/*-------------------------------------------------------------------------
 *
 * pruneheap.c
 *	  heap page pruning and HOT-chain management code
 *
 * Portions Copyright (c) 1996-2026, 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/visibilitymap.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 "storage/freespace.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;
	/* whether to attempt setting the VM */
	bool		attempt_set_vm;
	struct VacuumCutoffs *cutoffs;
	Relation	relation;

	/*
	 * Keep the buffer, block, and page handy so that helpers needing to
	 * access them don't need to make repeated calls to BufferGetBlockNumber()
	 * and BufferGetPage().
	 */
	BlockNumber block;
	Buffer		buffer;
	Page		page;

	/*-------------------------------------------------------
	 * 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];

	/*
	 * set_all_visible and set_all_frozen indicate if the all-visible and
	 * all-frozen bits in the visibility map can be set for this page after
	 * pruning. They are only tracked when the caller requests VM updates
	 * (attempt_set_vm); otherwise they remain false throughout.
	 *
	 * NOTE: set_all_visible and set_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 opportunistically freeze the page or not.
	 * The set_all_visible and set_all_frozen values ultimately used to set
	 * the VM are adjusted to include LP_DEAD items after we determine whether
	 * or not to opportunistically freeze.
	 */
	bool		set_all_visible;
	bool		set_all_frozen;

	/*-------------------------------------------------------
	 * 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];

	/*-------------------------------------------------------
	 * Working state for freezing
	 *-------------------------------------------------------
	 */
	HeapPageFreeze pagefrz;

	/*-------------------------------------------------------
	 * Working state for visibility map processing
	 *-------------------------------------------------------
	 */

	/*
	 * Caller must provide a pinned vmbuffer corresponding to the heap block
	 * passed to heap_page_prune_and_freeze(). We will fix any corruption
	 * found in the VM and set the VM if the page is all-visible/all-frozen.
	 */
	Buffer		vmbuffer;

	/*
	 * The state of the VM bits at the beginning of pruning and the state they
	 * will be in at the end.
	 */
	uint8		old_vmbits;
	uint8		new_vmbits;

	/* The newest xmin of live tuples on the page */
	TransactionId newest_live_xid;

	/*-------------------------------------------------------
	 * 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 */
} PruneState;

/*
 * Type of visibility map corruption detected on a heap page and its
 * associated VM page. Passed to heap_page_fix_vm_corruption() so the caller
 * can specify what it found rather than having the function rederive the
 * corruption from page state.
 */
typedef enum VMCorruptionType
{
	/* VM bits are set but the heap page-level PD_ALL_VISIBLE flag is not */
	VM_CORRUPT_MISSING_PAGE_HINT,
	/* LP_DEAD line pointers found on a page marked all-visible */
	VM_CORRUPT_LPDEAD,
	/* Tuple not visible to all transactions on a page marked all-visible */
	VM_CORRUPT_TUPLE_VISIBILITY,
} VMCorruptionType;

/* Local functions */
static void prune_freeze_setup(PruneFreezeParams *params,
							   TransactionId *new_relfrozen_xid,
							   MultiXactId *new_relmin_mxid,
							   PruneFreezeResult *presult,
							   PruneState *prstate);
static void heap_page_fix_vm_corruption(PruneState *prstate,
										OffsetNumber offnum,
										VMCorruptionType corruption_type);
static void prune_freeze_fast_path(PruneState *prstate,
								   PruneFreezeResult *presult);
static void prune_freeze_plan(PruneState *prstate,
							  OffsetNumber *off_loc);
static HTSV_Result heap_prune_satisfies_vacuum(PruneState *prstate,
											   HeapTuple tup);
static inline HTSV_Result htsv_get_valid_status(int status);
static void heap_prune_chain(OffsetNumber maxoff,
							 OffsetNumber rootoffnum, PruneState *prstate);
static void heap_prune_record_prunable(PruneState *prstate, TransactionId xid,
									   OffsetNumber offnum);
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(PruneState *prstate, OffsetNumber offnum);
static void heap_prune_record_unchanged_lp_normal(PruneState *prstate, OffsetNumber offnum);
static void heap_prune_record_unchanged_lp_dead(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(bool did_tuple_hint_fpi, bool do_prune, bool do_hint_prune,
								  PruneState *prstate);
static bool heap_page_will_set_vm(PruneState *prstate, PruneReason reason,
								  bool do_prune, bool do_freeze);


/*
 * 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.
 *
 * This function may pin *vmbuffer. It's passed by reference so the caller can
 * reuse the pin across calls, avoiding repeated pin/unpin cycles. If we find
 * VM corruption during pruning, we will fix it. Caller is responsible for
 * unpinning *vmbuffer.
 *
 * rel_read_only is true if we determined at plan time that the query does not
 * modify the relation. It is counterproductive to set the VM if the query
 * will immediately clear it.
 *
 * As noted in ScanRelIsReadOnly(), INSERT ... SELECT from the same table will
 * report the scan relation as read-only. This is usually harmless in
 * practice. It is useful to set scanned pages all-visible that won't be
 * inserted into. Pages it does insert to will rarely meet the criteria for
 * pruning, and those that do are likely to contain in-progress inserts which
 * make the page not fully all-visible.
 */
void
heap_page_prune_opt(Relation relation, Buffer buffer, Buffer *vmbuffer,
					bool rel_read_only)
{
	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 and no
	 * inserts inserted new tuples that may be visible to all).
	 */
	prune_xid = PageGetPruneXid(page);
	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, true))
		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)
	{
		bool		record_free_space = false;
		Size		freespace = 0;

		/* 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;
			PruneFreezeParams params;

			visibilitymap_pin(relation, BufferGetBlockNumber(buffer),
							  vmbuffer);

			params.relation = relation;
			params.buffer = buffer;
			params.vmbuffer = *vmbuffer;
			params.reason = PRUNE_ON_ACCESS;
			params.vistest = vistest;
			params.cutoffs = NULL;

			/*
			 * 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.
			 */
			params.options = HEAP_PAGE_PRUNE_ALLOW_FAST_PATH;
			if (rel_read_only)
				params.options |= HEAP_PAGE_PRUNE_SET_VM;

			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);

			/*
			 * If this prune newly set the page all-visible, VACUUM may later
			 * skip the page and not update the free space map (FSM) for it.
			 * Keep the FSM from going stale by recording it now. We do not
			 * want to update the freespace map otherwise, to reserve
			 * freespace on this page for HOT updates.
			 */
			if (presult.newly_all_visible)
			{
				record_free_space = true;
				freespace = PageGetHeapFreeSpace(page);
			}
		}

		/* And release buffer lock */
		LockBuffer(buffer, BUFFER_LOCK_UNLOCK);

		/*
		 * RecordPageWithFreeSpace() only dirties the FSM when the recorded
		 * free-space category actually changes. Note that vacuum will still
		 * do FreeSpaceMapVacuum() for ranges of pages that are skipped, so we
		 * don't have to worry about that here.
		 */
		if (record_free_space)
			RecordPageWithFreeSpace(relation, BufferGetBlockNumber(buffer), freespace);
	}
}

/*
 * Helper for heap_page_prune_and_freeze() to initialize the PruneState using
 * the provided parameters.
 *
 * params, new_relfrozen_xid, new_relmin_mxid, and presult are input
 * parameters and are not modified by this function. Only prstate is modified.
 */
static void
prune_freeze_setup(PruneFreezeParams *params,
				   TransactionId *new_relfrozen_xid,
				   MultiXactId *new_relmin_mxid,
				   PruneFreezeResult *presult,
				   PruneState *prstate)
{
	/* Copy parameters to prstate */
	prstate->vistest = params->vistest;
	prstate->mark_unused_now =
		(params->options & HEAP_PAGE_PRUNE_MARK_UNUSED_NOW) != 0;

	/* cutoffs must be provided if we will attempt freezing */
	Assert(!(params->options & HEAP_PAGE_PRUNE_FREEZE) || params->cutoffs);
	prstate->attempt_freeze = (params->options & HEAP_PAGE_PRUNE_FREEZE) != 0;
	prstate->attempt_set_vm = (params->options & HEAP_PAGE_PRUNE_SET_VM) != 0;
	prstate->cutoffs = params->cutoffs;
	prstate->relation = params->relation;
	prstate->block = BufferGetBlockNumber(params->buffer);
	prstate->buffer = params->buffer;
	prstate->page = BufferGetPage(params->buffer);

	Assert(BufferIsValid(params->vmbuffer));
	prstate->vmbuffer = params->vmbuffer;
	prstate->new_vmbits = 0;
	prstate->old_vmbits = visibilitymap_get_status(prstate->relation,
												   prstate->block,
												   &prstate->vmbuffer);

	/*
	 * 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 = 0;
	prstate->nfrozen = 0;
	prstate->nroot_items = 0;
	prstate->nheaponly_items = 0;

	/* initialize page freezing working state */
	prstate->pagefrz.freeze_required = false;
	prstate->pagefrz.FreezePageConflictXid = InvalidTransactionId;
	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 && !new_relmin_mxid);
		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;

	/*
	 * deadoffsets are filled in during pruning but are only used to populate
	 * PruneFreezeResult->deadoffsets. To avoid needing two copies of the
	 * array, just save a pointer to the result offsets array in the
	 * PruneState.
	 */
	prstate->deadoffsets = presult->deadoffsets;

	/*
	 * We track whether the page will be all-visible/all-frozen at the end of
	 * pruning and freezing. While examining tuple visibility, we'll set
	 * set_all_visible to false if there are tuples on the page not visible to
	 * all running and future transactions. If setting the VM is enabled for
	 * this scan, we will do so if the page ends up being all-visible.
	 *
	 * We also keep track of the newest live XID, which is used to calculate
	 * the snapshot conflict horizon for a WAL record setting the VM.
	 */
	prstate->set_all_visible = prstate->attempt_set_vm;
	prstate->newest_live_xid = InvalidTransactionId;

	/*
	 * Currently, only VACUUM performs freezing, but other callers may in the
	 * future. We must initialize set_all_frozen based on whether or not the
	 * caller passed HEAP_PAGE_PRUNE_FREEZE, because if they did not, we won't
	 * call heap_prepare_freeze_tuple() for each tuple, and set_all_frozen
	 * will never be cleared for tuples that need freezing. This would lead to
	 * incorrectly setting the visibility map all-frozen for this page. We
	 * can't set the page all-frozen in the VM if the caller didn't pass
	 * HEAP_PAGE_PRUNE_SET_VM.
	 *
	 * When freezing is not required (no XIDs/MXIDs older than the freeze
	 * cutoff), we may still choose to "opportunistically" freeze if doing so
	 * would make the page all-frozen.
	 *
	 * We will not be able to freeze the whole page at the end of vacuum if
	 * there are tuples present that are not visible to everyone or if there
	 * are dead tuples which will not be removable. However, dead tuples that
	 * will be removed by the end of vacuum should not prevent this
	 * opportunistic freezing.
	 *
	 * Therefore, we do not clear set_all_visible and set_all_frozen when we
	 * encounter LP_DEAD items. Instead, we correct them after deciding
	 * whether to freeze, but before updating the VM, to avoid setting the VM
	 * bits incorrectly.
	 */
	prstate->set_all_frozen = prstate->attempt_freeze && prstate->attempt_set_vm;
}

/*
 * Helper for heap_page_prune_and_freeze(). Iterates over every tuple on the
 * page, examines its visibility information, and determines the appropriate
 * action for each tuple. All tuples are processed and classified during this
 * phase, but no modifications are made to the page until the later execution
 * stage.
 *
 * *off_loc is used for error callback and cleared before returning.
 */
static void
prune_freeze_plan(PruneState *prstate, OffsetNumber *off_loc)
{
	Page		page = prstate->page;
	BlockNumber blockno = prstate->block;
	OffsetNumber maxoff = PageGetMaxOffsetNumber(prstate->page);
	OffsetNumber offnum;
	HeapTupleData tup;

	tup.t_tableOid = RelationGetRelid(prstate->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(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(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);

		if (!HeapTupleHeaderIsHeapOnly(htup))
			prstate->root_items[prstate->nroot_items++] = offnum;
		else
			prstate->heaponly_items[prstate->nheaponly_items++] = offnum;
	}

	/*
	 * 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(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(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;
}

/*
 * Decide whether to proceed with freezing according to the freeze plans
 * prepared for the current 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(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->set_all_frozen && prstate->nfrozen == 0);
		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->set_all_frozen && prstate->nfrozen > 0)
		{
			Assert(prstate->set_all_visible);

			/*
			 * Freezing would make the page all-frozen.  Have already emitted
			 * an FPI or will do so anyway?
			 */
			if (RelationNeedsWAL(prstate->relation))
			{
				if (did_tuple_hint_fpi)
					do_freeze = true;
				else if (do_prune)
				{
					if (XLogCheckBufferNeedsBackup(prstate->buffer))
						do_freeze = true;
				}
				else if (do_hint_prune)
				{
					if (XLogHintBitIsNeeded() &&
						XLogCheckBufferNeedsBackup(prstate->buffer))
						do_freeze = true;
				}
			}
		}
	}

	if (do_freeze)
	{
		/*
		 * Validate the tuples we will be freezing before entering the
		 * critical section.
		 */
		heap_pre_freeze_checks(prstate->buffer, prstate->frozen, prstate->nfrozen);
		Assert(TransactionIdPrecedes(prstate->pagefrz.FreezePageConflictXid,
									 prstate->cutoffs->OldestXmin));
	}
	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->set_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;
}

/*
 * Emit a warning about and fix visibility map corruption on the given page.
 *
 * The caller specifies the type of corruption it has already detected via
 * corruption_type, so that we can emit the appropriate warning. All cases
 * result in the VM bits being cleared; corruption types where PD_ALL_VISIBLE
 * is incorrectly set also clear PD_ALL_VISIBLE.
 *
 * Must be called while holding an exclusive lock on the heap buffer. Dead
 * items and not all-visible tuples must have been discovered under that same
 * lock. Although we do not hold a lock on the VM buffer, it is pinned, and
 * the heap buffer is exclusively locked, ensuring that no other backend can
 * update the VM bits corresponding to this heap page.
 *
 * This function makes changes to the VM and, potentially, the heap page, but
 * it does not need to be done in a critical section.
 */
static void
heap_page_fix_vm_corruption(PruneState *prstate, OffsetNumber offnum,
							VMCorruptionType corruption_type)
{
	const char *relname = RelationGetRelationName(prstate->relation);
	bool		do_clear_vm = false;
	bool		do_clear_heap = false;

	Assert(BufferIsLockedByMeInMode(prstate->buffer, BUFFER_LOCK_EXCLUSIVE));

	switch (corruption_type)
	{
		case VM_CORRUPT_LPDEAD:
			ereport(WARNING,
					(errcode(ERRCODE_DATA_CORRUPTED),
					 errmsg("dead line pointer found on page marked all-visible"),
					 errcontext("relation \"%s\", page %u, tuple %u",
								relname, prstate->block, offnum)));
			do_clear_vm = true;
			do_clear_heap = true;
			break;

		case VM_CORRUPT_TUPLE_VISIBILITY:

			/*
			 * A HEAPTUPLE_LIVE tuple on an all-visible page can appear to not
			 * be visible to everyone when
			 * GetOldestNonRemovableTransactionId() returns a conservative
			 * value that's older than the real safe xmin. That is not
			 * corruption -- the PD_ALL_VISIBLE flag is still correct.
			 *
			 * However, dead tuple versions, in-progress inserts, and
			 * in-progress deletes should never appear on a page marked
			 * all-visible. That indicates real corruption. PD_ALL_VISIBLE
			 * should have been cleared by the DML operation that deleted or
			 * inserted the tuple.
			 */
			ereport(WARNING,
					(errcode(ERRCODE_DATA_CORRUPTED),
					 errmsg("tuple not visible to all transactions found on page marked all-visible"),
					 errcontext("relation \"%s\", page %u, tuple %u",
								relname, prstate->block, offnum)));
			do_clear_vm = true;
			do_clear_heap = true;
			break;

		case VM_CORRUPT_MISSING_PAGE_HINT:

			/*
			 * As of PostgreSQL 9.2, the visibility map bit should never be
			 * set if the page-level bit is clear. However, for vacuum, it's
			 * possible that the bit got cleared after
			 * heap_vac_scan_next_block() was called, so we must recheck now
			 * that we have the buffer lock before concluding that the VM is
			 * corrupt.
			 */
			Assert(!PageIsAllVisible(prstate->page));
			Assert(prstate->old_vmbits & VISIBILITYMAP_VALID_BITS);
			ereport(WARNING,
					(errcode(ERRCODE_DATA_CORRUPTED),
					 errmsg("page is not marked all-visible but visibility map bit is set"),
					 errcontext("relation \"%s\", page %u",
								relname, prstate->block)));
			do_clear_vm = true;
			break;
	}

	Assert(do_clear_heap || do_clear_vm);

	/* Avoid marking the buffer dirty if PD_ALL_VISIBLE is already clear */
	if (do_clear_heap)
	{
		Assert(PageIsAllVisible(prstate->page));
		PageClearAllVisible(prstate->page);
		MarkBufferDirtyHint(prstate->buffer, true);
	}

	if (do_clear_vm)
	{
		LockBuffer(prstate->vmbuffer, BUFFER_LOCK_EXCLUSIVE);
		/* This VM clear is not WAL-logged, so its return value is not needed. */
		(void) visibilitymap_clear(prstate->relation, prstate->block,
								   prstate->vmbuffer,
								   VISIBILITYMAP_VALID_BITS);
		LockBuffer(prstate->vmbuffer, BUFFER_LOCK_UNLOCK);
		prstate->old_vmbits = 0;
	}
}

/*
 * Decide whether to set the visibility map bits (all-visible and all-frozen)
 * for the current page using information from the PruneState and VM.
 *
 * This function does not actually set the VM bits or page-level visibility
 * hint, PD_ALL_VISIBLE.
 *
 * This should be called only after do_freeze has been decided (and do_prune
 * has been set), as these factor into our heuristic-based decision.
 *
 * Returns true if one or both VM bits should be set and false otherwise.
 */
static bool
heap_page_will_set_vm(PruneState *prstate, PruneReason reason,
					  bool do_prune, bool do_freeze)
{
	if (!prstate->attempt_set_vm)
		return false;

	if (!prstate->set_all_visible)
		return false;

	/*
	 * If this is an on-access call and we're not actually pruning, avoid
	 * setting the visibility map if it would newly dirty the heap page or, if
	 * the page is already dirty, if doing so would require including a
	 * full-page image (FPI) of the heap page in the WAL.
	 */
	if (reason == PRUNE_ON_ACCESS && !do_prune && !do_freeze &&
		(!BufferIsDirty(prstate->buffer) || XLogCheckBufferNeedsBackup(prstate->buffer)))
	{
		prstate->set_all_visible = prstate->set_all_frozen = false;
		return false;
	}

	prstate->new_vmbits = VISIBILITYMAP_ALL_VISIBLE;

	if (prstate->set_all_frozen)
		prstate->new_vmbits |= VISIBILITYMAP_ALL_FROZEN;

	if (prstate->new_vmbits == prstate->old_vmbits)
	{
		prstate->new_vmbits = 0;
		return false;
	}

	return true;
}

/*
 * If the page is already all-frozen, or already all-visible and freezing
 * won't be attempted, there is no remaining work and we can use the fast path
 * to avoid the expensive overhead of heap_page_prune_and_freeze().
 *
 * This can happen when the page has a stale prune hint, or if VACUUM is
 * scanning an already all-frozen page due to SKIP_PAGES_THRESHOLD.
 *
 * The caller must already have examined the visibility map and saved the
 * status of the page's VM bits in prstate->old_vmbits. Caller must hold a
 * content lock on the heap page since it will examine line pointers.
 *
 * Before calling prune_freeze_fast_path(), the caller should first
 * check for and fix any discrepancy between the page-level visibility hint
 * and the visibility map. Otherwise, the fast path will always prevent us
 * from getting them in sync. Note that if there are tuples on the page that
 * are not visible to all but the VM is incorrectly marked
 * all-visible/all-frozen, we will not get the chance to fix that corruption
 * when using the fast path.
 */
static void
prune_freeze_fast_path(PruneState *prstate, PruneFreezeResult *presult)
{
	OffsetNumber maxoff = PageGetMaxOffsetNumber(prstate->page);
	Page		page = prstate->page;

	Assert((prstate->old_vmbits & VISIBILITYMAP_ALL_FROZEN) ||
		   ((prstate->old_vmbits & VISIBILITYMAP_ALL_VISIBLE) &&
			!prstate->attempt_freeze));

	/* We'll fill in presult for the caller */
	memset(presult, 0, sizeof(PruneFreezeResult));

	/* Clear any stale prune hint */
	if (TransactionIdIsValid(PageGetPruneXid(page)))
	{
		PageClearPrunable(page);
		MarkBufferDirtyHint(prstate->buffer, true);
	}

	if (PageIsEmpty(page))
		return;

	/*
	 * Since the page is all-visible, a count of the normal ItemIds on the
	 * page should be sufficient for vacuum's live tuple count.
	 */
	for (OffsetNumber off = FirstOffsetNumber;
		 off <= maxoff;
		 off = OffsetNumberNext(off))
	{
		ItemId		lp = PageGetItemId(page, off);

		if (!ItemIdIsUsed(lp))
			continue;

		presult->hastup = true;

		if (ItemIdIsNormal(lp))
			prstate->live_tuples++;
	}

	presult->live_tuples = prstate->live_tuples;
}

/*
 * Prune and repair fragmentation and potentially freeze tuples on the
 * specified page. If the page's visibility status has changed, update it in
 * the VM.
 *
 * 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.
 *
 * A vmbuffer corresponding to the heap page is also passed and if the page is
 * found to be all-visible/all-frozen, we will set it in the VM.
 *
 * 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 be 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 the 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)
{
	PruneState	prstate;
	bool		do_freeze;
	bool		do_prune;
	bool		do_hint_prune;
	bool		do_set_vm;
	bool		did_tuple_hint_fpi;
	int64		fpi_before = pgWalUsage.wal_fpi;
	TransactionId conflict_xid;

	/* Initialize prstate */
	prune_freeze_setup(params,
					   new_relfrozen_xid, new_relmin_mxid,
					   presult, &prstate);

	/*
	 * If the VM is set but PD_ALL_VISIBLE is clear, fix that corruption
	 * before pruning and freezing so that the page and VM start out in a
	 * consistent state.
	 */
	if ((prstate.old_vmbits & VISIBILITYMAP_VALID_BITS) &&
		!PageIsAllVisible(prstate.page))
		heap_page_fix_vm_corruption(&prstate, InvalidOffsetNumber,
									VM_CORRUPT_MISSING_PAGE_HINT);

	/*
	 * If the page is already all-frozen, or already all-visible when freezing
	 * is not being attempted, take the fast path, skipping pruning and
	 * freezing code entirely. This must be done after fixing any discrepancy
	 * between the page-level visibility hint and the VM, since that may have
	 * cleared old_vmbits.
	 */
	if ((params->options & HEAP_PAGE_PRUNE_ALLOW_FAST_PATH) != 0 &&
		((prstate.old_vmbits & VISIBILITYMAP_ALL_FROZEN) ||
		 ((prstate.old_vmbits & VISIBILITYMAP_ALL_VISIBLE) &&
		  !prstate.attempt_freeze)))
	{
		prune_freeze_fast_path(&prstate, presult);
		return;
	}

	/*
	 * Examine all line pointers and tuple visibility information to determine
	 * which line pointers should change state and which tuples may be frozen.
	 * Prepare queue of state changes to later be executed in a critical
	 * section.
	 */
	prune_freeze_plan(&prstate, off_loc);

	/*
	 * After processing all the live tuples on the page, if the newest xmin
	 * amongst them may be considered running by any snapshot, the page cannot
	 * be all-visible. This should be done before determining whether or not
	 * to opportunistically freeze.
	 */
	if (prstate.set_all_visible &&
		TransactionIdIsNormal(prstate.newest_live_xid) &&
		GlobalVisTestXidConsideredRunning(prstate.vistest,
										  prstate.newest_live_xid,
										  true))
		prstate.set_all_visible = prstate.set_all_frozen = false;

	/*
	 * If checksums are enabled, calling heap_prune_satisfies_vacuum() while
	 * checking tuple visibility information in prune_freeze_plan() may have
	 * caused an FPI to be emitted.
	 */
	did_tuple_hint_fpi = fpi_before != pgWalUsage.wal_fpi;

	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 = PageGetPruneXid(prstate.page) != prstate.new_prune_xid ||
		PageIsFull(prstate.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(did_tuple_hint_fpi,
									  do_prune,
									  do_hint_prune,
									  &prstate);

	/*
	 * While scanning the line pointers, we did not clear
	 * set_all_visible/set_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 set_all_visible and set_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.set_all_visible = prstate.set_all_frozen = false;

	Assert(!prstate.set_all_frozen || prstate.set_all_visible);
	Assert(!prstate.set_all_visible || prstate.attempt_set_vm);
	Assert(!prstate.set_all_visible || (prstate.lpdead_items == 0));

	do_set_vm = heap_page_will_set_vm(&prstate, params->reason, do_prune, do_freeze);

	/*
	 * new_vmbits should be 0 regardless of whether or not the page is
	 * all-visible if we do not intend to set the VM.
	 */
	Assert(do_set_vm || prstate.new_vmbits == 0);

	/*
	 * The snapshot conflict horizon for the whole record is the most
	 * conservative (newest) horizon required by any change in the record.
	 */
	conflict_xid = InvalidTransactionId;
	if (do_set_vm)
		conflict_xid = prstate.newest_live_xid;
	if (do_freeze && TransactionIdFollows(prstate.pagefrz.FreezePageConflictXid, conflict_xid))
		conflict_xid = prstate.pagefrz.FreezePageConflictXid;
	if (do_prune && TransactionIdFollows(prstate.latest_xid_removed, conflict_xid))
		conflict_xid = prstate.latest_xid_removed;

	/* Lock vmbuffer before entering a critical section */
	if (do_set_vm)
		LockBuffer(prstate.vmbuffer, BUFFER_LOCK_EXCLUSIVE);

	/* 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) prstate.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(prstate.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 or set
		 * PD_ALL_VISIBLE, we will mark the buffer dirty below.
		 *
		 * Setting PD_ALL_VISIBLE is fully WAL-logged because it is forbidden
		 * for the VM to be set and PD_ALL_VISIBLE to be clear.
		 */
		if (!do_freeze && !do_prune && !do_set_vm)
			MarkBufferDirtyHint(prstate.buffer, true);
	}

	if (do_prune || do_freeze || do_set_vm)
	{
		/* Apply the planned item changes and repair page fragmentation. */
		if (do_prune)
		{
			heap_page_prune_execute(prstate.buffer, false,
									prstate.redirected, prstate.nredirected,
									prstate.nowdead, prstate.ndead,
									prstate.nowunused, prstate.nunused);
		}

		if (do_freeze)
			heap_freeze_prepared_tuples(prstate.buffer, prstate.frozen, prstate.nfrozen);

		/* Set the visibility map and page visibility hint */
		if (do_set_vm)
		{
			/*
			 * While it is valid for PD_ALL_VISIBLE to be set when the
			 * corresponding VM bit is clear, we strongly prefer to keep them
			 * in sync.
			 *
			 * The heap buffer must be marked dirty before adding it to the
			 * WAL chain when setting the VM. We don't worry about
			 * unnecessarily dirtying the heap buffer if PD_ALL_VISIBLE is
			 * already set, though. It is extremely rare to have a clean heap
			 * buffer with PD_ALL_VISIBLE already set and the VM bits clear,
			 * so there is no point in optimizing it.
			 */
			PageSetAllVisible(prstate.page);
			PageClearPrunable(prstate.page);
			visibilitymap_set(prstate.block, prstate.vmbuffer, prstate.new_vmbits,
							  prstate.relation->rd_locator);
		}

		MarkBufferDirty(prstate.buffer);

		/*
		 * Emit a WAL XLOG_HEAP2_PRUNE* record showing what we did
		 */
		if (RelationNeedsWAL(prstate.relation))
		{
			log_heap_prune_and_freeze(prstate.relation, prstate.buffer,
									  do_set_vm ? prstate.vmbuffer : InvalidBuffer,
									  do_set_vm ? prstate.new_vmbits : 0,
									  conflict_xid,
									  true, /* cleanup lock */
									  params->reason,
									  prstate.frozen, prstate.nfrozen,
									  prstate.redirected, prstate.nredirected,
									  prstate.nowdead, prstate.ndead,
									  prstate.nowunused, prstate.nunused);
		}
	}

	END_CRIT_SECTION();

	if (do_set_vm)
		LockBuffer(prstate.vmbuffer, BUFFER_LOCK_UNLOCK);

	/*
	 * During its second pass over the heap, VACUUM calls
	 * heap_page_would_be_all_visible() to determine whether a page is
	 * all-visible and all-frozen. The logic here is similar. After completing
	 * pruning and freezing, use an assertion to verify that our results
	 * remain consistent with heap_page_would_be_all_visible(). It's also a
	 * valuable cross-check of the page state after pruning and freezing.
	 */
#ifdef USE_ASSERT_CHECKING
	if (prstate.set_all_visible)
	{
		TransactionId debug_cutoff;
		bool		debug_all_frozen;

		Assert(prstate.lpdead_items == 0);

		Assert(heap_page_is_all_visible(prstate.relation, prstate.buffer,
										prstate.vistest,
										&debug_all_frozen,
										&debug_cutoff, off_loc));

		Assert(!TransactionIdIsValid(debug_cutoff) ||
			   debug_cutoff == prstate.newest_live_xid);

		/*
		 * It's possible the page is composed entirely of frozen tuples but is
		 * not set all-frozen in the VM and did not pass
		 * HEAP_PAGE_PRUNE_FREEZE. In this case, it's possible
		 * heap_page_is_all_visible() finds the page completely frozen, even
		 * though prstate.set_all_frozen is false.
		 */
		Assert(!prstate.set_all_frozen || debug_all_frozen);
	}
#endif

	/* 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->hastup = prstate.hastup;

	presult->lpdead_items = prstate.lpdead_items;
	/* the presult->deadoffsets array was already filled in */

	presult->newly_all_visible = false;
	presult->newly_all_frozen = false;
	presult->newly_all_visible_frozen = false;
	if (do_set_vm)
	{
		if ((prstate.old_vmbits & VISIBILITYMAP_ALL_VISIBLE) == 0)
		{
			presult->newly_all_visible = true;
			if (prstate.set_all_frozen)
				presult->newly_all_visible_frozen = true;
		}
		else if ((prstate.old_vmbits & VISIBILITYMAP_ALL_FROZEN) == 0 &&
				 prstate.set_all_frozen)
			presult->newly_all_frozen = true;
	}

	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)
{
	HTSV_Result res;
	TransactionId dead_after;

	res = HeapTupleSatisfiesVacuumHorizon(tup, prstate->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, true))
		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(OffsetNumber maxoff, OffsetNumber rootoffnum,
				 PruneState *prstate)
{
	TransactionId priorXmax = InvalidTransactionId;
	ItemId		rootlp;
	OffsetNumber offnum;
	OffsetNumber chainitems[MaxHeapTuplesPerPage];
	Page		page = prstate->page;

	/*
	 * 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) == prstate->block);
		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(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(prstate, chainitems[i]);
	}
}

/* Record lowest soon-prunable XID */
static void
heap_prune_record_prunable(PruneState *prstate, TransactionId xid,
						   OffsetNumber offnum)
{
	/*
	 * 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;

	/*
	 * It's incorrect for a page to be marked all-visible if it contains
	 * prunable items.
	 */
	if (PageIsAllVisible(prstate->page))
		heap_page_fix_vm_corruption(prstate, offnum,
									VM_CORRUPT_TUPLE_VISIBILITY);
}

/* 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 set_all_visible and set_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);

	/*
	 * It's incorrect for the page to be set all-visible if it contains dead
	 * items. Fix that on the heap page and check the VM for corruption as
	 * well. Do that here rather than in heap_prune_record_dead() so we also
	 * cover tuples that are directly marked LP_UNUSED via mark_unused_now.
	 */
	if (PageIsAllVisible(prstate->page))
		heap_page_fix_vm_corruption(prstate, offnum, VM_CORRUPT_LPDEAD);
}

/* 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(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(PruneState *prstate, OffsetNumber offnum)
{
	HeapTupleHeader htup;
	TransactionId xmin;
	Page		page = prstate->page;

	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 (!HeapTupleHeaderXminCommitted(htup))
			{
				prstate->set_all_visible = false;
				prstate->set_all_frozen = false;
				break;
			}

			/*
			 * The inserter definitely committed. But we don't know if it is
			 * old enough that everyone sees it as committed. Later, after
			 * processing all the tuples on the page, we'll check if there is
			 * any snapshot that still considers the newest xid on the page to
			 * be running. If so, we don't consider the page all-visible.
			 */
			xmin = HeapTupleHeaderGetXmin(htup);

			/* Track newest xmin on page. */
			if (TransactionIdFollows(xmin, prstate->newest_live_xid) &&
				TransactionIdIsNormal(xmin))
				prstate->newest_live_xid = xmin;

			break;

		case HEAPTUPLE_RECENTLY_DEAD:
			prstate->recently_dead_tuples++;
			prstate->set_all_visible = false;
			prstate->set_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),
									   offnum);
			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->set_all_visible = false;
			prstate->set_all_frozen = false;

			/*
			 * Though there is nothing "prunable" on the page, we maintain
			 * pd_prune_xid for inserts so that we have the opportunity to
			 * mark them all-visible during the next round of pruning.
			 */
			heap_prune_record_prunable(prstate,
									   HeapTupleHeaderGetXmin(htup),
									   offnum);
			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->set_all_visible = false;
			prstate->set_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),
									   offnum);
			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->set_all_frozen = false;
	}
}


/*
 * Record line pointer that was already LP_DEAD and is left unchanged.
 */
static void
heap_prune_record_unchanged_lp_dead(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 set_all_visible and set_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;

	/*
	 * It's incorrect for a page to be marked all-visible if it contains dead
	 * items.
	 */
	if (PageIsAllVisible(prstate->page))
		heap_page_fix_vm_corruption(prstate, offnum, VM_CORRUPT_LPDEAD);
}

/*
 * 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 (nextoffnum < FirstOffsetNumber)
				break;

			/*
			 * An offset past the end of page's line pointer array is possible
			 * when the array was truncated
			 */
			if (nextoffnum > 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)
{
	const HeapTupleFreeze *frz1 = arg1;
	const HeapTupleFreeze *frz2 = 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;

	Page		heap_page = BufferGetPage(buffer);

	/* 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;
	bool		heap_fpi_allowed = true;

	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.
	 *
	 * However, if the page has never been WAL-logged (LSN is invalid), we
	 * must force an FPI regardless.  This can happen when another backend
	 * extends the heap, initializes the page, and then fails before WAL-
	 * logging it.  Since heap extension is not WAL-logged, recovery might try
	 * to replay our record and find that the page isn't initialized, which
	 * would cause a PANIC.
	 */
	if (!XLogRecPtrIsValid(PageGetLSN(heap_page)))
		regbuf_flags_heap |= REGBUF_FORCE_IMAGE;
	else if (!do_prune && nfrozen == 0 && (!do_set_vm || !XLogHintBitIsNeeded()))
	{
		regbuf_flags_heap |= REGBUF_NO_IMAGE;
		heap_fpi_allowed = false;
	}

	/*
	 * 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);
	}

	/*
	 * 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 (heap_fpi_allowed)
	{
		Assert(BufferIsDirty(buffer));
		PageSetLSN(heap_page, recptr);
	}
}
