index access methods that need it (see also indexfsm.c). See README for
 *      more information.
 *
 *-------------------------------------------------------------------------
 */
#include "postgres.h"

#include "access/htup_details.h"
#include "access/xloginsert.h"
#include "access/xlogutils.h"
#include "miscadmin.h"
#include "storage/freespace.h"
#include "storage/fsm_internals.h"
#include "storage/smgr.h"
#include "utils/rel.h"


/*
 * We use just one byte to store the amount of free space on a page, so we
 * divide the amount of free space a page can have into 256 different
 * categories. The highest category, 255, represents a page with at least
 * MaxFSMRequestSize bytes of free space, and the second highest category
 * represents the range from 254 * FSM_CAT_STEP, inclusive, to
 * MaxFSMRequestSize, exclusive.
 *
 * MaxFSMRequestSize depends on the architecture and BLCKSZ, but assuming
 * default 8k BLCKSZ, and that MaxFSMRequestSize is 8164 bytes, the
 * categories look like this:
 *
 *
 * Range         Category
 * 0    - 31   0
 * 32   - 63   1
 * ...    ...  ...
 * 8096 - 8127 253
 * 8128 - 8163 254
 * 8164 - 8192 255
 *
 * The reason that MaxFSMRequestSize is special is that if MaxFSMRequestSize
 * isn't equal to a range boundary, a page with exactly MaxFSMRequestSize
 * bytes of free space wouldn't satisfy a request for MaxFSMRequestSize
 * bytes. If there isn't more than MaxFSMRequestSize bytes of free space on a
 * completely empty page, that would mean that we could never satisfy a
 * request of exactly MaxFSMRequestSize bytes.
 */
#define FSM_CATEGORIES  256
#define FSM_CAT_STEP    (BLCKSZ / FSM_CATEGORIES)
#define MaxFSMRequestSize       MaxHeapTupleSize

/*
 * Depth of the on-disk tree. We need to be able to address 2^32-1 blocks,
 * and 1626 is the smallest number that satisfies X^3 >= 2^32-1. Likewise,
 * 256 is the smallest number that satisfies X^4 >= 2^32-1. In practice,
 * this means that 4096 bytes is the smallest BLCKSZ that we can get away
 * with a 3-level tree, and 512 is the smallest we support.
 */
#define FSM_TREE_DEPTH  ((SlotsPerFSMPage >= 1626) ? 3 : 4)

#define FSM_ROOT_LEVEL  (FSM_TREE_DEPTH - 1)
#define FSM_BOTTOM_LEVEL 0

/*
 * The internal FSM routines work on a logical addressing scheme. Each
 * level of the tree can be thought of as a separately addressable file.
 */
typedef struct
{
        int                     level;                  /* level */
        int                     logpageno;              /* page number within the level */
} FSMAddress;

/* Address of the root page. */
static const FSMAddress FSM_ROOT_ADDRESS = {FSM_ROOT_LEVEL, 0};

/* functions to navigate the tree */
static FSMAddress fsm_get_child(FSMAddress parent, uint16 slot);
static FSMAddress fsm_get_parent(FSMAddress child, uint16 *slot);
static FSMAddress fsm_get_location(BlockNumber heapblk, uint16 *slot);
static BlockNumber fsm_get_heap_blk(FSMAddress addr, uint16 slot);
static BlockNumber fsm_logical_to_physical(FSMAddress addr);

static Buffer fsm_readbuf(Relation rel, FSMAddress addr, bool extend);
static Buffer fsm_extend(Relation rel, BlockNumber fsm_nblocks);

/* functions to convert amount of free space to a FSM category */
static uint8 fsm_space_avail_to_cat(Size avail);
static uint8 fsm_space_needed_to_cat(Size needed);
static Size fsm_space_cat_to_avail(uint8 cat);

/* workhorse functions for various operations */
static int      fsm_set_and_search(Relation rel, FSMAddress addr, uint16 slot,
                                                           uint8 newValue, uint8 minValue);
static BlockNumber fsm_search(Relation rel, uint8 min_cat);
static uint8 fsm_vacuum_page(Relation rel, FSMAddress addr,
                                                         BlockNumber start, BlockNumber end,
                                                         bool *eof_p);
static bool fsm_does_block_exist(Relation rel, BlockNumber blknumber);


/******** Public API ********/

/*
 * GetPageWithFreeSpace - try to find a page in the given relation with
 *              at least the specified amount of free space.
 *
 * If successful, return the block number; if not, return InvalidBlockNumber.
 *
 * The caller must be prepared for the possibility that the returned page
 * will turn out to have too little space available by the time the caller
 * gets a lock on it.  In that case, the caller should report the actual
 * amount of free space available on that page and then try again (see
 * RecordAndGetPageWithFreeSpace).  If InvalidBlockNumber is returned,
 * extend the relation.
 *
 * This can trigger FSM updates if any FSM entry is found to point to a block
 * past the end of the relation.
 */
BlockNumber
GetPageWithFreeSpace(Relation rel, Size spaceNeeded)
{
        uint8           min_cat = fsm_space_needed_to_cat(spaceNeeded);

        return fsm_search(rel, min_cat);
}

/*
 * RecordAndGetPageWithFreeSpace - update info about a page and try again.
 *
 * We provide this combo form to save some locking overhead, compared to
 * separate RecordPageWithFreeSpace + GetPageWithFreeSpace calls. There's
 * also some effort to return a page close to the old page; if there's a
 * page with enough free space on the same FSM page where the old one page
 * is located, it is preferred.
 */
BlockNumber
RecordAndGetPageWithFreeSpace(Relation rel, BlockNumber oldPage,
                                                          Size oldSpaceAvail, Size spaceNeeded)
{
        int                     old_cat = fsm_space_avail_to_cat(oldSpaceAvail);
        int                     search_cat = fsm_space_needed_to_cat(spaceNeeded);
        FSMAddress      addr;
        uint16          slot;
        int                     search_slot;

        /* Get the location of the FSM byte representing the heap block */
        addr = fsm_get_location(oldPage, &slot);

        search_slot = fsm_set_and_search(rel, addr, slot, old_cat, search_cat);

        /*
         * If fsm_set_and_search found a suitable new block, return that.
         * Otherwise, search as usual.
         */
        if (search_slot != -1)
        {
                BlockNumber blknum = fsm_get_heap_blk(addr, search_slot);

                /*
                 * Check that the blknum is actually in the relation. Don't try to
                 * update the FSM in that case, just fall back to the other case
                 */
                if (fsm_does_block_exist(rel, blknum))
                        return blknum;
        }
        return fsm_search(rel, search_cat);
}

/*
 * RecordPageWithFreeSpace - update info about a page.
 *
 * Note that if the new spaceAvail value is higher than the old value stored
 * in the FSM, the space might not become visible to searchers until the next
 * FreeSpaceMapVacuum call, which updates the upper level pages.
 */
void
RecordPageWithFreeSpace(Relation rel, BlockNumber heapBlk, Size spaceAvail)
{
        int                     new_cat = fsm_space_avail_to_cat(spaceAvail);
        FSMAddress      addr;
        uint16          slot;

        /* Get the location of the FSM byte representing the heap block */
        addr = fsm_get_location(heapBlk, &slot);

        fsm_set_and_search(rel, addr, slot, new_cat, 0);
}

/*
 * XLogRecordPageWithFreeSpace - like RecordPageWithFreeSpace, for use in
 *              WAL replay
 */
void
XLogRecordPageWithFreeSpace(RelFileLocator rlocator, BlockNumber heapBlk,
                                                        Size spaceAvail)
{
        int                     new_cat = fsm_space_avail_to_cat(spaceAvail);
        FSMAddress      addr;
        uint16          slot;
        BlockNumber blkno;
        Buffer          buf;
        Page            page;

        /* Get the location of the FSM byte representing the heap block */
        addr = fsm_get_location(heapBlk, &slot);
        blkno = fsm_logical_to_physical(addr);

        /* If the page doesn't exist already, extend */
        buf = XLogReadBufferExtended(rlocator, FSM_FORKNUM, blkno,
                                                                 RBM_ZERO_ON_ERROR, InvalidBuffer);
        LockBuffer(buf, BUFFER_LOCK_EXCLUSIVE);

        page = BufferGetPage(buf);
        if (PageIsNew(page))
                PageInit(page, BLCKSZ, 0);

        if (fsm_set_avail(page, slot, new_cat))
                MarkBufferDirtyHint(buf, false);
        UnlockReleaseBuffer(buf);
}

/*
 * GetRecordedFreeSpace - return the amount of free space on a particular page,
 *              according to the FSM.
 */
Size
GetRecordedFreeSpace(Relation rel, BlockNumber heapBlk)
{
        FSMAddress      addr;
        uint16          slot;
        Buffer          buf;
        uint8           cat;

        /* Get the location of the FSM byte representing the heap block */
        addr = fsm_get_location(heapBlk, &slot);

        buf = fsm_readbuf(rel, addr, false);
        if (!BufferIsValid(buf))
                return 0;
        cat = fsm_get_avail(BufferGetPage(buf), slot);
        ReleaseBuffer(buf);

        return fsm_space_cat_to_avail(cat);
}

/*
 * FreeSpaceMapPrepareTruncateRel - prepare for truncation of a relation.
 *
 * nblocks is the new size of the heap.
 *
 * Return the number of blocks of new FSM.
 * If it's InvalidBlockNumber, there is nothing to truncate;
 * otherwise the caller is responsible for calling smgrtruncate()
 * to truncate the FSM pages, and FreeSpaceMapVacuumRange()
 * to update upper-level pages in the FSM.
 */
BlockNumber
FreeSpaceMapPrepareTruncateRel(Relation rel, BlockNumber nblocks)
{
        BlockNumber new_nfsmblocks;
        FSMAddress      first_removed_address;
        uint16          first_removed_slot;
        Buffer          buf;

        /*
         * If no FSM has been created yet for this relation, there's nothing to
         * truncate.
         */
        if (!smgrexists(RelationGetSmgr(rel), FSM_FORKNUM))
                return InvalidBlockNumber;

        /* Get the location in the FSM of the first removed heap block */
        first_removed_address = fsm_get_location(nblocks, &first_removed_slot);

        /*
         * Zero out the tail of the last remaining FSM page. If the slot
         * representing the first removed heap block is at a page boundary, as the
         * first slot on the FSM page that first_removed_address points to, we can
         * just truncate that page altogether.
         */
        if (first_removed_slot > 0)
        {
                buf = fsm_readbuf(rel, first_removed_address, false);
                if (!BufferIsValid(buf))
                        return InvalidBlockNumber;      /* nothing to do; the FSM was already
                                                                                 * smaller */
                LockBuffer(buf, BUFFER_LOCK_EXCLUSIVE);

                /* NO EREPORT(ERROR) from here till changes are logged */
                START_CRIT_SECTION();

                fsm_truncate_avail(BufferGetPage(buf), first_removed_slot);

                /*
                 * This change is non-critical, because fsm_does_block_exist() would
                 * stop us from returning a truncated-away block.  However, since this
                 * may remove up to SlotsPerFSMPage slots, it's nice to avoid the cost
                 * of that many fsm_does_block_exist() rejections.  Use a full
                 * MarkBufferDirty(), not MarkBufferDirtyHint().
                 */
                MarkBufferDirty(buf);

                /*
                 * WAL-log like MarkBufferDirtyHint() might have done, just to avoid
                 * differing from the rest of the file in this respect.  This is
                 * optional; see README mention of full page images.  XXX consider
                 * XLogSaveBufferForHint() for even closer similarity.
                 *
                 * A higher-level operation calls us at WAL replay.  If we crash
                 * before the XLOG_SMGR_TRUNCATE flushes to disk, main fork length has
                 * not changed, and our fork remains valid.  If we crash after that
                 * flush, redo will return here.
                 */
                if (!InRecovery && RelationNeedsWAL(rel) && XLogHintBitIsNeeded())
                        log_newpage_buffer(buf, false);

                END_CRIT_SECTION();

                UnlockReleaseBuffer(buf);

                new_nfsmblocks = fsm_logical_to_physical(first_removed_address) + 1;
        }
        else
        {
                new_nfsmblocks = fsm_logical_to_physical(first_removed_address);
                if (smgrnblocks(RelationGetSmgr(rel), FSM_FORKNUM) <= new_nfsmblocks)
                        return InvalidBlockNumber;      /* nothing to do; the FSM was already
                                                                                 * smaller */
        }

        return new_nfsmblocks;
}

/*
 * FreeSpaceMapVacuum - update upper-level pages in the rel's FSM
 *
 * We assume that the bottom-level pages have already been updated with
 * new free-space information.
 */
void
FreeSpaceMapVacuum(Relation rel)
{
        bool            dummy;

        /* Recursively scan the tree, starting at the root */
        (void) fsm_vacuum_page(rel, FSM_ROOT_ADDRESS,
                                                   (BlockNumber) 0, InvalidBlockNumber,
                                                   &dummy);
}

/*
 * FreeSpaceMapVacuumRange - update upper-level pages in the rel's FSM
 *
 * As above, but assume that only heap pages between start and end-1 inclusive
 * have new free-space information, so update only the upper-level slots
 * covering that block range.  end == InvalidBlockNumber is equivalent to
 * "all the rest of the relation".
 */
void
FreeSpaceMapVacuumRange(Relation rel, BlockNumber start, BlockNumber end)
{
        bool            dummy;

        /* Recursively scan the tree, starting at the root */
        if (end > start)
                (void) fsm_vacuum_page(rel, FSM_ROOT_ADDRESS, start, end, &dummy);
}

/******** Internal routines ********/

/*
 * Return category corresponding x bytes of free space
 */
static uint8
fsm_space_avail_to_cat(Size avail)
{
        int                     cat;

        Assert(avail < BLCKSZ);

        if (avail >= MaxFSMRequestSize)
                return 255;

        cat = avail / FSM_CAT_STEP;

        /*
         * The highest category, 255, is reserved for MaxFSMRequestSize bytes or
         * more.
         */
        if (cat > 254)
                cat = 254;

        return (uint8) cat;
}

/*
 * Return the lower bound of the range of free space represented by given
 * category.
 */
static Size
fsm_space_cat_to_avail(uint8 cat)
{
        /* The highest category represents exactly MaxFSMRequestSize bytes. */
        if (cat == 255)
                return MaxFSMRequestSize;
        else
                return cat * FSM_CAT_STEP;
}

/*
 * Which category does a page need to have, to accommodate x bytes of data?
 * While fsm_space_avail_to_cat() rounds down, this needs to round up.
 */
static uint8
fsm_space_needed_to_cat(Size needed)
{
        int                     cat;

        /* Can't ask for more space than the highest category represents */
        if (needed > MaxFSMRequestSize)
                elog(ERROR, "invalid FSM request size %zu", needed);

        if (needed == 0)
                return 1;

        cat = (needed + FSM_CAT_STEP - 1) / FSM_CAT_STEP;

        if (cat > 255)
                cat = 255;

        return (uint8) cat;
}

/*
 * Returns the physical block number of a FSM page
 */
static BlockNumber
fsm_logical_to_physical(FSMAddress addr)
{
        BlockNumber pages;
        int                     leafno;
        int                     l;

        /*
         * Calculate the logical page number of the first leaf page below the
         * given page.
         */
        leafno = addr.logpageno;
        for (l = 0; l < addr.level; l++)
                leafno *= SlotsPerFSMPage;

        /* Count upper level nodes required to address the leaf page */
        pages = 0;
        for (l = 0; l < FSM_TREE_DEPTH; l++)
        {
                pages += leafno + 1;
                leafno /= SlotsPerFSMPage;
        }

        /*
         * If the page we were asked for wasn't at the bottom level, subtract the
         * additional lower level pages we counted above.
         */
        pages -= addr.level;

        /* Turn the page count into 0-based block number */
        return pages - 1;
}

/*
 * Return the FSM location corresponding to given heap block.
 */
static FSMAddress
fsm_get_location(BlockNumber heapblk, uint16 *slot)
{
        FSMAddress      addr;

        addr.level = FSM_BOTTOM_LEVEL;
        addr.logpageno = heapblk / SlotsPerFSMPage;
        *slot = heapblk % SlotsPerFSMPage;

        return addr;
}

/*
 * Return the heap block number corresponding to given location in the FSM.
 */
static BlockNumber
fsm_get_heap_blk(FSMAddress addr, uint16 slot)
{
        Assert(addr.level == FSM_BOTTOM_LEVEL);
        return ((unsigned int) addr.logpageno) * SlotsPerFSMPage + slot;
}

/*
 * Given a logical address of a child page, get the logical page number of
 * the parent, and the slot within the parent corresponding to the child.
 */
static FSMAddress
fsm_get_parent(FSMAddress child, uint16 *slot)
{
        FSMAddress      parent;

        Assert(child.level < FSM_ROOT_LEVEL);

        parent.level = child.level + 1;
        parent.logpageno = child.logpageno / SlotsPerFSMPage;
        *slot = child.logpageno % SlotsPerFSMPage;

        return parent;
}

/*
 * Given a logical address of a parent page and a slot number, get the
 * logical address of the corresponding child page.
 */
static FSMAddress
fsm_get_child(FSMAddress parent, uint16 slot)
{
        FSMAddress      child;

        Assert(parent.level > FSM_BOTTOM_LEVEL);

        child.level = parent.level - 1;
        child.logpageno = parent.logpageno * SlotsPerFSMPage + slot;

        return child;
}

/*
 * Read a FSM page.
 *
 * If the page doesn't exist, InvalidBuffer is returned, or if 'extend' is
 * true, the FSM file is extended.
 */
static Buffer
fsm_readbuf(Relation rel, FSMAddress addr, bool extend)
{
        BlockNumber blkno = fsm_logical_to_physical(addr);
        Buffer          buf;
        SMgrRelation reln = RelationGetSmgr(rel);

        /*
         * If we haven't cached the size of the FSM yet, check it first.  Also
         * recheck if the requested block seems to be past end, since our cached
         * value might be stale.  (We send smgr inval messages on truncation, but
         * not on extension.)
         */
        if (reln->smgr_cached_nblocks[FSM_FORKNUM] == InvalidBlockNumber ||
                blkno >= reln->smgr_cached_nblocks[FSM_FORKNUM])
        {
                /* Invalidate the cache so smgrnblocks asks the kernel. */
                reln->smgr_cached_nblocks[FSM_FORKNUM] = InvalidBlockNumber;
                if (smgrexists(reln, FSM_FORKNUM))
                        smgrnblocks(reln, FSM_FORKNUM);
                else
                        reln->smgr_cached_nblocks[FSM_FORKNUM] = 0;
        }

        /*
         * For reading we use ZERO_ON_ERROR mode, and initialize the page if
         * necessary.  The FSM information is not accurate anyway, so it's better
         * to clear corrupt pages than error out. Since the FSM changes are not
         * WAL-logged, the so-called torn page problem on crash can lead to pages
         * with corrupt headers, for example.
         *
         * We use the same path below to initialize pages when extending the
         * relation, as a concurrent extension can end up with vm_extend()
         * returning an already-initialized page.
         */
        if (blkno >= reln->smgr_cached_nblocks[FSM_FORKNUM])
        {
                if (extend)
                        buf = fsm_extend(rel, blkno + 1);
                else
                        return InvalidBuffer;
        }
        else
                buf = ReadBufferExtended(rel, FSM_FORKNUM, blkno, RBM_ZERO_ON_ERROR, NULL);

        /*
         * Initializing the page when needed is trickier than it looks, because of
         * the possibility of multiple backends doing this concurrently, and our
         * desire to not uselessly take the buffer lock in the normal path where
         * the page is OK.  We must take the lock to initialize the page, so
         * recheck page newness after we have the lock, in case someone else
         * already did it.  Also, because we initially check PageIsNew with no
         * lock, it's possible to fall through and return the buffer while someone
         * else is still initializing the page (i.e., we might see pd_upper as set
         * but other page header fields are still zeroes).  This is harmless for
         * callers that will take a buffer lock themselves, but some callers
         * inspect the page without any lock at all.  The latter is OK only so
         * long as it doesn't depend on the page header having correct contents.
         * Current usage is safe because PageGetContents() does not require that.
         */
        if (PageIsNew(BufferGetPage(buf)))
        {
                LockBuffer(buf, BUFFER_LOCK_EXCLUSIVE);
                if (PageIsNew(BufferGetPage(buf)))
                        PageInit(BufferGetPage(buf), BLCKSZ, 0);
                LockBuffer(buf, BUFFER_LOCK_UNLOCK);
        }
        return buf;
}

/*
 * Ensure that the FSM fork is at least fsm_nblocks long, extending
 * it if necessary with empty pages. And by empty, I mean pages filled
 * with zeros, meaning there's no free space.
 */
static Buffer
fsm_extend(Relation rel, BlockNumber fsm_nblocks)
{
        return ExtendBufferedRelTo(BMR_REL(rel), FSM_FORKNUM, NULL,
                                                           EB_CREATE_FORK_IF_NEEDED |
                                                           EB_CLEAR_SIZE_CACHE,
                                                           fsm_nblocks,
                                                           RBM_ZERO_ON_ERROR);
}

/*
 * Set value in given FSM page and slot.
 *
 * If minValue > 0, the updated page is also searched for a page with at
 * least minValue of free space. If one is found, its slot number is
 * returned, -1 otherwise.
 */
static int
fsm_set_and_search(Relation rel, FSMAddress addr, uint16 slot,
                                   uint8 newValue, uint8 minValue)
{
        Buffer          buf;
        Page            page;
        int                     newslot = -1;

        buf = fsm_readbuf(rel, addr, true);
        LockBuffer(buf, BUFFER_LOCK_EXCLUSIVE);

        page = BufferGetPage(buf);

        if (fsm_set_avail(page, slot, newValue))
                MarkBufferDirtyHint(buf, false);

        if (minValue != 0)
        {
                /* Search while we still hold the lock */
                newslot = fsm_search_avail(buf, minValue,
                                                                   addr.level == FSM_BOTTOM_LEVEL,
                                                                   true);
        }

        UnlockReleaseBuffer(buf);

        return newslot;
}

/*
 * Search the tree for a heap page with at least min_cat of free space
 */
static BlockNumber
fsm_search(Relation rel, uint8 min_cat)
{
        int                     restarts = 0;
        FSMAddress      addr = FSM_ROOT_ADDRESS;

        for (;;)
        {
                int                     slot;
                Buffer          buf;
                uint8           max_avail = 0;

                /* Read the FSM page. */
                buf = fsm_readbuf(rel, addr, false);

                /* Search within the page */
                if (BufferIsValid(buf))
                {
                        LockBuffer(buf, BUFFER_LOCK_SHARE);
                        slot = fsm_search_avail(buf, min_cat,
                                                                        (addr.level == FSM_BOTTOM_LEVEL),
                                                                        false);
                        if (slot == -1)
                        {
                                max_avail = fsm_get_max_avail(BufferGetPage(buf));
                                UnlockReleaseBuffer(buf);
                        }
                        else
                        {
                                /* Keep the pin for possible update below */
                                LockBuffer(buf, BUFFER_LOCK_UNLOCK);
                        }
                }
                else
                        slot = -1;

                if (slot != -1)
                {
                        /*
                         * Descend the tree, or return the found block if we're at the
                         * bottom.
                         */
                        if (addr.level == FSM_BOTTOM_LEVEL)
                        {
                                BlockNumber blkno = fsm_get_heap_blk(addr, slot);
                                Page            page;

                                if (fsm_does_block_exist(rel, blkno))
                                {
                                        ReleaseBuffer(buf);
                                        return blkno;
                                }

                                /*
                                 * Block is past the end of the relation.  Update FSM, and
                                 * restart from root.  The usual "advancenext" behavior is
                                 * pessimal for this rare scenario, since every later slot is
                                 * unusable in the same way.  We could zero all affected slots
                                 * on the same FSM page, but don't bet on the benefits of that
                                 * optimization justifying its compiled code bulk.
                                 */
                                page = BufferGetPage(buf);
                                LockBuffer(buf, BUFFER_LOCK_EXCLUSIVE);
                                fsm_set_avail(page, slot, 0);
                                MarkBufferDirtyHint(buf, false);
                                UnlockReleaseBuffer(buf);
                                if (restarts++ > 10000) /* same rationale as below */
                                        return InvalidBlockNumber;
                                addr = FSM_ROOT_ADDRESS;
                        }
                        else
                        {
                                ReleaseBuffer(buf);
                        }
                        addr = fsm_get_child(addr, slot);
                }
                else if (addr.level == FSM_ROOT_LEVEL)
                {
                        /*
                         * At the root, failure means there's no page with enough free
                         * space in the FSM. Give up.
                         */
                        return InvalidBlockNumber;
                }
                else
                {
                        uint16          parentslot;
                        FSMAddress      parent;

                        /*
                         * At lower level, failure can happen if the value in the upper-
                         * level node didn't reflect the value on the lower page. Update
                         * the upper node, to avoid falling into the same trap again, and
                         * start over.
                         *
                         * There's a race condition here, if another backend updates this
                         * page right after we release it, and gets the lock on the parent
                         * page before us. We'll then update the parent page with the now
                         * stale information we had. It's OK, because it should happen
                         * rarely, and will be fixed by the next vacuum.
                         */
                        parent = fsm_get_parent(addr, &parentslot);
                        fsm_set_and_search(rel, parent, parentslot, max_avail, 0);

                        /*
                         * If the upper pages are badly out of date, we might need to loop
                         * quite a few times, updating them as we go. Any inconsistencies
                         * should eventually be corrected and the loop should end. Looping
                         * indefinitely is nevertheless scary, so provide an emergency
                         * valve.
                         */
                        if (restarts++ > 10000)
                                return InvalidBlockNumber;

                        /* Start search all over from the root */
                        addr = FSM_ROOT_ADDRESS;
                }
        }
}


/*
 * Recursive guts of FreeSpaceMapVacuum
 *
 * Examine the FSM page indicated by addr, as well as its children, updating
 * upper-level nodes that cover the heap block range from start to end-1.
 * (It's okay if end is beyond the actual end of the map.)
 * Return the maximum freespace value on this page.
 *
 * If addr is past the end of the FSM, set *eof_p to true and return 0.
 *
 * This traverses the tree in depth-first order.  The tree is stored
 * physically in depth-first order, so this should be pretty I/O efficient.
 */
static uint8
fsm_vacuum_page(Relation rel, FSMAddress addr,
                                BlockNumber start, BlockNumber end,
                                bool *eof_p)
{
        Buffer          buf;
        Page            page;
        uint8           max_avail;

        /* Read the page if it exists, or return EOF */
        buf = fsm_readbuf(rel, addr, false);
        if (!BufferIsValid(buf))
        {
                *eof_p = true;
                return 0;
        }
        else
                *eof_p = false;

        page = BufferGetPage(buf);

        /*
         * If we're above the bottom level, recurse into children, and fix the
         * information stored about them at this level.
         */
        if (addr.level > FSM_BOTTOM_LEVEL)
        {
                FSMAddress      fsm_start,
                                        fsm_end;
                uint16          fsm_start_slot,
                                        fsm_end_slot;
                int                     slot,
                                        start_slot,
                                        end_slot;
                bool            eof = false;

                /*
                 * Compute the range of slots we need to update on this page, given
                 * the requested range of heap blocks to consider.  The first slot to
                 * update is the one covering the "start" block, and the last slot is
                 * the one covering "end - 1".  (Some of this work will be duplicated
                 * in each recursive call, but it's cheap enough to not worry about.)
                 */
                fsm_start = fsm_get_location(start, &fsm_start_slot);
                fsm_end = fsm_get_location(end - 1, &fsm_end_slot);

                while (fsm_start.level < addr.level)
                {
                        fsm_start = fsm_get_parent(fsm_start, &fsm_start_slot);
                        fsm_end = fsm_get_parent(fsm_end, &fsm_end_slot);
                }
                Assert(fsm_start.level == addr.level);

                if (fsm_start.logpageno == addr.logpageno)
                        start_slot = fsm_start_slot;
                else if (fsm_start.logpageno > addr.logpageno)
                        start_slot = SlotsPerFSMPage;   /* shouldn't get here... */
                else
                        start_slot = 0;

                if (fsm_end.logpageno == addr.logpageno)
                        end_slot = fsm_end_slot;
                else if (fsm_end.logpageno > addr.logpageno)
                        end_slot = SlotsPerFSMPage - 1;
                else
                        end_slot = -1;          /* shouldn't get here... */

                for (slot = start_slot; slot <= end_slot; slot++)
                {
                        int                     child_avail;

                        CHECK_FOR_INTERRUPTS();

                        /* After we hit end-of-file, just clear the rest of the slots */
                        if (!eof)
                                child_avail = fsm_vacuum_page(rel, fsm_get_child(addr, slot),
                                                                                          start, end,
                                                                                          &eof);
                        else
                                child_avail = 0;

                        /* Update information about the child */
                        if (fsm_get_avail(page, slot) != child_avail)
                        {
                                LockBuffer(buf, BUFFER_LOCK_EXCLUSIVE);
                                fsm_set_avail(page, slot, child_avail);
                                MarkBufferDirtyHint(buf, false);
                                LockBuffer(buf, BUFFER_LOCK_UNLOCK);
                        }
                }
        }

        /* Now get the maximum value on the page, to return to caller */
        max_avail = fsm_get_max_avail(page);

        /*
         * Reset the next slot pointer. This encourages the use of low-numbered
         * pages, increasing the chances that a later vacuum can truncate the
         * relation.  We don't bother with a lock here, nor with marking the page
         * dirty if it wasn't already, since this is just a hint.
         */
        ((FSMPage) PageGetContents(page))->fp_next_slot = 0;

        ReleaseBuffer(buf);

        return max_avail;
}


/*
 * Check whether a block number is past the end of the relation.  This can
 * happen after WAL replay, if the FSM reached disk but newly-extended pages
 * it refers to did not.
 */
static bool
fsm_does_block_exist(Relation rel, BlockNumber blknumber)
{
        SMgrRelation smgr = RelationGetSmgr(rel);

        /*
         * If below the cached nblocks, the block surely exists.  Otherwise, we
         * face a trade-off.  We opt to compare to a fresh nblocks, incurring
         * lseek() overhead.  The alternative would be to assume the block does
         * not exist, but that would cause FSM to set zero space available for
         * blocks that main fork extension just recorded.
         */
        return ((BlockNumberIsValid(smgr->smgr_cached_nblocks[MAIN_FORKNUM]) &&
                         blknumber < smgr->smgr_cached_nblocks[MAIN_FORKNUM]) ||
                        blknumber < RelationGetNumberOfBlocks(rel));
}