pruneheap.c

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/*-------------------------------------------------------------------------
 *
 * pruneheap.c
 *    heap page pruning and HOT-chain management code
 *
 * Portions Copyright (c) 1996-2025, PostgreSQL Global Development Group
 * Portions Copyright (c) 1994, Regents of the University of California
 *
 *
 * IDENTIFICATION
 *    src/backend/access/heap/pruneheap.c
 *
 *-------------------------------------------------------------------------
 */
#include "postgres.h"
 
#include "access/heapam.h"
#include "access/heapam_xlog.h"
#include "access/htup_details.h"
#include "access/multixact.h"
#include "access/transam.h"
#include "access/visibilitymapdefs.h"
#include "access/xlog.h"
#include "access/xloginsert.h"
#include "commands/vacuum.h"
#include "executor/instrument.h"
#include "miscadmin.h"
#include "pgstat.h"
#include "storage/bufmgr.h"
#include "utils/rel.h"
#include "utils/snapmgr.h"
 
/* Working data for heap_page_prune_and_freeze() and subroutines */
typedef struct
{
    /*-------------------------------------------------------
     * Arguments passed to heap_page_prune_and_freeze()
     *-------------------------------------------------------
     */
 
    /* tuple visibility test, initialized for the relation */
    GlobalVisState *vistest;
    /* whether or not dead items can be set LP_UNUSED during pruning */
    bool        mark_unused_now;
    /* whether to attempt freezing tuples */
    bool        attempt_freeze;
    struct VacuumCutoffs *cutoffs;
 
    /*-------------------------------------------------------
     * Fields describing what to do to the page
     *-------------------------------------------------------
     */
    TransactionId new_prune_xid;    /* new prune hint value */
    TransactionId latest_xid_removed;
    int         nredirected;    /* numbers of entries in arrays below */
    int         ndead;
    int         nunused;
    int         nfrozen;
    /* arrays that accumulate indexes of items to be changed */
    OffsetNumber redirected[MaxHeapTuplesPerPage * 2];
    OffsetNumber nowdead[MaxHeapTuplesPerPage];
    OffsetNumber nowunused[MaxHeapTuplesPerPage];
    HeapTupleFreeze frozen[MaxHeapTuplesPerPage];
 
    /*-------------------------------------------------------
     * Working state for HOT chain processing
     *-------------------------------------------------------
     */
 
    /*
     * 'root_items' contains offsets of all LP_REDIRECT line pointers and
     * normal non-HOT tuples.  They can be stand-alone items or the first item
     * in a HOT chain.  'heaponly_items' contains heap-only tuples which can
     * only be removed as part of a HOT chain.
     */
    int         nroot_items;
    OffsetNumber root_items[MaxHeapTuplesPerPage];
    int         nheaponly_items;
    OffsetNumber heaponly_items[MaxHeapTuplesPerPage];
 
    /*
     * processed[offnum] is true if item at offnum has been processed.
     *
     * This needs to be MaxHeapTuplesPerPage + 1 long as FirstOffsetNumber is
     * 1. Otherwise every access would need to subtract 1.
     */
    bool        processed[MaxHeapTuplesPerPage + 1];
 
    /*
     * Tuple visibility is only computed once for each tuple, for correctness
     * and efficiency reasons; see comment in heap_page_prune_and_freeze() for
     * details.  This is of type int8[], instead of HTSV_Result[], so we can
     * use -1 to indicate no visibility has been computed, e.g. for LP_DEAD
     * items.
     *
     * This needs to be MaxHeapTuplesPerPage + 1 long as FirstOffsetNumber is
     * 1. Otherwise every access would need to subtract 1.
     */
    int8        htsv[MaxHeapTuplesPerPage + 1];
 
    /*
     * Freezing-related state.
     */
    HeapPageFreeze pagefrz;
 
    /*-------------------------------------------------------
     * Information about what was done
     *
     * These fields are not used by pruning itself for the most part, but are
     * used to collect information about what was pruned and what state the
     * page is in after pruning, for the benefit of the caller.  They are
     * copied to the caller's PruneFreezeResult at the end.
     * -------------------------------------------------------
     */
 
    int         ndeleted;       /* Number of tuples deleted from the page */
 
    /* Number of live and recently dead tuples, after pruning */
    int         live_tuples;
    int         recently_dead_tuples;
 
    /* Whether or not the page makes rel truncation unsafe */
    bool        hastup;
 
    /*
     * LP_DEAD items on the page after pruning.  Includes existing LP_DEAD
     * items
     */
    int         lpdead_items;   /* number of items in the array */
    OffsetNumber *deadoffsets;  /* points directly to presult->deadoffsets */
 
    /*
     * The snapshot conflict horizon used when freezing tuples. The final
     * snapshot conflict horizon for the record may be newer if pruning
     * removes newer transaction IDs.
     */
    TransactionId frz_conflict_horizon;
 
    /*
     * all_visible and all_frozen indicate if the all-visible and all-frozen
     * bits in the visibility map can be set for this page after pruning.
     *
     * visibility_cutoff_xid is the newest xmin of live tuples on the page.
     * The caller can use it as the conflict horizon, when setting the VM
     * bits.  It is only valid if we froze some tuples, and all_frozen is
     * true.
     *
     * NOTE: all_visible and all_frozen initially don't include LP_DEAD items.
     * That's convenient for heap_page_prune_and_freeze() to use them to
     * decide whether to freeze the page or not.  The all_visible and
     * all_frozen values returned to the caller are adjusted to include
     * LP_DEAD items after we determine whether to opportunistically freeze.
     */
    bool        all_visible;
    bool        all_frozen;
    TransactionId visibility_cutoff_xid;
} PruneState;
 
/* Local functions */
static HTSV_Result heap_prune_satisfies_vacuum(PruneState *prstate,
                                               HeapTuple tup,
                                               Buffer buffer);
static inline HTSV_Result htsv_get_valid_status(int status);
static void heap_prune_chain(Page page, BlockNumber blockno, OffsetNumber maxoff,
                             OffsetNumber rootoffnum, PruneState *prstate);
static void heap_prune_record_prunable(PruneState *prstate, TransactionId xid);
static void heap_prune_record_redirect(PruneState *prstate,
                                       OffsetNumber offnum, OffsetNumber rdoffnum,
                                       bool was_normal);
static void heap_prune_record_dead(PruneState *prstate, OffsetNumber offnum,
                                   bool was_normal);
static void heap_prune_record_dead_or_unused(PruneState *prstate, OffsetNumber offnum,
                                             bool was_normal);
static void heap_prune_record_unused(PruneState *prstate, OffsetNumber offnum, bool was_normal);
 
static void heap_prune_record_unchanged_lp_unused(Page page, PruneState *prstate, OffsetNumber offnum);
static void heap_prune_record_unchanged_lp_normal(Page page, PruneState *prstate, OffsetNumber offnum);
static void heap_prune_record_unchanged_lp_dead(Page page, PruneState *prstate, OffsetNumber offnum);
static void heap_prune_record_unchanged_lp_redirect(PruneState *prstate, OffsetNumber offnum);
 
static void page_verify_redirects(Page page);
 
static bool heap_page_will_freeze(Relation relation, Buffer buffer,
                                  bool did_tuple_hint_fpi, bool do_prune, bool do_hint_prune,
                                  PruneState *prstate);
 
 
/*
 * Optionally prune and repair fragmentation in the specified page.
 *
 * This is an opportunistic function.  It will perform housekeeping
 * only if the page heuristically looks like a candidate for pruning and we
 * can acquire buffer cleanup lock without blocking.
 *
 * Note: this is called quite often.  It's important that it fall out quickly
 * if there's not any use in pruning.
 *
 * Caller must have pin on the buffer, and must *not* have a lock on it.
 */
void
heap_page_prune_opt(Relation relation, Buffer buffer)
{
    Page        page = BufferGetPage(buffer);
    TransactionId prune_xid;
    GlobalVisState *vistest;
    Size        minfree;
 
    /*
     * We can't write WAL in recovery mode, so there's no point trying to
     * clean the page. The primary will likely issue a cleaning WAL record
     * soon anyway, so this is no particular loss.
     */
    if (RecoveryInProgress())
        return;
 
    /*
     * First check whether there's any chance there's something to prune,
     * determining the appropriate horizon is a waste if there's no prune_xid
     * (i.e. no updates/deletes left potentially dead tuples around).
     */
    prune_xid = ((PageHeader) page)->pd_prune_xid;
    if (!TransactionIdIsValid(prune_xid))
        return;
 
    /*
     * Check whether prune_xid indicates that there may be dead rows that can
     * be cleaned up.
     */
    vistest = GlobalVisTestFor(relation);
 
    if (!GlobalVisTestIsRemovableXid(vistest, prune_xid))
        return;
 
    /*
     * We prune when a previous UPDATE failed to find enough space on the page
     * for a new tuple version, or when free space falls below the relation's
     * fill-factor target (but not less than 10%).
     *
     * Checking free space here is questionable since we aren't holding any
     * lock on the buffer; in the worst case we could get a bogus answer. It's
     * unlikely to be *seriously* wrong, though, since reading either pd_lower
     * or pd_upper is probably atomic.  Avoiding taking a lock seems more
     * important than sometimes getting a wrong answer in what is after all
     * just a heuristic estimate.
     */
    minfree = RelationGetTargetPageFreeSpace(relation,
                                             HEAP_DEFAULT_FILLFACTOR);
    minfree = Max(minfree, BLCKSZ / 10);
 
    if (PageIsFull(page) || PageGetHeapFreeSpace(page) < minfree)
    {
        /* OK, try to get exclusive buffer lock */
        if (!ConditionalLockBufferForCleanup(buffer))
            return;
 
        /*
         * Now that we have buffer lock, get accurate information about the
         * page's free space, and recheck the heuristic about whether to
         * prune.
         */
        if (PageIsFull(page) || PageGetHeapFreeSpace(page) < minfree)
        {
            OffsetNumber dummy_off_loc;
            PruneFreezeResult presult;
 
            /*
             * We don't pass the HEAP_PAGE_PRUNE_MARK_UNUSED_NOW option
             * regardless of whether or not the relation has indexes, since we
             * cannot safely determine that during on-access pruning with the
             * current implementation.
             */
            PruneFreezeParams params = {
                .relation = relation,
                .buffer = buffer,
                .reason = PRUNE_ON_ACCESS,
                .options = 0,
                .vistest = vistest,
                .cutoffs = NULL,
            };
 
            heap_page_prune_and_freeze(&params, &presult, &dummy_off_loc,
                                       NULL, NULL);
 
            /*
             * Report the number of tuples reclaimed to pgstats.  This is
             * presult.ndeleted minus the number of newly-LP_DEAD-set items.
             *
             * We derive the number of dead tuples like this to avoid totally
             * forgetting about items that were set to LP_DEAD, since they
             * still need to be cleaned up by VACUUM.  We only want to count
             * heap-only tuples that just became LP_UNUSED in our report,
             * which don't.
             *
             * VACUUM doesn't have to compensate in the same way when it
             * tracks ndeleted, since it will set the same LP_DEAD items to
             * LP_UNUSED separately.
             */
            if (presult.ndeleted > presult.nnewlpdead)
                pgstat_update_heap_dead_tuples(relation,
                                               presult.ndeleted - presult.nnewlpdead);
        }
 
        /* And release buffer lock */
        LockBuffer(buffer, BUFFER_LOCK_UNLOCK);
 
        /*
         * We avoid reuse of any free space created on the page by unrelated
         * UPDATEs/INSERTs by opting to not update the FSM at this point.  The
         * free space should be reused by UPDATEs to *this* page.
         */
    }
}
 
/*
 * Decide whether to proceed with freezing according to the freeze plans
 * prepared for the given heap buffer. If freezing is chosen, this function
 * performs several pre-freeze checks.
 *
 * The values of do_prune, do_hint_prune, and did_tuple_hint_fpi must be
 * determined before calling this function.
 *
 * prstate is both an input and output parameter.
 *
 * Returns true if we should apply the freeze plans and freeze tuples on the
 * page, and false otherwise.
 */
static bool
heap_page_will_freeze(Relation relation, Buffer buffer,
                      bool did_tuple_hint_fpi,
                      bool do_prune,
                      bool do_hint_prune,
                      PruneState *prstate)
{
    bool        do_freeze = false;
 
    /*
     * If the caller specified we should not attempt to freeze any tuples,
     * validate that everything is in the right state and return.
     */
    if (!prstate->attempt_freeze)
    {
        Assert(!prstate->all_frozen && prstate->nfrozen == 0);
        Assert(prstate->lpdead_items == 0 || !prstate->all_visible);
        return false;
    }
 
    if (prstate->pagefrz.freeze_required)
    {
        /*
         * heap_prepare_freeze_tuple indicated that at least one XID/MXID from
         * before FreezeLimit/MultiXactCutoff is present.  Must freeze to
         * advance relfrozenxid/relminmxid.
         */
        do_freeze = true;
    }
    else
    {
        /*
         * Opportunistically freeze the page if we are generating an FPI
         * anyway and if doing so means that we can set the page all-frozen
         * afterwards (might not happen until VACUUM's final heap pass).
         *
         * XXX: Previously, we knew if pruning emitted an FPI by checking
         * pgWalUsage.wal_fpi before and after pruning.  Once the freeze and
         * prune records were combined, this heuristic couldn't be used
         * anymore.  The opportunistic freeze heuristic must be improved;
         * however, for now, try to approximate the old logic.
         */
        if (prstate->all_frozen && prstate->nfrozen > 0)
        {
            Assert(prstate->all_visible);
 
            /*
             * Freezing would make the page all-frozen.  Have already emitted
             * an FPI or will do so anyway?
             */
            if (RelationNeedsWAL(relation))
            {
                if (did_tuple_hint_fpi)
                    do_freeze = true;
                else if (do_prune)
                {
                    if (XLogCheckBufferNeedsBackup(buffer))
                        do_freeze = true;
                }
                else if (do_hint_prune)
                {
                    if (XLogHintBitIsNeeded() && XLogCheckBufferNeedsBackup(buffer))
                        do_freeze = true;
                }
            }
        }
    }
 
    if (do_freeze)
    {
        /*
         * Validate the tuples we will be freezing before entering the
         * critical section.
         */
        heap_pre_freeze_checks(buffer, prstate->frozen, prstate->nfrozen);
 
        /*
         * Calculate what the snapshot conflict horizon should be for a record
         * freezing tuples. We can use the visibility_cutoff_xid as our cutoff
         * for conflicts when the whole page is eligible to become all-frozen
         * in the VM once we're done with it. Otherwise, we generate a
         * conservative cutoff by stepping back from OldestXmin.
         */
        if (prstate->all_frozen)
            prstate->frz_conflict_horizon = prstate->visibility_cutoff_xid;
        else
        {
            /* Avoids false conflicts when hot_standby_feedback in use */
            prstate->frz_conflict_horizon = prstate->cutoffs->OldestXmin;
            TransactionIdRetreat(prstate->frz_conflict_horizon);
        }
    }
    else if (prstate->nfrozen > 0)
    {
        /*
         * The page contained some tuples that were not already frozen, and we
         * chose not to freeze them now.  The page won't be all-frozen then.
         */
        Assert(!prstate->pagefrz.freeze_required);
 
        prstate->all_frozen = false;
        prstate->nfrozen = 0;   /* avoid miscounts in instrumentation */
    }
    else
    {
        /*
         * We have no freeze plans to execute.  The page might already be
         * all-frozen (perhaps only following pruning), though.  Such pages
         * can be marked all-frozen in the VM by our caller, even though none
         * of its tuples were newly frozen here.
         */
    }
 
    return do_freeze;
}
 
 
/*
 * Prune and repair fragmentation and potentially freeze tuples on the
 * specified page.
 *
 * Caller must have pin and buffer cleanup lock on the page.  Note that we
 * don't update the FSM information for page on caller's behalf.  Caller might
 * also need to account for a reduction in the length of the line pointer
 * array following array truncation by us.
 *
 * params contains the input parameters used to control freezing and pruning
 * behavior. See the definition of PruneFreezeParams for more on what each
 * parameter does.
 *
 * If the HEAP_PAGE_PRUNE_FREEZE option is set in params, we will freeze
 * tuples if it's required in order to advance relfrozenxid / relminmxid, or
 * if it's considered advantageous for overall system performance to do so
 * now.  The 'params.cutoffs', 'presult', 'new_relfrozen_xid' and
 * 'new_relmin_mxid' arguments are required when freezing.  When
 * HEAP_PAGE_PRUNE_FREEZE option is passed, we also set presult->all_visible
 * and presult->all_frozen after determining whether or not to
 * opporunistically freeze, to indicate if the VM bits can be set.  They are
 * always set to false when the HEAP_PAGE_PRUNE_FREEZE option is not passed,
 * because at the moment only callers that also freeze need that information.
 *
 * presult contains output parameters needed by callers, such as the number of
 * tuples removed and the offsets of dead items on the page after pruning.
 * heap_page_prune_and_freeze() is responsible for initializing it.  Required
 * by all callers.
 *
 * off_loc is the offset location required by the caller to use in error
 * callback.
 *
 * new_relfrozen_xid and new_relmin_mxid must provided by the caller if the
 * HEAP_PAGE_PRUNE_FREEZE option is set in params.  On entry, they contain the
 * oldest XID and multi-XID seen on the relation so far.  They will be updated
 * with oldest values present on the page after pruning.  After processing the
 * whole relation, VACUUM can use these values as the new
 * relfrozenxid/relminmxid for the relation.
 */
void
heap_page_prune_and_freeze(PruneFreezeParams *params,
                           PruneFreezeResult *presult,
                           OffsetNumber *off_loc,
                           TransactionId *new_relfrozen_xid,
                           MultiXactId *new_relmin_mxid)
{
    Buffer      buffer = params->buffer;
    Page        page = BufferGetPage(buffer);
    BlockNumber blockno = BufferGetBlockNumber(buffer);
    OffsetNumber offnum,
                maxoff;
    PruneState  prstate;
    HeapTupleData tup;
    bool        do_freeze;
    bool        do_prune;
    bool        do_hint_prune;
    bool        did_tuple_hint_fpi;
    int64       fpi_before = pgWalUsage.wal_fpi;
 
    /* Copy parameters to prstate */
    prstate.vistest = params->vistest;
    prstate.mark_unused_now =
        (params->options & HEAP_PAGE_PRUNE_MARK_UNUSED_NOW) != 0;
    prstate.attempt_freeze = (params->options & HEAP_PAGE_PRUNE_FREEZE) != 0;
    prstate.cutoffs = params->cutoffs;
 
    /*
     * Our strategy is to scan the page and make lists of items to change,
     * then apply the changes within a critical section.  This keeps as much
     * logic as possible out of the critical section, and also ensures that
     * WAL replay will work the same as the normal case.
     *
     * First, initialize the new pd_prune_xid value to zero (indicating no
     * prunable tuples).  If we find any tuples which may soon become
     * prunable, we will save the lowest relevant XID in new_prune_xid. Also
     * initialize the rest of our working state.
     */
    prstate.new_prune_xid = InvalidTransactionId;
    prstate.latest_xid_removed = InvalidTransactionId;
    prstate.nredirected = prstate.ndead = prstate.nunused = prstate.nfrozen = 0;
    prstate.nroot_items = 0;
    prstate.nheaponly_items = 0;
 
    /* initialize page freezing working state */
    prstate.pagefrz.freeze_required = false;
    if (prstate.attempt_freeze)
    {
        Assert(new_relfrozen_xid && new_relmin_mxid);
        prstate.pagefrz.FreezePageRelfrozenXid = *new_relfrozen_xid;
        prstate.pagefrz.NoFreezePageRelfrozenXid = *new_relfrozen_xid;
        prstate.pagefrz.FreezePageRelminMxid = *new_relmin_mxid;
        prstate.pagefrz.NoFreezePageRelminMxid = *new_relmin_mxid;
    }
    else
    {
        Assert(new_relfrozen_xid == NULL && new_relmin_mxid == NULL);
        prstate.pagefrz.FreezePageRelminMxid = InvalidMultiXactId;
        prstate.pagefrz.NoFreezePageRelminMxid = InvalidMultiXactId;
        prstate.pagefrz.FreezePageRelfrozenXid = InvalidTransactionId;
        prstate.pagefrz.NoFreezePageRelfrozenXid = InvalidTransactionId;
    }
 
    prstate.ndeleted = 0;
    prstate.live_tuples = 0;
    prstate.recently_dead_tuples = 0;
    prstate.hastup = false;
    prstate.lpdead_items = 0;
    prstate.deadoffsets = presult->deadoffsets;
    prstate.frz_conflict_horizon = InvalidTransactionId;
 
    /*
     * Caller may update the VM after we're done.  We can keep track of
     * whether the page will be all-visible and all-frozen after pruning and
     * freezing to help the caller to do that.
     *
     * Currently, only VACUUM sets the VM bits.  To save the effort, only do
     * the bookkeeping if the caller needs it.  Currently, that's tied to
     * HEAP_PAGE_PRUNE_FREEZE, but it could be a separate flag if you wanted
     * to update the VM bits without also freezing or freeze without also
     * setting the VM bits.
     *
     * In addition to telling the caller whether it can set the VM bit, we
     * also use 'all_visible' and 'all_frozen' for our own decision-making. If
     * the whole page would become frozen, we consider opportunistically
     * freezing tuples.  We will not be able to freeze the whole page if there
     * are tuples present that are not visible to everyone or if there are
     * dead tuples which are not yet removable.  However, dead tuples which
     * will be removed by the end of vacuuming should not preclude us from
     * opportunistically freezing.  Because of that, we do not immediately
     * clear all_visible and all_frozen when we see LP_DEAD items.  We fix
     * that after scanning the line pointers. We must correct all_visible and
     * all_frozen before we return them to the caller, so that the caller
     * doesn't set the VM bits incorrectly.
     */
    if (prstate.attempt_freeze)
    {
        prstate.all_visible = true;
        prstate.all_frozen = true;
    }
    else
    {
        /*
         * Initializing to false allows skipping the work to update them in
         * heap_prune_record_unchanged_lp_normal().
         */
        prstate.all_visible = false;
        prstate.all_frozen = false;
    }
 
    /*
     * The visibility cutoff xid is the newest xmin of live tuples on the
     * page.  In the common case, this will be set as the conflict horizon the
     * caller can use for updating the VM.  If, at the end of freezing and
     * pruning, the page is all-frozen, there is no possibility that any
     * running transaction on the standby does not see tuples on the page as
     * all-visible, so the conflict horizon remains InvalidTransactionId.
     */
    prstate.visibility_cutoff_xid = InvalidTransactionId;
 
    maxoff = PageGetMaxOffsetNumber(page);
    tup.t_tableOid = RelationGetRelid(params->relation);
 
    /*
     * Determine HTSV for all tuples, and queue them up for processing as HOT
     * chain roots or as heap-only items.
     *
     * Determining HTSV only once for each tuple is required for correctness,
     * to deal with cases where running HTSV twice could result in different
     * results.  For example, RECENTLY_DEAD can turn to DEAD if another
     * checked item causes GlobalVisTestIsRemovableFullXid() to update the
     * horizon, or INSERT_IN_PROGRESS can change to DEAD if the inserting
     * transaction aborts.
     *
     * It's also good for performance. Most commonly tuples within a page are
     * stored at decreasing offsets (while the items are stored at increasing
     * offsets). When processing all tuples on a page this leads to reading
     * memory at decreasing offsets within a page, with a variable stride.
     * That's hard for CPU prefetchers to deal with. Processing the items in
     * reverse order (and thus the tuples in increasing order) increases
     * prefetching efficiency significantly / decreases the number of cache
     * misses.
     */
    for (offnum = maxoff;
         offnum >= FirstOffsetNumber;
         offnum = OffsetNumberPrev(offnum))
    {
        ItemId      itemid = PageGetItemId(page, offnum);
        HeapTupleHeader htup;
 
        /*
         * Set the offset number so that we can display it along with any
         * error that occurred while processing this tuple.
         */
        *off_loc = offnum;
 
        prstate.processed[offnum] = false;
        prstate.htsv[offnum] = -1;
 
        /* Nothing to do if slot doesn't contain a tuple */
        if (!ItemIdIsUsed(itemid))
        {
            heap_prune_record_unchanged_lp_unused(page, &prstate, offnum);
            continue;
        }
 
        if (ItemIdIsDead(itemid))
        {
            /*
             * If the caller set mark_unused_now true, we can set dead line
             * pointers LP_UNUSED now.
             */
            if (unlikely(prstate.mark_unused_now))
                heap_prune_record_unused(&prstate, offnum, false);
            else
                heap_prune_record_unchanged_lp_dead(page, &prstate, offnum);
            continue;
        }
 
        if (ItemIdIsRedirected(itemid))
        {
            /* This is the start of a HOT chain */
            prstate.root_items[prstate.nroot_items++] = offnum;
            continue;
        }
 
        Assert(ItemIdIsNormal(itemid));
 
        /*
         * Get the tuple's visibility status and queue it up for processing.
         */
        htup = (HeapTupleHeader) PageGetItem(page, itemid);
        tup.t_data = htup;
        tup.t_len = ItemIdGetLength(itemid);
        ItemPointerSet(&tup.t_self, blockno, offnum);
 
        prstate.htsv[offnum] = heap_prune_satisfies_vacuum(&prstate, &tup,
                                                           buffer);
 
        if (!HeapTupleHeaderIsHeapOnly(htup))
            prstate.root_items[prstate.nroot_items++] = offnum;
        else
            prstate.heaponly_items[prstate.nheaponly_items++] = offnum;
    }
 
    /*
     * If checksums are enabled, heap_prune_satisfies_vacuum() may have caused
     * an FPI to be emitted.
     */
    did_tuple_hint_fpi = fpi_before != pgWalUsage.wal_fpi;
 
    /*
     * Process HOT chains.
     *
     * We added the items to the array starting from 'maxoff', so by
     * processing the array in reverse order, we process the items in
     * ascending offset number order.  The order doesn't matter for
     * correctness, but some quick micro-benchmarking suggests that this is
     * faster.  (Earlier PostgreSQL versions, which scanned all the items on
     * the page instead of using the root_items array, also did it in
     * ascending offset number order.)
     */
    for (int i = prstate.nroot_items - 1; i >= 0; i--)
    {
        offnum = prstate.root_items[i];
 
        /* Ignore items already processed as part of an earlier chain */
        if (prstate.processed[offnum])
            continue;
 
        /* see preceding loop */
        *off_loc = offnum;
 
        /* Process this item or chain of items */
        heap_prune_chain(page, blockno, maxoff, offnum, &prstate);
    }
 
    /*
     * Process any heap-only tuples that were not already processed as part of
     * a HOT chain.
     */
    for (int i = prstate.nheaponly_items - 1; i >= 0; i--)
    {
        offnum = prstate.heaponly_items[i];
 
        if (prstate.processed[offnum])
            continue;
 
        /* see preceding loop */
        *off_loc = offnum;
 
        /*
         * If the tuple is DEAD and doesn't chain to anything else, mark it
         * unused.  (If it does chain, we can only remove it as part of
         * pruning its chain.)
         *
         * We need this primarily to handle aborted HOT updates, that is,
         * XMIN_INVALID heap-only tuples.  Those might not be linked to by any
         * chain, since the parent tuple might be re-updated before any
         * pruning occurs.  So we have to be able to reap them separately from
         * chain-pruning.  (Note that HeapTupleHeaderIsHotUpdated will never
         * return true for an XMIN_INVALID tuple, so this code will work even
         * when there were sequential updates within the aborted transaction.)
         */
        if (prstate.htsv[offnum] == HEAPTUPLE_DEAD)
        {
            ItemId      itemid = PageGetItemId(page, offnum);
            HeapTupleHeader htup = (HeapTupleHeader) PageGetItem(page, itemid);
 
            if (likely(!HeapTupleHeaderIsHotUpdated(htup)))
            {
                HeapTupleHeaderAdvanceConflictHorizon(htup,
                                                      &prstate.latest_xid_removed);
                heap_prune_record_unused(&prstate, offnum, true);
            }
            else
            {
                /*
                 * This tuple should've been processed and removed as part of
                 * a HOT chain, so something's wrong.  To preserve evidence,
                 * we don't dare to remove it.  We cannot leave behind a DEAD
                 * tuple either, because that will cause VACUUM to error out.
                 * Throwing an error with a distinct error message seems like
                 * the least bad option.
                 */
                elog(ERROR, "dead heap-only tuple (%u, %d) is not linked to from any HOT chain",
                     blockno, offnum);
            }
        }
        else
            heap_prune_record_unchanged_lp_normal(page, &prstate, offnum);
    }
 
    /* We should now have processed every tuple exactly once  */
#ifdef USE_ASSERT_CHECKING
    for (offnum = FirstOffsetNumber;
         offnum <= maxoff;
         offnum = OffsetNumberNext(offnum))
    {
        *off_loc = offnum;
 
        Assert(prstate.processed[offnum]);
    }
#endif
 
    /* Clear the offset information once we have processed the given page. */
    *off_loc = InvalidOffsetNumber;
 
    do_prune = prstate.nredirected > 0 ||
        prstate.ndead > 0 ||
        prstate.nunused > 0;
 
    /*
     * Even if we don't prune anything, if we found a new value for the
     * pd_prune_xid field or the page was marked full, we will update the hint
     * bit.
     */
    do_hint_prune = ((PageHeader) page)->pd_prune_xid != prstate.new_prune_xid ||
        PageIsFull(page);
 
    /*
     * Decide if we want to go ahead with freezing according to the freeze
     * plans we prepared, or not.
     */
    do_freeze = heap_page_will_freeze(params->relation, buffer,
                                      did_tuple_hint_fpi,
                                      do_prune,
                                      do_hint_prune,
                                      &prstate);
 
    /*
     * While scanning the line pointers, we did not clear
     * all_visible/all_frozen when encountering LP_DEAD items because we
     * wanted the decision whether or not to freeze the page to be unaffected
     * by the short-term presence of LP_DEAD items.  These LP_DEAD items are
     * effectively assumed to be LP_UNUSED items in the making.  It doesn't
     * matter which vacuum heap pass (initial pass or final pass) ends up
     * setting the page all-frozen, as long as the ongoing VACUUM does it.
     *
     * Now that we finished determining whether or not to freeze the page,
     * update all_visible and all_frozen so that they reflect the true state
     * of the page for setting PD_ALL_VISIBLE and VM bits.
     */
    if (prstate.lpdead_items > 0)
        prstate.all_visible = prstate.all_frozen = false;
 
    Assert(!prstate.all_frozen || prstate.all_visible);
 
    /* Any error while applying the changes is critical */
    START_CRIT_SECTION();
 
    if (do_hint_prune)
    {
        /*
         * Update the page's pd_prune_xid field to either zero, or the lowest
         * XID of any soon-prunable tuple.
         */
        ((PageHeader) page)->pd_prune_xid = prstate.new_prune_xid;
 
        /*
         * Also clear the "page is full" flag, since there's no point in
         * repeating the prune/defrag process until something else happens to
         * the page.
         */
        PageClearFull(page);
 
        /*
         * If that's all we had to do to the page, this is a non-WAL-logged
         * hint.  If we are going to freeze or prune the page, we will mark
         * the buffer dirty below.
         */
        if (!do_freeze && !do_prune)
            MarkBufferDirtyHint(buffer, true);
    }
 
    if (do_prune || do_freeze)
    {
        /* Apply the planned item changes and repair page fragmentation. */
        if (do_prune)
        {
            heap_page_prune_execute(buffer, false,
                                    prstate.redirected, prstate.nredirected,
                                    prstate.nowdead, prstate.ndead,
                                    prstate.nowunused, prstate.nunused);
        }
 
        if (do_freeze)
            heap_freeze_prepared_tuples(buffer, prstate.frozen, prstate.nfrozen);
 
        MarkBufferDirty(buffer);
 
        /*
         * Emit a WAL XLOG_HEAP2_PRUNE* record showing what we did
         */
        if (RelationNeedsWAL(params->relation))
        {
            /*
             * The snapshotConflictHorizon for the whole record should be the
             * most conservative of all the horizons calculated for any of the
             * possible modifications.  If this record will prune tuples, any
             * transactions on the standby older than the youngest xmax of the
             * most recently removed tuple this record will prune will
             * conflict.  If this record will freeze tuples, any transactions
             * on the standby with xids older than the youngest tuple this
             * record will freeze will conflict.
             */
            TransactionId conflict_xid;
 
            if (TransactionIdFollows(prstate.frz_conflict_horizon,
                                     prstate.latest_xid_removed))
                conflict_xid = prstate.frz_conflict_horizon;
            else
                conflict_xid = prstate.latest_xid_removed;
 
            log_heap_prune_and_freeze(params->relation, buffer,
                                      InvalidBuffer,    /* vmbuffer */
                                      0,    /* vmflags */
                                      conflict_xid,
                                      true, params->reason,
                                      prstate.frozen, prstate.nfrozen,
                                      prstate.redirected, prstate.nredirected,
                                      prstate.nowdead, prstate.ndead,
                                      prstate.nowunused, prstate.nunused);
        }
    }
 
    END_CRIT_SECTION();
 
    /* Copy information back for caller */
    presult->ndeleted = prstate.ndeleted;
    presult->nnewlpdead = prstate.ndead;
    presult->nfrozen = prstate.nfrozen;
    presult->live_tuples = prstate.live_tuples;
    presult->recently_dead_tuples = prstate.recently_dead_tuples;
    presult->all_visible = prstate.all_visible;
    presult->all_frozen = prstate.all_frozen;
    presult->hastup = prstate.hastup;
 
    /*
     * For callers planning to update the visibility map, the conflict horizon
     * for that record must be the newest xmin on the page.  However, if the
     * page is completely frozen, there can be no conflict and the
     * vm_conflict_horizon should remain InvalidTransactionId.  This includes
     * the case that we just froze all the tuples; the prune-freeze record
     * included the conflict XID already so the caller doesn't need it.
     */
    if (presult->all_frozen)
        presult->vm_conflict_horizon = InvalidTransactionId;
    else
        presult->vm_conflict_horizon = prstate.visibility_cutoff_xid;
 
    presult->lpdead_items = prstate.lpdead_items;
    /* the presult->deadoffsets array was already filled in */
 
    if (prstate.attempt_freeze)
    {
        if (presult->nfrozen > 0)
        {
            *new_relfrozen_xid = prstate.pagefrz.FreezePageRelfrozenXid;
            *new_relmin_mxid = prstate.pagefrz.FreezePageRelminMxid;
        }
        else
        {
            *new_relfrozen_xid = prstate.pagefrz.NoFreezePageRelfrozenXid;
            *new_relmin_mxid = prstate.pagefrz.NoFreezePageRelminMxid;
        }
    }
}
 
 
/*
 * Perform visibility checks for heap pruning.
 */
static HTSV_Result
heap_prune_satisfies_vacuum(PruneState *prstate, HeapTuple tup, Buffer buffer)
{
    HTSV_Result res;
    TransactionId dead_after;
 
    res = HeapTupleSatisfiesVacuumHorizon(tup, buffer, &dead_after);
 
    if (res != HEAPTUPLE_RECENTLY_DEAD)
        return res;
 
    /*
     * For VACUUM, we must be sure to prune tuples with xmax older than
     * OldestXmin -- a visibility cutoff determined at the beginning of
     * vacuuming the relation. OldestXmin is used for freezing determination
     * and we cannot freeze dead tuples' xmaxes.
     */
    if (prstate->cutoffs &&
        TransactionIdIsValid(prstate->cutoffs->OldestXmin) &&
        NormalTransactionIdPrecedes(dead_after, prstate->cutoffs->OldestXmin))
        return HEAPTUPLE_DEAD;
 
    /*
     * Determine whether or not the tuple is considered dead when compared
     * with the provided GlobalVisState. On-access pruning does not provide
     * VacuumCutoffs. And for vacuum, even if the tuple's xmax is not older
     * than OldestXmin, GlobalVisTestIsRemovableXid() could find the row dead
     * if the GlobalVisState has been updated since the beginning of vacuuming
     * the relation.
     */
    if (GlobalVisTestIsRemovableXid(prstate->vistest, dead_after))
        return HEAPTUPLE_DEAD;
 
    return res;
}
 
 
/*
 * Pruning calculates tuple visibility once and saves the results in an array
 * of int8.  See PruneState.htsv for details.  This helper function is meant
 * to guard against examining visibility status array members which have not
 * yet been computed.
 */
static inline HTSV_Result
htsv_get_valid_status(int status)
{
    Assert(status >= HEAPTUPLE_DEAD &&
           status <= HEAPTUPLE_DELETE_IN_PROGRESS);
    return (HTSV_Result) status;
}
 
/*
 * Prune specified line pointer or a HOT chain originating at line pointer.
 *
 * Tuple visibility information is provided in prstate->htsv.
 *
 * If the item is an index-referenced tuple (i.e. not a heap-only tuple),
 * the HOT chain is pruned by removing all DEAD tuples at the start of the HOT
 * chain.  We also prune any RECENTLY_DEAD tuples preceding a DEAD tuple.
 * This is OK because a RECENTLY_DEAD tuple preceding a DEAD tuple is really
 * DEAD, our visibility test is just too coarse to detect it.
 *
 * Pruning must never leave behind a DEAD tuple that still has tuple storage.
 * VACUUM isn't prepared to deal with that case.
 *
 * The root line pointer is redirected to the tuple immediately after the
 * latest DEAD tuple.  If all tuples in the chain are DEAD, the root line
 * pointer is marked LP_DEAD.  (This includes the case of a DEAD simple
 * tuple, which we treat as a chain of length 1.)
 *
 * We don't actually change the page here. We just add entries to the arrays in
 * prstate showing the changes to be made.  Items to be redirected are added
 * to the redirected[] array (two entries per redirection); items to be set to
 * LP_DEAD state are added to nowdead[]; and items to be set to LP_UNUSED
 * state are added to nowunused[].  We perform bookkeeping of live tuples,
 * visibility etc. based on what the page will look like after the changes
 * applied.  All that bookkeeping is performed in the heap_prune_record_*()
 * subroutines.  The division of labor is that heap_prune_chain() decides the
 * fate of each tuple, ie. whether it's going to be removed, redirected or
 * left unchanged, and the heap_prune_record_*() subroutines update PruneState
 * based on that outcome.
 */
static void
heap_prune_chain(Page page, BlockNumber blockno, OffsetNumber maxoff,
                 OffsetNumber rootoffnum, PruneState *prstate)
{
    TransactionId priorXmax = InvalidTransactionId;
    ItemId      rootlp;
    OffsetNumber offnum;
    OffsetNumber chainitems[MaxHeapTuplesPerPage];
 
    /*
     * After traversing the HOT chain, ndeadchain is the index in chainitems
     * of the first live successor after the last dead item.
     */
    int         ndeadchain = 0,
                nchain = 0;
 
    rootlp = PageGetItemId(page, rootoffnum);
 
    /* Start from the root tuple */
    offnum = rootoffnum;
 
    /* while not end of the chain */
    for (;;)
    {
        HeapTupleHeader htup;
        ItemId      lp;
 
        /* Sanity check (pure paranoia) */
        if (offnum < FirstOffsetNumber)
            break;
 
        /*
         * An offset past the end of page's line pointer array is possible
         * when the array was truncated (original item must have been unused)
         */
        if (offnum > maxoff)
            break;
 
        /* If item is already processed, stop --- it must not be same chain */
        if (prstate->processed[offnum])
            break;
 
        lp = PageGetItemId(page, offnum);
 
        /*
         * Unused item obviously isn't part of the chain. Likewise, a dead
         * line pointer can't be part of the chain.  Both of those cases were
         * already marked as processed.
         */
        Assert(ItemIdIsUsed(lp));
        Assert(!ItemIdIsDead(lp));
 
        /*
         * If we are looking at the redirected root line pointer, jump to the
         * first normal tuple in the chain.  If we find a redirect somewhere
         * else, stop --- it must not be same chain.
         */
        if (ItemIdIsRedirected(lp))
        {
            if (nchain > 0)
                break;          /* not at start of chain */
            chainitems[nchain++] = offnum;
            offnum = ItemIdGetRedirect(rootlp);
            continue;
        }
 
        Assert(ItemIdIsNormal(lp));
 
        htup = (HeapTupleHeader) PageGetItem(page, lp);
 
        /*
         * Check the tuple XMIN against prior XMAX, if any
         */
        if (TransactionIdIsValid(priorXmax) &&
            !TransactionIdEquals(HeapTupleHeaderGetXmin(htup), priorXmax))
            break;
 
        /*
         * OK, this tuple is indeed a member of the chain.
         */
        chainitems[nchain++] = offnum;
 
        switch (htsv_get_valid_status(prstate->htsv[offnum]))
        {
            case HEAPTUPLE_DEAD:
 
                /* Remember the last DEAD tuple seen */
                ndeadchain = nchain;
                HeapTupleHeaderAdvanceConflictHorizon(htup,
                                                      &prstate->latest_xid_removed);
                /* Advance to next chain member */
                break;
 
            case HEAPTUPLE_RECENTLY_DEAD:
 
                /*
                 * We don't need to advance the conflict horizon for
                 * RECENTLY_DEAD tuples, even if we are removing them.  This
                 * is because we only remove RECENTLY_DEAD tuples if they
                 * precede a DEAD tuple, and the DEAD tuple must have been
                 * inserted by a newer transaction than the RECENTLY_DEAD
                 * tuple by virtue of being later in the chain.  We will have
                 * advanced the conflict horizon for the DEAD tuple.
                 */
 
                /*
                 * Advance past RECENTLY_DEAD tuples just in case there's a
                 * DEAD one after them.  We have to make sure that we don't
                 * miss any DEAD tuples, since DEAD tuples that still have
                 * tuple storage after pruning will confuse VACUUM.
                 */
                break;
 
            case HEAPTUPLE_DELETE_IN_PROGRESS:
            case HEAPTUPLE_LIVE:
            case HEAPTUPLE_INSERT_IN_PROGRESS:
                goto process_chain;
 
            default:
                elog(ERROR, "unexpected HeapTupleSatisfiesVacuum result");
                goto process_chain;
        }
 
        /*
         * If the tuple is not HOT-updated, then we are at the end of this
         * HOT-update chain.
         */
        if (!HeapTupleHeaderIsHotUpdated(htup))
            goto process_chain;
 
        /* HOT implies it can't have moved to different partition */
        Assert(!HeapTupleHeaderIndicatesMovedPartitions(htup));
 
        /*
         * Advance to next chain member.
         */
        Assert(ItemPointerGetBlockNumber(&htup->t_ctid) == blockno);
        offnum = ItemPointerGetOffsetNumber(&htup->t_ctid);
        priorXmax = HeapTupleHeaderGetUpdateXid(htup);
    }
 
    if (ItemIdIsRedirected(rootlp) && nchain < 2)
    {
        /*
         * We found a redirect item that doesn't point to a valid follow-on
         * item.  This can happen if the loop in heap_page_prune_and_freeze()
         * caused us to visit the dead successor of a redirect item before
         * visiting the redirect item.  We can clean up by setting the
         * redirect item to LP_DEAD state or LP_UNUSED if the caller
         * indicated.
         */
        heap_prune_record_dead_or_unused(prstate, rootoffnum, false);
        return;
    }
 
process_chain:
 
    if (ndeadchain == 0)
    {
        /*
         * No DEAD tuple was found, so the chain is entirely composed of
         * normal, unchanged tuples.  Leave it alone.
         */
        int         i = 0;
 
        if (ItemIdIsRedirected(rootlp))
        {
            heap_prune_record_unchanged_lp_redirect(prstate, rootoffnum);
            i++;
        }
        for (; i < nchain; i++)
            heap_prune_record_unchanged_lp_normal(page, prstate, chainitems[i]);
    }
    else if (ndeadchain == nchain)
    {
        /*
         * The entire chain is dead.  Mark the root line pointer LP_DEAD, and
         * fully remove the other tuples in the chain.
         */
        heap_prune_record_dead_or_unused(prstate, rootoffnum, ItemIdIsNormal(rootlp));
        for (int i = 1; i < nchain; i++)
            heap_prune_record_unused(prstate, chainitems[i], true);
    }
    else
    {
        /*
         * We found a DEAD tuple in the chain.  Redirect the root line pointer
         * to the first non-DEAD tuple, and mark as unused each intermediate
         * item that we are able to remove from the chain.
         */
        heap_prune_record_redirect(prstate, rootoffnum, chainitems[ndeadchain],
                                   ItemIdIsNormal(rootlp));
        for (int i = 1; i < ndeadchain; i++)
            heap_prune_record_unused(prstate, chainitems[i], true);
 
        /* the rest of tuples in the chain are normal, unchanged tuples */
        for (int i = ndeadchain; i < nchain; i++)
            heap_prune_record_unchanged_lp_normal(page, prstate, chainitems[i]);
    }
}
 
/* Record lowest soon-prunable XID */
static void
heap_prune_record_prunable(PruneState *prstate, TransactionId xid)
{
    /*
     * This should exactly match the PageSetPrunable macro.  We can't store
     * directly into the page header yet, so we update working state.
     */
    Assert(TransactionIdIsNormal(xid));
    if (!TransactionIdIsValid(prstate->new_prune_xid) ||
        TransactionIdPrecedes(xid, prstate->new_prune_xid))
        prstate->new_prune_xid = xid;
}
 
/* Record line pointer to be redirected */
static void
heap_prune_record_redirect(PruneState *prstate,
                           OffsetNumber offnum, OffsetNumber rdoffnum,
                           bool was_normal)
{
    Assert(!prstate->processed[offnum]);
    prstate->processed[offnum] = true;
 
    /*
     * Do not mark the redirect target here.  It needs to be counted
     * separately as an unchanged tuple.
     */
 
    Assert(prstate->nredirected < MaxHeapTuplesPerPage);
    prstate->redirected[prstate->nredirected * 2] = offnum;
    prstate->redirected[prstate->nredirected * 2 + 1] = rdoffnum;
 
    prstate->nredirected++;
 
    /*
     * If the root entry had been a normal tuple, we are deleting it, so count
     * it in the result.  But changing a redirect (even to DEAD state) doesn't
     * count.
     */
    if (was_normal)
        prstate->ndeleted++;
 
    prstate->hastup = true;
}
 
/* Record line pointer to be marked dead */
static void
heap_prune_record_dead(PruneState *prstate, OffsetNumber offnum,
                       bool was_normal)
{
    Assert(!prstate->processed[offnum]);
    prstate->processed[offnum] = true;
 
    Assert(prstate->ndead < MaxHeapTuplesPerPage);
    prstate->nowdead[prstate->ndead] = offnum;
    prstate->ndead++;
 
    /*
     * Deliberately delay unsetting all_visible and all_frozen until later
     * during pruning. Removable dead tuples shouldn't preclude freezing the
     * page.
     */
 
    /* Record the dead offset for vacuum */
    prstate->deadoffsets[prstate->lpdead_items++] = offnum;
 
    /*
     * If the root entry had been a normal tuple, we are deleting it, so count
     * it in the result.  But changing a redirect (even to DEAD state) doesn't
     * count.
     */
    if (was_normal)
        prstate->ndeleted++;
}
 
/*
 * Depending on whether or not the caller set mark_unused_now to true, record that a
 * line pointer should be marked LP_DEAD or LP_UNUSED. There are other cases in
 * which we will mark line pointers LP_UNUSED, but we will not mark line
 * pointers LP_DEAD if mark_unused_now is true.
 */
static void
heap_prune_record_dead_or_unused(PruneState *prstate, OffsetNumber offnum,
                                 bool was_normal)
{
    /*
     * If the caller set mark_unused_now to true, we can remove dead tuples
     * during pruning instead of marking their line pointers dead. Set this
     * tuple's line pointer LP_UNUSED. We hint that this option is less
     * likely.
     */
    if (unlikely(prstate->mark_unused_now))
        heap_prune_record_unused(prstate, offnum, was_normal);
    else
        heap_prune_record_dead(prstate, offnum, was_normal);
}
 
/* Record line pointer to be marked unused */
static void
heap_prune_record_unused(PruneState *prstate, OffsetNumber offnum, bool was_normal)
{
    Assert(!prstate->processed[offnum]);
    prstate->processed[offnum] = true;
 
    Assert(prstate->nunused < MaxHeapTuplesPerPage);
    prstate->nowunused[prstate->nunused] = offnum;
    prstate->nunused++;
 
    /*
     * If the root entry had been a normal tuple, we are deleting it, so count
     * it in the result.  But changing a redirect (even to DEAD state) doesn't
     * count.
     */
    if (was_normal)
        prstate->ndeleted++;
}
 
/*
 * Record an unused line pointer that is left unchanged.
 */
static void
heap_prune_record_unchanged_lp_unused(Page page, PruneState *prstate, OffsetNumber offnum)
{
    Assert(!prstate->processed[offnum]);
    prstate->processed[offnum] = true;
}
 
/*
 * Record line pointer that is left unchanged.  We consider freezing it, and
 * update bookkeeping of tuple counts and page visibility.
 */
static void
heap_prune_record_unchanged_lp_normal(Page page, PruneState *prstate, OffsetNumber offnum)
{
    HeapTupleHeader htup;
 
    Assert(!prstate->processed[offnum]);
    prstate->processed[offnum] = true;
 
    prstate->hastup = true;     /* the page is not empty */
 
    /*
     * The criteria for counting a tuple as live in this block need to match
     * what analyze.c's acquire_sample_rows() does, otherwise VACUUM and
     * ANALYZE may produce wildly different reltuples values, e.g. when there
     * are many recently-dead tuples.
     *
     * The logic here is a bit simpler than acquire_sample_rows(), as VACUUM
     * can't run inside a transaction block, which makes some cases impossible
     * (e.g. in-progress insert from the same transaction).
     *
     * HEAPTUPLE_DEAD are handled by the other heap_prune_record_*()
     * subroutines.  They don't count dead items like acquire_sample_rows()
     * does, because we assume that all dead items will become LP_UNUSED
     * before VACUUM finishes.  This difference is only superficial.  VACUUM
     * effectively agrees with ANALYZE about DEAD items, in the end.  VACUUM
     * won't remember LP_DEAD items, but only because they're not supposed to
     * be left behind when it is done. (Cases where we bypass index vacuuming
     * will violate this optimistic assumption, but the overall impact of that
     * should be negligible.)
     */
    htup = (HeapTupleHeader) PageGetItem(page, PageGetItemId(page, offnum));
 
    switch (prstate->htsv[offnum])
    {
        case HEAPTUPLE_LIVE:
 
            /*
             * Count it as live.  Not only is this natural, but it's also what
             * acquire_sample_rows() does.
             */
            prstate->live_tuples++;
 
            /*
             * Is the tuple definitely visible to all transactions?
             *
             * NB: Like with per-tuple hint bits, we can't set the
             * PD_ALL_VISIBLE flag if the inserter committed asynchronously.
             * See SetHintBits for more info.  Check that the tuple is hinted
             * xmin-committed because of that.
             */
            if (prstate->all_visible)
            {
                TransactionId xmin;
 
                if (!HeapTupleHeaderXminCommitted(htup))
                {
                    prstate->all_visible = false;
                    prstate->all_frozen = false;
                    break;
                }
 
                /*
                 * The inserter definitely committed.  But is it old enough
                 * that everyone sees it as committed?  A FrozenTransactionId
                 * is seen as committed to everyone.  Otherwise, we check if
                 * there is a snapshot that considers this xid to still be
                 * running, and if so, we don't consider the page all-visible.
                 */
                xmin = HeapTupleHeaderGetXmin(htup);
 
                /*
                 * For now always use prstate->cutoffs for this test, because
                 * we only update 'all_visible' and 'all_frozen' when freezing
                 * is requested. We could use GlobalVisTestIsRemovableXid
                 * instead, if a non-freezing caller wanted to set the VM bit.
                 */
                Assert(prstate->cutoffs);
                if (!TransactionIdPrecedes(xmin, prstate->cutoffs->OldestXmin))
                {
                    prstate->all_visible = false;
                    prstate->all_frozen = false;
                    break;
                }
 
                /* Track newest xmin on page. */
                if (TransactionIdFollows(xmin, prstate->visibility_cutoff_xid) &&
                    TransactionIdIsNormal(xmin))
                    prstate->visibility_cutoff_xid = xmin;
            }
            break;
 
        case HEAPTUPLE_RECENTLY_DEAD:
            prstate->recently_dead_tuples++;
            prstate->all_visible = false;
            prstate->all_frozen = false;
 
            /*
             * This tuple will soon become DEAD.  Update the hint field so
             * that the page is reconsidered for pruning in future.
             */
            heap_prune_record_prunable(prstate,
                                       HeapTupleHeaderGetUpdateXid(htup));
            break;
 
        case HEAPTUPLE_INSERT_IN_PROGRESS:
 
            /*
             * We do not count these rows as live, because we expect the
             * inserting transaction to update the counters at commit, and we
             * assume that will happen only after we report our results.  This
             * assumption is a bit shaky, but it is what acquire_sample_rows()
             * does, so be consistent.
             */
            prstate->all_visible = false;
            prstate->all_frozen = false;
 
            /*
             * If we wanted to optimize for aborts, we might consider marking
             * the page prunable when we see INSERT_IN_PROGRESS.  But we
             * don't.  See related decisions about when to mark the page
             * prunable in heapam.c.
             */
            break;
 
        case HEAPTUPLE_DELETE_IN_PROGRESS:
 
            /*
             * This an expected case during concurrent vacuum.  Count such
             * rows as live.  As above, we assume the deleting transaction
             * will commit and update the counters after we report.
             */
            prstate->live_tuples++;
            prstate->all_visible = false;
            prstate->all_frozen = false;
 
            /*
             * This tuple may soon become DEAD.  Update the hint field so that
             * the page is reconsidered for pruning in future.
             */
            heap_prune_record_prunable(prstate,
                                       HeapTupleHeaderGetUpdateXid(htup));
            break;
 
        default:
 
            /*
             * DEAD tuples should've been passed to heap_prune_record_dead()
             * or heap_prune_record_unused() instead.
             */
            elog(ERROR, "unexpected HeapTupleSatisfiesVacuum result %d",
                 prstate->htsv[offnum]);
            break;
    }
 
    /* Consider freezing any normal tuples which will not be removed */
    if (prstate->attempt_freeze)
    {
        bool        totally_frozen;
 
        if ((heap_prepare_freeze_tuple(htup,
                                       prstate->cutoffs,
                                       &prstate->pagefrz,
                                       &prstate->frozen[prstate->nfrozen],
                                       &totally_frozen)))
        {
            /* Save prepared freeze plan for later */
            prstate->frozen[prstate->nfrozen++].offset = offnum;
        }
 
        /*
         * If any tuple isn't either totally frozen already or eligible to
         * become totally frozen (according to its freeze plan), then the page
         * definitely cannot be set all-frozen in the visibility map later on.
         */
        if (!totally_frozen)
            prstate->all_frozen = false;
    }
}
 
 
/*
 * Record line pointer that was already LP_DEAD and is left unchanged.
 */
static void
heap_prune_record_unchanged_lp_dead(Page page, PruneState *prstate, OffsetNumber offnum)
{
    Assert(!prstate->processed[offnum]);
    prstate->processed[offnum] = true;
 
    /*
     * Deliberately don't set hastup for LP_DEAD items.  We make the soft
     * assumption that any LP_DEAD items encountered here will become
     * LP_UNUSED later on, before count_nondeletable_pages is reached.  If we
     * don't make this assumption then rel truncation will only happen every
     * other VACUUM, at most.  Besides, VACUUM must treat
     * hastup/nonempty_pages as provisional no matter how LP_DEAD items are
     * handled (handled here, or handled later on).
     *
     * Similarly, don't unset all_visible and all_frozen until later, at the
     * end of heap_page_prune_and_freeze().  This will allow us to attempt to
     * freeze the page after pruning.  As long as we unset it before updating
     * the visibility map, this will be correct.
     */
 
    /* Record the dead offset for vacuum */
    prstate->deadoffsets[prstate->lpdead_items++] = offnum;
}
 
/*
 * Record LP_REDIRECT that is left unchanged.
 */
static void
heap_prune_record_unchanged_lp_redirect(PruneState *prstate, OffsetNumber offnum)
{
    /*
     * A redirect line pointer doesn't count as a live tuple.
     *
     * If we leave a redirect line pointer in place, there will be another
     * tuple on the page that it points to.  We will do the bookkeeping for
     * that separately.  So we have nothing to do here, except remember that
     * we processed this item.
     */
    Assert(!prstate->processed[offnum]);
    prstate->processed[offnum] = true;
}
 
/*
 * Perform the actual page changes needed by heap_page_prune_and_freeze().
 *
 * If 'lp_truncate_only' is set, we are merely marking LP_DEAD line pointers
 * as unused, not redirecting or removing anything else.  The
 * PageRepairFragmentation() call is skipped in that case.
 *
 * If 'lp_truncate_only' is not set, the caller must hold a cleanup lock on
 * the buffer.  If it is set, an ordinary exclusive lock suffices.
 */
void
heap_page_prune_execute(Buffer buffer, bool lp_truncate_only,
                        OffsetNumber *redirected, int nredirected,
                        OffsetNumber *nowdead, int ndead,
                        OffsetNumber *nowunused, int nunused)
{
    Page        page = BufferGetPage(buffer);
    OffsetNumber *offnum;
    HeapTupleHeader htup PG_USED_FOR_ASSERTS_ONLY;
 
    /* Shouldn't be called unless there's something to do */
    Assert(nredirected > 0 || ndead > 0 || nunused > 0);
 
    /* If 'lp_truncate_only', we can only remove already-dead line pointers */
    Assert(!lp_truncate_only || (nredirected == 0 && ndead == 0));
 
    /* Update all redirected line pointers */
    offnum = redirected;
    for (int i = 0; i < nredirected; i++)
    {
        OffsetNumber fromoff = *offnum++;
        OffsetNumber tooff = *offnum++;
        ItemId      fromlp = PageGetItemId(page, fromoff);
        ItemId      tolp PG_USED_FOR_ASSERTS_ONLY;
 
#ifdef USE_ASSERT_CHECKING
 
        /*
         * Any existing item that we set as an LP_REDIRECT (any 'from' item)
         * must be the first item from a HOT chain.  If the item has tuple
         * storage then it can't be a heap-only tuple.  Otherwise we are just
         * maintaining an existing LP_REDIRECT from an existing HOT chain that
         * has been pruned at least once before now.
         */
        if (!ItemIdIsRedirected(fromlp))
        {
            Assert(ItemIdHasStorage(fromlp) && ItemIdIsNormal(fromlp));
 
            htup = (HeapTupleHeader) PageGetItem(page, fromlp);
            Assert(!HeapTupleHeaderIsHeapOnly(htup));
        }
        else
        {
            /* We shouldn't need to redundantly set the redirect */
            Assert(ItemIdGetRedirect(fromlp) != tooff);
        }
 
        /*
         * The item that we're about to set as an LP_REDIRECT (the 'from'
         * item) will point to an existing item (the 'to' item) that is
         * already a heap-only tuple.  There can be at most one LP_REDIRECT
         * item per HOT chain.
         *
         * We need to keep around an LP_REDIRECT item (after original
         * non-heap-only root tuple gets pruned away) so that it's always
         * possible for VACUUM to easily figure out what TID to delete from
         * indexes when an entire HOT chain becomes dead.  A heap-only tuple
         * can never become LP_DEAD; an LP_REDIRECT item or a regular heap
         * tuple can.
         *
         * This check may miss problems, e.g. the target of a redirect could
         * be marked as unused subsequently. The page_verify_redirects() check
         * below will catch such problems.
         */
        tolp = PageGetItemId(page, tooff);
        Assert(ItemIdHasStorage(tolp) && ItemIdIsNormal(tolp));
        htup = (HeapTupleHeader) PageGetItem(page, tolp);
        Assert(HeapTupleHeaderIsHeapOnly(htup));
#endif
 
        ItemIdSetRedirect(fromlp, tooff);
    }
 
    /* Update all now-dead line pointers */
    offnum = nowdead;
    for (int i = 0; i < ndead; i++)
    {
        OffsetNumber off = *offnum++;
        ItemId      lp = PageGetItemId(page, off);
 
#ifdef USE_ASSERT_CHECKING
 
        /*
         * An LP_DEAD line pointer must be left behind when the original item
         * (which is dead to everybody) could still be referenced by a TID in
         * an index.  This should never be necessary with any individual
         * heap-only tuple item, though. (It's not clear how much of a problem
         * that would be, but there is no reason to allow it.)
         */
        if (ItemIdHasStorage(lp))
        {
            Assert(ItemIdIsNormal(lp));
            htup = (HeapTupleHeader) PageGetItem(page, lp);
            Assert(!HeapTupleHeaderIsHeapOnly(htup));
        }
        else
        {
            /* Whole HOT chain becomes dead */
            Assert(ItemIdIsRedirected(lp));
        }
#endif
 
        ItemIdSetDead(lp);
    }
 
    /* Update all now-unused line pointers */
    offnum = nowunused;
    for (int i = 0; i < nunused; i++)
    {
        OffsetNumber off = *offnum++;
        ItemId      lp = PageGetItemId(page, off);
 
#ifdef USE_ASSERT_CHECKING
 
        if (lp_truncate_only)
        {
            /* Setting LP_DEAD to LP_UNUSED in vacuum's second pass */
            Assert(ItemIdIsDead(lp) && !ItemIdHasStorage(lp));
        }
        else
        {
            /*
             * When heap_page_prune_and_freeze() was called, mark_unused_now
             * may have been passed as true, which allows would-be LP_DEAD
             * items to be made LP_UNUSED instead.  This is only possible if
             * the relation has no indexes.  If there are any dead items, then
             * mark_unused_now was not true and every item being marked
             * LP_UNUSED must refer to a heap-only tuple.
             */
            if (ndead > 0)
            {
                Assert(ItemIdHasStorage(lp) && ItemIdIsNormal(lp));
                htup = (HeapTupleHeader) PageGetItem(page, lp);
                Assert(HeapTupleHeaderIsHeapOnly(htup));
            }
            else
                Assert(ItemIdIsUsed(lp));
        }
 
#endif
 
        ItemIdSetUnused(lp);
    }
 
    if (lp_truncate_only)
        PageTruncateLinePointerArray(page);
    else
    {
        /*
         * Finally, repair any fragmentation, and update the page's hint bit
         * about whether it has free pointers.
         */
        PageRepairFragmentation(page);
 
        /*
         * Now that the page has been modified, assert that redirect items
         * still point to valid targets.
         */
        page_verify_redirects(page);
    }
}
 
 
/*
 * If built with assertions, verify that all LP_REDIRECT items point to a
 * valid item.
 *
 * One way that bugs related to HOT pruning show is redirect items pointing to
 * removed tuples. It's not trivial to reliably check that marking an item
 * unused will not orphan a redirect item during heap_prune_chain() /
 * heap_page_prune_execute(), so we additionally check the whole page after
 * pruning. Without this check such bugs would typically only cause asserts
 * later, potentially well after the corruption has been introduced.
 *
 * Also check comments in heap_page_prune_execute()'s redirection loop.
 */
static void
page_verify_redirects(Page page)
{
#ifdef USE_ASSERT_CHECKING
    OffsetNumber offnum;
    OffsetNumber maxoff;
 
    maxoff = PageGetMaxOffsetNumber(page);
    for (offnum = FirstOffsetNumber;
         offnum <= maxoff;
         offnum = OffsetNumberNext(offnum))
    {
        ItemId      itemid = PageGetItemId(page, offnum);
        OffsetNumber targoff;
        ItemId      targitem;
        HeapTupleHeader htup;
 
        if (!ItemIdIsRedirected(itemid))
            continue;
 
        targoff = ItemIdGetRedirect(itemid);
        targitem = PageGetItemId(page, targoff);
 
        Assert(ItemIdIsUsed(targitem));
        Assert(ItemIdIsNormal(targitem));
        Assert(ItemIdHasStorage(targitem));
        htup = (HeapTupleHeader) PageGetItem(page, targitem);
        Assert(HeapTupleHeaderIsHeapOnly(htup));
    }
#endif
}
 
 
/*
 * For all items in this page, find their respective root line pointers.
 * If item k is part of a HOT-chain with root at item j, then we set
 * root_offsets[k - 1] = j.
 *
 * The passed-in root_offsets array must have MaxHeapTuplesPerPage entries.
 * Unused entries are filled with InvalidOffsetNumber (zero).
 *
 * The function must be called with at least share lock on the buffer, to
 * prevent concurrent prune operations.
 *
 * Note: The information collected here is valid only as long as the caller
 * holds a pin on the buffer. Once pin is released, a tuple might be pruned
 * and reused by a completely unrelated tuple.
 */
void
heap_get_root_tuples(Page page, OffsetNumber *root_offsets)
{
    OffsetNumber offnum,
                maxoff;
 
    MemSet(root_offsets, InvalidOffsetNumber,
           MaxHeapTuplesPerPage * sizeof(OffsetNumber));
 
    maxoff = PageGetMaxOffsetNumber(page);
    for (offnum = FirstOffsetNumber; offnum <= maxoff; offnum = OffsetNumberNext(offnum))
    {
        ItemId      lp = PageGetItemId(page, offnum);
        HeapTupleHeader htup;
        OffsetNumber nextoffnum;
        TransactionId priorXmax;
 
        /* skip unused and dead items */
        if (!ItemIdIsUsed(lp) || ItemIdIsDead(lp))
            continue;
 
        if (ItemIdIsNormal(lp))
        {
            htup = (HeapTupleHeader) PageGetItem(page, lp);
 
            /*
             * Check if this tuple is part of a HOT-chain rooted at some other
             * tuple. If so, skip it for now; we'll process it when we find
             * its root.
             */
            if (HeapTupleHeaderIsHeapOnly(htup))
                continue;
 
            /*
             * This is either a plain tuple or the root of a HOT-chain.
             * Remember it in the mapping.
             */
            root_offsets[offnum - 1] = offnum;
 
            /* If it's not the start of a HOT-chain, we're done with it */
            if (!HeapTupleHeaderIsHotUpdated(htup))
                continue;
 
            /* Set up to scan the HOT-chain */
            nextoffnum = ItemPointerGetOffsetNumber(&htup->t_ctid);
            priorXmax = HeapTupleHeaderGetUpdateXid(htup);
        }
        else
        {
            /* Must be a redirect item. We do not set its root_offsets entry */
            Assert(ItemIdIsRedirected(lp));
            /* Set up to scan the HOT-chain */
            nextoffnum = ItemIdGetRedirect(lp);
            priorXmax = InvalidTransactionId;
        }
 
        /*
         * Now follow the HOT-chain and collect other tuples in the chain.
         *
         * Note: Even though this is a nested loop, the complexity of the
         * function is O(N) because a tuple in the page should be visited not
         * more than twice, once in the outer loop and once in HOT-chain
         * chases.
         */
        for (;;)
        {
            /* Sanity check (pure paranoia) */
            if (offnum < FirstOffsetNumber)
                break;
 
            /*
             * An offset past the end of page's line pointer array is possible
             * when the array was truncated
             */
            if (offnum > maxoff)
                break;
 
            lp = PageGetItemId(page, nextoffnum);
 
            /* Check for broken chains */
            if (!ItemIdIsNormal(lp))
                break;
 
            htup = (HeapTupleHeader) PageGetItem(page, lp);
 
            if (TransactionIdIsValid(priorXmax) &&
                !TransactionIdEquals(priorXmax, HeapTupleHeaderGetXmin(htup)))
                break;
 
            /* Remember the root line pointer for this item */
            root_offsets[nextoffnum - 1] = offnum;
 
            /* Advance to next chain member, if any */
            if (!HeapTupleHeaderIsHotUpdated(htup))
                break;
 
            /* HOT implies it can't have moved to different partition */
            Assert(!HeapTupleHeaderIndicatesMovedPartitions(htup));
 
            nextoffnum = ItemPointerGetOffsetNumber(&htup->t_ctid);
            priorXmax = HeapTupleHeaderGetUpdateXid(htup);
        }
    }
}
 
 
/*
 * Compare fields that describe actions required to freeze tuple with caller's
 * open plan.  If everything matches then the frz tuple plan is equivalent to
 * caller's plan.
 */
static inline bool
heap_log_freeze_eq(xlhp_freeze_plan *plan, HeapTupleFreeze *frz)
{
    if (plan->xmax == frz->xmax &&
        plan->t_infomask2 == frz->t_infomask2 &&
        plan->t_infomask == frz->t_infomask &&
        plan->frzflags == frz->frzflags)
        return true;
 
    /* Caller must call heap_log_freeze_new_plan again for frz */
    return false;
}
 
/*
 * Comparator used to deduplicate the freeze plans used in WAL records.
 */
static int
heap_log_freeze_cmp(const void *arg1, const void *arg2)
{
    HeapTupleFreeze *frz1 = (HeapTupleFreeze *) arg1;
    HeapTupleFreeze *frz2 = (HeapTupleFreeze *) arg2;
 
    if (frz1->xmax < frz2->xmax)
        return -1;
    else if (frz1->xmax > frz2->xmax)
        return 1;
 
    if (frz1->t_infomask2 < frz2->t_infomask2)
        return -1;
    else if (frz1->t_infomask2 > frz2->t_infomask2)
        return 1;
 
    if (frz1->t_infomask < frz2->t_infomask)
        return -1;
    else if (frz1->t_infomask > frz2->t_infomask)
        return 1;
 
    if (frz1->frzflags < frz2->frzflags)
        return -1;
    else if (frz1->frzflags > frz2->frzflags)
        return 1;
 
    /*
     * heap_log_freeze_eq would consider these tuple-wise plans to be equal.
     * (So the tuples will share a single canonical freeze plan.)
     *
     * We tiebreak on page offset number to keep each freeze plan's page
     * offset number array individually sorted. (Unnecessary, but be tidy.)
     */
    if (frz1->offset < frz2->offset)
        return -1;
    else if (frz1->offset > frz2->offset)
        return 1;
 
    Assert(false);
    return 0;
}
 
/*
 * Start new plan initialized using tuple-level actions.  At least one tuple
 * will have steps required to freeze described by caller's plan during REDO.
 */
static inline void
heap_log_freeze_new_plan(xlhp_freeze_plan *plan, HeapTupleFreeze *frz)
{
    plan->xmax = frz->xmax;
    plan->t_infomask2 = frz->t_infomask2;
    plan->t_infomask = frz->t_infomask;
    plan->frzflags = frz->frzflags;
    plan->ntuples = 1;          /* for now */
}
 
/*
 * Deduplicate tuple-based freeze plans so that each distinct set of
 * processing steps is only stored once in the WAL record.
 * Called during original execution of freezing (for logged relations).
 *
 * Return value is number of plans set in *plans_out for caller.  Also writes
 * an array of offset numbers into *offsets_out output argument for caller
 * (actually there is one array per freeze plan, but that's not of immediate
 * concern to our caller).
 */
static int
heap_log_freeze_plan(HeapTupleFreeze *tuples, int ntuples,
                     xlhp_freeze_plan *plans_out,
                     OffsetNumber *offsets_out)
{
    int         nplans = 0;
 
    /* Sort tuple-based freeze plans in the order required to deduplicate */
    qsort(tuples, ntuples, sizeof(HeapTupleFreeze), heap_log_freeze_cmp);
 
    for (int i = 0; i < ntuples; i++)
    {
        HeapTupleFreeze *frz = tuples + i;
 
        if (i == 0)
        {
            /* New canonical freeze plan starting with first tup */
            heap_log_freeze_new_plan(plans_out, frz);
            nplans++;
        }
        else if (heap_log_freeze_eq(plans_out, frz))
        {
            /* tup matches open canonical plan -- include tup in it */
            Assert(offsets_out[i - 1] < frz->offset);
            plans_out->ntuples++;
        }
        else
        {
            /* Tup doesn't match current plan -- done with it now */
            plans_out++;
 
            /* New canonical freeze plan starting with this tup */
            heap_log_freeze_new_plan(plans_out, frz);
            nplans++;
        }
 
        /*
         * Save page offset number in dedicated buffer in passing.
         *
         * REDO routine relies on the record's offset numbers array grouping
         * offset numbers by freeze plan.  The sort order within each grouping
         * is ascending offset number order, just to keep things tidy.
         */
        offsets_out[i] = frz->offset;
    }
 
    Assert(nplans > 0 && nplans <= ntuples);
 
    return nplans;
}
 
/*
 * Write an XLOG_HEAP2_PRUNE* WAL record
 *
 * This is used for several different page maintenance operations:
 *
 * - Page pruning, in VACUUM's 1st pass or on access: Some items are
 *   redirected, some marked dead, and some removed altogether.
 *
 * - Freezing: Items are marked as 'frozen'.
 *
 * - Vacuum, 2nd pass: Items that are already LP_DEAD are marked as unused.
 *
 * They have enough commonalities that we use a single WAL record for them
 * all.
 *
 * If replaying the record requires a cleanup lock, pass cleanup_lock = true.
 * Replaying 'redirected' or 'dead' items always requires a cleanup lock, but
 * replaying 'unused' items depends on whether they were all previously marked
 * as dead.
 *
 * If the VM is being updated, vmflags will contain the bits to set. In this
 * case, vmbuffer should already have been updated and marked dirty and should
 * still be pinned and locked.
 *
 * Note: This function scribbles on the 'frozen' array.
 *
 * Note: This is called in a critical section, so careful what you do here.
 */
void
log_heap_prune_and_freeze(Relation relation, Buffer buffer,
                          Buffer vmbuffer, uint8 vmflags,
                          TransactionId conflict_xid,
                          bool cleanup_lock,
                          PruneReason reason,
                          HeapTupleFreeze *frozen, int nfrozen,
                          OffsetNumber *redirected, int nredirected,
                          OffsetNumber *dead, int ndead,
                          OffsetNumber *unused, int nunused)
{
    xl_heap_prune xlrec;
    XLogRecPtr  recptr;
    uint8       info;
    uint8       regbuf_flags_heap;
 
    /* The following local variables hold data registered in the WAL record: */
    xlhp_freeze_plan plans[MaxHeapTuplesPerPage];
    xlhp_freeze_plans freeze_plans;
    xlhp_prune_items redirect_items;
    xlhp_prune_items dead_items;
    xlhp_prune_items unused_items;
    OffsetNumber frz_offsets[MaxHeapTuplesPerPage];
    bool        do_prune = nredirected > 0 || ndead > 0 || nunused > 0;
    bool        do_set_vm = vmflags & VISIBILITYMAP_VALID_BITS;
 
    Assert((vmflags & VISIBILITYMAP_VALID_BITS) == vmflags);
 
    xlrec.flags = 0;
    regbuf_flags_heap = REGBUF_STANDARD;
 
    /*
     * We can avoid an FPI of the heap page if the only modification we are
     * making to it is to set PD_ALL_VISIBLE and checksums/wal_log_hints are
     * disabled. Note that if we explicitly skip an FPI, we must not stamp the
     * heap page with this record's LSN. Recovery skips records <= the stamped
     * LSN, so this could lead to skipping an earlier FPI needed to repair a
     * torn page.
     */
    if (!do_prune &&
        nfrozen == 0 &&
        (!do_set_vm || !XLogHintBitIsNeeded()))
        regbuf_flags_heap |= REGBUF_NO_IMAGE;
 
    /*
     * Prepare data for the buffer.  The arrays are not actually in the
     * buffer, but we pretend that they are.  When XLogInsert stores a full
     * page image, the arrays can be omitted.
     */
    XLogBeginInsert();
    XLogRegisterBuffer(0, buffer, regbuf_flags_heap);
 
    if (do_set_vm)
        XLogRegisterBuffer(1, vmbuffer, 0);
 
    if (nfrozen > 0)
    {
        int         nplans;
 
        xlrec.flags |= XLHP_HAS_FREEZE_PLANS;
 
        /*
         * Prepare deduplicated representation for use in the WAL record. This
         * destructively sorts frozen tuples array in-place.
         */
        nplans = heap_log_freeze_plan(frozen, nfrozen, plans, frz_offsets);
 
        freeze_plans.nplans = nplans;
        XLogRegisterBufData(0, &freeze_plans,
                            offsetof(xlhp_freeze_plans, plans));
        XLogRegisterBufData(0, plans,
                            sizeof(xlhp_freeze_plan) * nplans);
    }
    if (nredirected > 0)
    {
        xlrec.flags |= XLHP_HAS_REDIRECTIONS;
 
        redirect_items.ntargets = nredirected;
        XLogRegisterBufData(0, &redirect_items,
                            offsetof(xlhp_prune_items, data));
        XLogRegisterBufData(0, redirected,
                            sizeof(OffsetNumber[2]) * nredirected);
    }
    if (ndead > 0)
    {
        xlrec.flags |= XLHP_HAS_DEAD_ITEMS;
 
        dead_items.ntargets = ndead;
        XLogRegisterBufData(0, &dead_items,
                            offsetof(xlhp_prune_items, data));
        XLogRegisterBufData(0, dead,
                            sizeof(OffsetNumber) * ndead);
    }
    if (nunused > 0)
    {
        xlrec.flags |= XLHP_HAS_NOW_UNUSED_ITEMS;
 
        unused_items.ntargets = nunused;
        XLogRegisterBufData(0, &unused_items,
                            offsetof(xlhp_prune_items, data));
        XLogRegisterBufData(0, unused,
                            sizeof(OffsetNumber) * nunused);
    }
    if (nfrozen > 0)
        XLogRegisterBufData(0, frz_offsets,
                            sizeof(OffsetNumber) * nfrozen);
 
    /*
     * Prepare the main xl_heap_prune record.  We already set the XLHP_HAS_*
     * flag above.
     */
    if (vmflags & VISIBILITYMAP_ALL_VISIBLE)
    {
        xlrec.flags |= XLHP_VM_ALL_VISIBLE;
        if (vmflags & VISIBILITYMAP_ALL_FROZEN)
            xlrec.flags |= XLHP_VM_ALL_FROZEN;
    }
    if (RelationIsAccessibleInLogicalDecoding(relation))
        xlrec.flags |= XLHP_IS_CATALOG_REL;
    if (TransactionIdIsValid(conflict_xid))
        xlrec.flags |= XLHP_HAS_CONFLICT_HORIZON;
    if (cleanup_lock)
        xlrec.flags |= XLHP_CLEANUP_LOCK;
    else
    {
        Assert(nredirected == 0 && ndead == 0);
        /* also, any items in 'unused' must've been LP_DEAD previously */
    }
    XLogRegisterData(&xlrec, SizeOfHeapPrune);
    if (TransactionIdIsValid(conflict_xid))
        XLogRegisterData(&conflict_xid, sizeof(TransactionId));
 
    switch (reason)
    {
        case PRUNE_ON_ACCESS:
            info = XLOG_HEAP2_PRUNE_ON_ACCESS;
            break;
        case PRUNE_VACUUM_SCAN:
            info = XLOG_HEAP2_PRUNE_VACUUM_SCAN;
            break;
        case PRUNE_VACUUM_CLEANUP:
            info = XLOG_HEAP2_PRUNE_VACUUM_CLEANUP;
            break;
        default:
            elog(ERROR, "unrecognized prune reason: %d", (int) reason);
            break;
    }
    recptr = XLogInsert(RM_HEAP2_ID, info);
 
    if (do_set_vm)
    {
        Assert(BufferIsDirty(vmbuffer));
        PageSetLSN(BufferGetPage(vmbuffer), recptr);
    }
 
    /*
     * See comment at the top of the function about regbuf_flags_heap for
     * details on when we can advance the page LSN.
     */
    if (do_prune || nfrozen > 0 || (do_set_vm && XLogHintBitIsNeeded()))
    {
        Assert(BufferIsDirty(buffer));
        PageSetLSN(BufferGetPage(buffer), recptr);
    }
}