postgres/src/backend/access/heap/pruneheap.c

/*-------------------------------------------------------------------------
 *
 * pruneheap.c
 *	  heap page pruning and HOT-chain management code
 *
 * Portions Copyright (c) 1996-2019, 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/transam.h"
#include "access/htup_details.h"
#include "access/xlog.h"
#include "catalog/catalog.h"
#include "miscadmin.h"
#include "pgstat.h"
#include "storage/bufmgr.h"
#include "utils/snapmgr.h"
#include "utils/rel.h"

/* Working data for heap_page_prune and subroutines */
typedef struct
{
	TransactionId new_prune_xid;	/* new prune hint value for page */
	TransactionId latestRemovedXid; /* latest xid to be removed by this prune */
	int			nredirected;	/* numbers of entries in arrays below */
	int			ndead;
	int			nunused;
	/* arrays that accumulate indexes of items to be changed */
	OffsetNumber redirected[MaxHeapTuplesPerPage * 2];
	OffsetNumber nowdead[MaxHeapTuplesPerPage];
	OffsetNumber nowunused[MaxHeapTuplesPerPage];
	/* marked[i] is true if item i is entered in one of the above arrays */
	bool		marked[MaxHeapTuplesPerPage + 1];
} PruneState;

/* Local functions */
static int	heap_prune_chain(Relation relation, Buffer buffer,
							 OffsetNumber rootoffnum,
							 TransactionId OldestXmin,
							 PruneState *prstate);
static void heap_prune_record_prunable(PruneState *prstate, TransactionId xid);
static void heap_prune_record_redirect(PruneState *prstate,
									   OffsetNumber offnum, OffsetNumber rdoffnum);
static void heap_prune_record_dead(PruneState *prstate, OffsetNumber offnum);
static void heap_prune_record_unused(PruneState *prstate, OffsetNumber offnum);


/*
 * 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.
 *
 * OldestXmin is the cutoff XID used to distinguish whether tuples are DEAD
 * or RECENTLY_DEAD (see HeapTupleSatisfiesVacuum).
 */
void
heap_page_prune_opt(Relation relation, Buffer buffer)
{
	Page		page = BufferGetPage(buffer);
	Size		minfree;
	TransactionId OldestXmin;

	/*
	 * We can't write WAL in recovery mode, so there's no point trying to
	 * clean the page. The master will likely issue a cleaning WAL record soon
	 * anyway, so this is no particular loss.
	 */
	if (RecoveryInProgress())
		return;

	/*
	 * Use the appropriate xmin horizon for this relation. If it's a proper
	 * catalog relation or a user defined, additional, catalog relation, we
	 * need to use the horizon that includes slots, otherwise the data-only
	 * horizon can be used. Note that the toast relation of user defined
	 * relations are *not* considered catalog relations.
	 *
	 * It is OK to apply the old snapshot limit before acquiring the cleanup
	 * lock because the worst that can happen is that we are not quite as
	 * aggressive about the cleanup (by however many transaction IDs are
	 * consumed between this point and acquiring the lock).  This allows us to
	 * save significant overhead in the case where the page is found not to be
	 * prunable.
	 */
	if (IsCatalogRelation(relation) ||
		RelationIsAccessibleInLogicalDecoding(relation))
		OldestXmin = RecentGlobalXmin;
	else
		OldestXmin =
			TransactionIdLimitedForOldSnapshots(RecentGlobalDataXmin,
												relation);

	Assert(TransactionIdIsValid(OldestXmin));

	/*
	 * Let's see if we really need pruning.
	 *
	 * Forget it if page is not hinted to contain something prunable that's
	 * older than OldestXmin.
	 */
	if (!PageIsPrunable(page, OldestXmin))
		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. (We needn't recheck PageIsPrunable, since no one else could
		 * have pruned while we hold pin.)
		 */
		if (PageIsFull(page) || PageGetHeapFreeSpace(page) < minfree)
		{
			TransactionId ignore = InvalidTransactionId;	/* return value not
															 * needed */

			/* OK to prune */
			(void) heap_page_prune(relation, buffer, OldestXmin, true, &ignore);
		}

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


/*
 * Prune and repair fragmentation in the specified page.
 *
 * Caller must have pin and buffer cleanup lock on the page.
 *
 * OldestXmin is the cutoff XID used to distinguish whether tuples are DEAD
 * or RECENTLY_DEAD (see HeapTupleSatisfiesVacuum).
 *
 * If report_stats is true then we send the number of reclaimed heap-only
 * tuples to pgstats.  (This must be false during vacuum, since vacuum will
 * send its own new total to pgstats, and we don't want this delta applied
 * on top of that.)
 *
 * Returns the number of tuples deleted from the page and sets
 * latestRemovedXid.
 */
int
heap_page_prune(Relation relation, Buffer buffer, TransactionId OldestXmin,
				bool report_stats, TransactionId *latestRemovedXid)
{
	int			ndeleted = 0;
	Page		page = BufferGetPage(buffer);
	OffsetNumber offnum,
				maxoff;
	PruneState	prstate;

	/*
	 * 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.latestRemovedXid = *latestRemovedXid;
	prstate.nredirected = prstate.ndead = prstate.nunused = 0;
	memset(prstate.marked, 0, sizeof(prstate.marked));

	/* Scan the page */
	maxoff = PageGetMaxOffsetNumber(page);
	for (offnum = FirstOffsetNumber;
		 offnum <= maxoff;
		 offnum = OffsetNumberNext(offnum))
	{
		ItemId		itemid;

		/* Ignore items already processed as part of an earlier chain */
		if (prstate.marked[offnum])
			continue;

		/* Nothing to do if slot is empty or already dead */
		itemid = PageGetItemId(page, offnum);
		if (!ItemIdIsUsed(itemid) || ItemIdIsDead(itemid))
			continue;

		/* Process this item or chain of items */
		ndeleted += heap_prune_chain(relation, buffer, offnum,
									 OldestXmin,
									 &prstate);
	}

	/* Any error while applying the changes is critical */
	START_CRIT_SECTION();

	/* Have we found any prunable items? */
	if (prstate.nredirected > 0 || prstate.ndead > 0 || prstate.nunused > 0)
	{
		/*
		 * Apply the planned item changes, then repair page fragmentation, and
		 * update the page's hint bit about whether it has free line pointers.
		 */
		heap_page_prune_execute(buffer,
								prstate.redirected, prstate.nredirected,
								prstate.nowdead, prstate.ndead,
								prstate.nowunused, prstate.nunused);

		/*
		 * 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);

		MarkBufferDirty(buffer);

		/*
		 * Emit a WAL XLOG_HEAP2_CLEAN record showing what we did
		 */
		if (RelationNeedsWAL(relation))
		{
			XLogRecPtr	recptr;

			recptr = log_heap_clean(relation, buffer,
									prstate.redirected, prstate.nredirected,
									prstate.nowdead, prstate.ndead,
									prstate.nowunused, prstate.nunused,
									prstate.latestRemovedXid);

			PageSetLSN(BufferGetPage(buffer), recptr);
		}
	}
	else
	{
		/*
		 * If we didn't prune anything, but have found a new value for the
		 * pd_prune_xid field, update it and mark the buffer dirty. This is
		 * treated as a non-WAL-logged hint.
		 *
		 * Also clear the "page is full" flag if it is set, since there's no
		 * point in repeating the prune/defrag process until something else
		 * happens to the page.
		 */
		if (((PageHeader) page)->pd_prune_xid != prstate.new_prune_xid ||
			PageIsFull(page))
		{
			((PageHeader) page)->pd_prune_xid = prstate.new_prune_xid;
			PageClearFull(page);
			MarkBufferDirtyHint(buffer, true);
		}
	}

	END_CRIT_SECTION();

	/*
	 * If requested, report the number of tuples reclaimed to pgstats. This is
	 * ndeleted minus ndead, because we don't want to count a now-DEAD root
	 * item as a deletion for this purpose.
	 */
	if (report_stats && ndeleted > prstate.ndead)
		pgstat_update_heap_dead_tuples(relation, ndeleted - prstate.ndead);

	*latestRemovedXid = prstate.latestRemovedXid;

	/*
	 * XXX Should we update the FSM information of this page ?
	 *
	 * There are two schools of thought here. We may not want to update FSM
	 * information so that the page is not used for unrelated UPDATEs/INSERTs
	 * and any free space in this page will remain available for further
	 * UPDATEs in *this* page, thus improving chances for doing HOT updates.
	 *
	 * But for a large table and where a page does not receive further UPDATEs
	 * for a long time, we might waste this space by not updating the FSM
	 * information. The relation may get extended and fragmented further.
	 *
	 * One possibility is to leave "fillfactor" worth of space in this page
	 * and update FSM with the remaining space.
	 */

	return ndeleted;
}


/*
 * Prune specified line pointer or a HOT chain originating at line pointer.
 *
 * 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, the OldestXmin test is just too coarse to detect it.
 *
 * 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.)
 *
 * OldestXmin is the cutoff XID used to identify dead tuples.
 *
 * We don't actually change the page here, except perhaps for hint-bit updates
 * caused by HeapTupleSatisfiesVacuum.  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[].
 *
 * Returns the number of tuples (to be) deleted from the page.
 */
static int
heap_prune_chain(Relation relation, Buffer buffer, OffsetNumber rootoffnum,
				 TransactionId OldestXmin,
				 PruneState *prstate)
{
	int			ndeleted = 0;
	Page		dp = (Page) BufferGetPage(buffer);
	TransactionId priorXmax = InvalidTransactionId;
	ItemId		rootlp;
	HeapTupleHeader htup;
	OffsetNumber latestdead = InvalidOffsetNumber,
				maxoff = PageGetMaxOffsetNumber(dp),
				offnum;
	OffsetNumber chainitems[MaxHeapTuplesPerPage];
	int			nchain = 0,
				i;
	HeapTupleData tup;

	tup.t_tableOid = RelationGetRelid(relation);

	rootlp = PageGetItemId(dp, rootoffnum);

	/*
	 * If it's a heap-only tuple, then it is not the start of a HOT chain.
	 */
	if (ItemIdIsNormal(rootlp))
	{
		htup = (HeapTupleHeader) PageGetItem(dp, rootlp);

		tup.t_data = htup;
		tup.t_len = ItemIdGetLength(rootlp);
		ItemPointerSet(&(tup.t_self), BufferGetBlockNumber(buffer), rootoffnum);

		if (HeapTupleHeaderIsHeapOnly(htup))
		{
			/*
			 * If the tuple is DEAD and doesn't chain to anything else, mark
			 * it unused immediately.  (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.)
			 *
			 * Note that we might first arrive at a dead heap-only tuple
			 * either here or while following a chain below.  Whichever path
			 * gets there first will mark the tuple unused.
			 */
			if (HeapTupleSatisfiesVacuum(&tup, OldestXmin, buffer)
				== HEAPTUPLE_DEAD && !HeapTupleHeaderIsHotUpdated(htup))
			{
				heap_prune_record_unused(prstate, rootoffnum);
				HeapTupleHeaderAdvanceLatestRemovedXid(htup,
													   &prstate->latestRemovedXid);
				ndeleted++;
			}

			/* Nothing more to do */
			return ndeleted;
		}
	}

	/* Start from the root tuple */
	offnum = rootoffnum;

	/* while not end of the chain */
	for (;;)
	{
		ItemId		lp;
		bool		tupdead,
					recent_dead;

		/* Some sanity checks */
		if (offnum < FirstOffsetNumber || offnum > maxoff)
			break;

		/* If item is already processed, stop --- it must not be same chain */
		if (prstate->marked[offnum])
			break;

		lp = PageGetItemId(dp, offnum);

		/* Unused item obviously isn't part of the chain */
		if (!ItemIdIsUsed(lp))
			break;

		/*
		 * 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;
		}

		/*
		 * Likewise, a dead line pointer can't be part of the chain. (We
		 * already eliminated the case of dead root tuple outside this
		 * function.)
		 */
		if (ItemIdIsDead(lp))
			break;

		Assert(ItemIdIsNormal(lp));
		htup = (HeapTupleHeader) PageGetItem(dp, lp);

		tup.t_data = htup;
		tup.t_len = ItemIdGetLength(lp);
		ItemPointerSet(&(tup.t_self), BufferGetBlockNumber(buffer), offnum);

		/*
		 * 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;

		/*
		 * Check tuple's visibility status.
		 */
		tupdead = recent_dead = false;

		switch (HeapTupleSatisfiesVacuum(&tup, OldestXmin, buffer))
		{
			case HEAPTUPLE_DEAD:
				tupdead = true;
				break;

			case HEAPTUPLE_RECENTLY_DEAD:
				recent_dead = true;

				/*
				 * 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;

			case HEAPTUPLE_DELETE_IN_PROGRESS:

				/*
				 * 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;

			case HEAPTUPLE_LIVE:
			case HEAPTUPLE_INSERT_IN_PROGRESS:

				/*
				 * 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;

			default:
				elog(ERROR, "unexpected HeapTupleSatisfiesVacuum result");
				break;
		}

		/*
		 * Remember the last DEAD tuple seen.  We will advance past
		 * RECENTLY_DEAD tuples just in case there's a DEAD one after them;
		 * but we can't advance past anything else.  (XXX is it really worth
		 * continuing to scan beyond RECENTLY_DEAD?  The case where we will
		 * find another DEAD tuple is a fairly unusual corner case.)
		 */
		if (tupdead)
		{
			latestdead = offnum;
			HeapTupleHeaderAdvanceLatestRemovedXid(htup,
												   &prstate->latestRemovedXid);
		}
		else if (!recent_dead)
			break;

		/*
		 * If the tuple is not HOT-updated, then we are at the end of this
		 * HOT-update chain.
		 */
		if (!HeapTupleHeaderIsHotUpdated(htup))
			break;

		/* HOT implies it can't have moved to different partition */
		Assert(!HeapTupleHeaderIndicatesMovedPartitions(htup));

		/*
		 * Advance to next chain member.
		 */
		Assert(ItemPointerGetBlockNumber(&htup->t_ctid) ==
			   BufferGetBlockNumber(buffer));
		offnum = ItemPointerGetOffsetNumber(&htup->t_ctid);
		priorXmax = HeapTupleHeaderGetUpdateXid(htup);
	}

	/*
	 * If we found a DEAD tuple in the chain, adjust the HOT chain so that all
	 * the DEAD tuples at the start of the chain are removed and the root line
	 * pointer is appropriately redirected.
	 */
	if (OffsetNumberIsValid(latestdead))
	{
		/*
		 * Mark as unused each intermediate item that we are able to remove
		 * from the chain.
		 *
		 * When the previous item is the last dead tuple seen, we are at the
		 * right candidate for redirection.
		 */
		for (i = 1; (i < nchain) && (chainitems[i - 1] != latestdead); i++)
		{
			heap_prune_record_unused(prstate, chainitems[i]);
			ndeleted++;
		}

		/*
		 * 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 (ItemIdIsNormal(rootlp))
			ndeleted++;

		/*
		 * If the DEAD tuple is at the end of the chain, the entire chain is
		 * dead and the root line pointer can be marked dead.  Otherwise just
		 * redirect the root to the correct chain member.
		 */
		if (i >= nchain)
			heap_prune_record_dead(prstate, rootoffnum);
		else
			heap_prune_record_redirect(prstate, rootoffnum, chainitems[i]);
	}
	else if (nchain < 2 && ItemIdIsRedirected(rootlp))
	{
		/*
		 * 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 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
		 * DEAD state.
		 */
		heap_prune_record_dead(prstate, rootoffnum);
	}

	return ndeleted;
}

/* 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)
{
	Assert(prstate->nredirected < MaxHeapTuplesPerPage);
	prstate->redirected[prstate->nredirected * 2] = offnum;
	prstate->redirected[prstate->nredirected * 2 + 1] = rdoffnum;
	prstate->nredirected++;
	Assert(!prstate->marked[offnum]);
	prstate->marked[offnum] = true;
	Assert(!prstate->marked[rdoffnum]);
	prstate->marked[rdoffnum] = true;
}

/* Record line pointer to be marked dead */
static void
heap_prune_record_dead(PruneState *prstate, OffsetNumber offnum)
{
	Assert(prstate->ndead < MaxHeapTuplesPerPage);
	prstate->nowdead[prstate->ndead] = offnum;
	prstate->ndead++;
	Assert(!prstate->marked[offnum]);
	prstate->marked[offnum] = true;
}

/* Record line pointer to be marked unused */
static void
heap_prune_record_unused(PruneState *prstate, OffsetNumber offnum)
{
	Assert(prstate->nunused < MaxHeapTuplesPerPage);
	prstate->nowunused[prstate->nunused] = offnum;
	prstate->nunused++;
	Assert(!prstate->marked[offnum]);
	prstate->marked[offnum] = true;
}


/*
 * Perform the actual page changes needed by heap_page_prune.
 * It is expected that the caller has suitable pin and lock on the
 * buffer, and is inside a critical section.
 *
 * This is split out because it is also used by heap_xlog_clean()
 * to replay the WAL record when needed after a crash.  Note that the
 * arguments are identical to those of log_heap_clean().
 */
void
heap_page_prune_execute(Buffer buffer,
						OffsetNumber *redirected, int nredirected,
						OffsetNumber *nowdead, int ndead,
						OffsetNumber *nowunused, int nunused)
{
	Page		page = (Page) BufferGetPage(buffer);
	OffsetNumber *offnum;
	int			i;

	/* Update all redirected line pointers */
	offnum = redirected;
	for (i = 0; i < nredirected; i++)
	{
		OffsetNumber fromoff = *offnum++;
		OffsetNumber tooff = *offnum++;
		ItemId		fromlp = PageGetItemId(page, fromoff);

		ItemIdSetRedirect(fromlp, tooff);
	}

	/* Update all now-dead line pointers */
	offnum = nowdead;
	for (i = 0; i < ndead; i++)
	{
		OffsetNumber off = *offnum++;
		ItemId		lp = PageGetItemId(page, off);

		ItemIdSetDead(lp);
	}

	/* Update all now-unused line pointers */
	offnum = nowunused;
	for (i = 0; i < nunused; i++)
	{
		OffsetNumber off = *offnum++;
		ItemId		lp = PageGetItemId(page, off);

		ItemIdSetUnused(lp);
	}

	/*
	 * Finally, repair any fragmentation, and update the page's hint bit about
	 * whether it has free pointers.
	 */
	PageRepairFragmentation(page);
}


/*
 * 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.
 * We zero out all unused entries.
 *
 * 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, 0, 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 (;;)
		{
			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);
		}
	}
}