Move vacuumlazy.c into access/heap.

It's heap table storage specific code that can't realistically be
generalized into table AM agnostic code.

Author: Andres Freund
Discussion: https://postgr.es/m/20180703070645.wchpu5muyto5n647@alap3.anarazel.de

2 files changed, 2298 insertions, 1 deletions

diff --git a/src/backend/access/heap/Makefile b/src/backend/access/heap/Makefile
index b83d496bcd7..7e7324a9166 100644
--- a/src/backend/access/heap/Makefile
+++ b/src/backend/access/heap/Makefile
@@ -12,6 +12,7 @@ subdir = src/backend/access/heap
 top_builddir = ../../../..
 include $(top_builddir)/src/Makefile.global
 
-OBJS = heapam.o hio.o pruneheap.o rewriteheap.o syncscan.o tuptoaster.o visibilitymap.o
+OBJS = heapam.o hio.o pruneheap.o rewriteheap.o syncscan.o tuptoaster.o \
+	vacuumlazy.o visibilitymap.o
 
 include $(top_srcdir)/src/backend/common.mk
diff --git a/src/backend/access/heap/vacuumlazy.c b/src/backend/access/heap/vacuumlazy.c
new file mode 100644
index 00000000000..2d317a94620
--- /dev/null
+++ b/src/backend/access/heap/vacuumlazy.c
@@ -0,0 +1,2296 @@
+/*-------------------------------------------------------------------------
+ *
+ * vacuumlazy.c
+ *	  Concurrent ("lazy") vacuuming.
+ *
+ *
+ * The major space usage for LAZY VACUUM is storage for the array of dead tuple
+ * TIDs.  We want to ensure we can vacuum even the very largest relations with
+ * finite memory space usage.  To do that, we set upper bounds on the number of
+ * tuples we will keep track of at once.
+ *
+ * We are willing to use at most maintenance_work_mem (or perhaps
+ * autovacuum_work_mem) memory space to keep track of dead tuples.  We
+ * initially allocate an array of TIDs of that size, with an upper limit that
+ * depends on table size (this limit ensures we don't allocate a huge area
+ * uselessly for vacuuming small tables).  If the array threatens to overflow,
+ * we suspend the heap scan phase and perform a pass of index cleanup and page
+ * compaction, then resume the heap scan with an empty TID array.
+ *
+ * If we're processing a table with no indexes, we can just vacuum each page
+ * as we go; there's no need to save up multiple tuples to minimize the number
+ * of index scans performed.  So we don't use maintenance_work_mem memory for
+ * the TID array, just enough to hold as many heap tuples as fit on one page.
+ *
+ *
+ * Portions Copyright (c) 1996-2019, PostgreSQL Global Development Group
+ * Portions Copyright (c) 1994, Regents of the University of California
+ *
+ *
+ * IDENTIFICATION
+ *	  src/backend/access/heap/vacuumlazy.c
+ *
+ *-------------------------------------------------------------------------
+ */
+#include "postgres.h"
+
+#include <math.h>
+
+#include "access/genam.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 "catalog/storage.h"
+#include "commands/dbcommands.h"
+#include "commands/progress.h"
+#include "commands/vacuum.h"
+#include "miscadmin.h"
+#include "pgstat.h"
+#include "portability/instr_time.h"
+#include "postmaster/autovacuum.h"
+#include "storage/bufmgr.h"
+#include "storage/freespace.h"
+#include "storage/lmgr.h"
+#include "utils/lsyscache.h"
+#include "utils/memutils.h"
+#include "utils/pg_rusage.h"
+#include "utils/timestamp.h"
+#include "utils/tqual.h"
+
+
+/*
+ * Space/time tradeoff parameters: do these need to be user-tunable?
+ *
+ * To consider truncating the relation, we want there to be at least
+ * REL_TRUNCATE_MINIMUM or (relsize / REL_TRUNCATE_FRACTION) (whichever
+ * is less) potentially-freeable pages.
+ */
+#define REL_TRUNCATE_MINIMUM	1000
+#define REL_TRUNCATE_FRACTION	16
+
+/*
+ * Timing parameters for truncate locking heuristics.
+ *
+ * These were not exposed as user tunable GUC values because it didn't seem
+ * that the potential for improvement was great enough to merit the cost of
+ * supporting them.
+ */
+#define VACUUM_TRUNCATE_LOCK_CHECK_INTERVAL		20	/* ms */
+#define VACUUM_TRUNCATE_LOCK_WAIT_INTERVAL		50	/* ms */
+#define VACUUM_TRUNCATE_LOCK_TIMEOUT			5000	/* ms */
+
+/*
+ * When a table has no indexes, vacuum the FSM after every 8GB, approximately
+ * (it won't be exact because we only vacuum FSM after processing a heap page
+ * that has some removable tuples).  When there are indexes, this is ignored,
+ * and we vacuum FSM after each index/heap cleaning pass.
+ */
+#define VACUUM_FSM_EVERY_PAGES \
+	((BlockNumber) (((uint64) 8 * 1024 * 1024 * 1024) / BLCKSZ))
+
+/*
+ * Guesstimation of number of dead tuples per page.  This is used to
+ * provide an upper limit to memory allocated when vacuuming small
+ * tables.
+ */
+#define LAZY_ALLOC_TUPLES		MaxHeapTuplesPerPage
+
+/*
+ * Before we consider skipping a page that's marked as clean in
+ * visibility map, we must've seen at least this many clean pages.
+ */
+#define SKIP_PAGES_THRESHOLD	((BlockNumber) 32)
+
+/*
+ * Size of the prefetch window for lazy vacuum backwards truncation scan.
+ * Needs to be a power of 2.
+ */
+#define PREFETCH_SIZE			((BlockNumber) 32)
+
+typedef struct LVRelStats
+{
+	/* hasindex = true means two-pass strategy; false means one-pass */
+	bool		hasindex;
+	/* Overall statistics about rel */
+	BlockNumber old_rel_pages;	/* previous value of pg_class.relpages */
+	BlockNumber rel_pages;		/* total number of pages */
+	BlockNumber scanned_pages;	/* number of pages we examined */
+	BlockNumber pinskipped_pages;	/* # of pages we skipped due to a pin */
+	BlockNumber frozenskipped_pages;	/* # of frozen pages we skipped */
+	BlockNumber tupcount_pages; /* pages whose tuples we counted */
+	double		old_live_tuples;	/* previous value of pg_class.reltuples */
+	double		new_rel_tuples; /* new estimated total # of tuples */
+	double		new_live_tuples;	/* new estimated total # of live tuples */
+	double		new_dead_tuples;	/* new estimated total # of dead tuples */
+	BlockNumber pages_removed;
+	double		tuples_deleted;
+	BlockNumber nonempty_pages; /* actually, last nonempty page + 1 */
+	/* List of TIDs of tuples we intend to delete */
+	/* NB: this list is ordered by TID address */
+	int			num_dead_tuples;	/* current # of entries */
+	int			max_dead_tuples;	/* # slots allocated in array */
+	ItemPointer dead_tuples;	/* array of ItemPointerData */
+	int			num_index_scans;
+	TransactionId latestRemovedXid;
+	bool		lock_waiter_detected;
+} LVRelStats;
+
+
+/* A few variables that don't seem worth passing around as parameters */
+static int	elevel = -1;
+
+static TransactionId OldestXmin;
+static TransactionId FreezeLimit;
+static MultiXactId MultiXactCutoff;
+
+static BufferAccessStrategy vac_strategy;
+
+
+/* non-export function prototypes */
+static void lazy_scan_heap(Relation onerel, int options,
+			   LVRelStats *vacrelstats, Relation *Irel, int nindexes,
+			   bool aggressive);
+static void lazy_vacuum_heap(Relation onerel, LVRelStats *vacrelstats);
+static bool lazy_check_needs_freeze(Buffer buf, bool *hastup);
+static void lazy_vacuum_index(Relation indrel,
+				  IndexBulkDeleteResult **stats,
+				  LVRelStats *vacrelstats);
+static void lazy_cleanup_index(Relation indrel,
+				   IndexBulkDeleteResult *stats,
+				   LVRelStats *vacrelstats);
+static int lazy_vacuum_page(Relation onerel, BlockNumber blkno, Buffer buffer,
+				 int tupindex, LVRelStats *vacrelstats, Buffer *vmbuffer);
+static bool should_attempt_truncation(LVRelStats *vacrelstats);
+static void lazy_truncate_heap(Relation onerel, LVRelStats *vacrelstats);
+static BlockNumber count_nondeletable_pages(Relation onerel,
+						 LVRelStats *vacrelstats);
+static void lazy_space_alloc(LVRelStats *vacrelstats, BlockNumber relblocks);
+static void lazy_record_dead_tuple(LVRelStats *vacrelstats,
+					   ItemPointer itemptr);
+static bool lazy_tid_reaped(ItemPointer itemptr, void *state);
+static int	vac_cmp_itemptr(const void *left, const void *right);
+static bool heap_page_is_all_visible(Relation rel, Buffer buf,
+						 TransactionId *visibility_cutoff_xid, bool *all_frozen);
+
+
+/*
+ *	vacuum_heap_rel() -- perform VACUUM for one heap relation
+ *
+ *		This routine vacuums a single heap, cleans out its indexes, and
+ *		updates its relpages and reltuples statistics.
+ *
+ *		At entry, we have already established a transaction and opened
+ *		and locked the relation.
+ */
+void
+heap_vacuum_rel(Relation onerel, int options, VacuumParams *params,
+				BufferAccessStrategy bstrategy)
+{
+	LVRelStats *vacrelstats;
+	Relation   *Irel;
+	int			nindexes;
+	PGRUsage	ru0;
+	TimestampTz starttime = 0;
+	long		secs;
+	int			usecs;
+	double		read_rate,
+				write_rate;
+	bool		aggressive;		/* should we scan all unfrozen pages? */
+	bool		scanned_all_unfrozen;	/* actually scanned all such pages? */
+	TransactionId xidFullScanLimit;
+	MultiXactId mxactFullScanLimit;
+	BlockNumber new_rel_pages;
+	BlockNumber new_rel_allvisible;
+	double		new_live_tuples;
+	TransactionId new_frozen_xid;
+	MultiXactId new_min_multi;
+
+	Assert(params != NULL);
+
+	/* measure elapsed time iff autovacuum logging requires it */
+	if (IsAutoVacuumWorkerProcess() && params->log_min_duration >= 0)
+	{
+		pg_rusage_init(&ru0);
+		starttime = GetCurrentTimestamp();
+	}
+
+	if (options & VACOPT_VERBOSE)
+		elevel = INFO;
+	else
+		elevel = DEBUG2;
+
+	pgstat_progress_start_command(PROGRESS_COMMAND_VACUUM,
+								  RelationGetRelid(onerel));
+
+	vac_strategy = bstrategy;
+
+	vacuum_set_xid_limits(onerel,
+						  params->freeze_min_age,
+						  params->freeze_table_age,
+						  params->multixact_freeze_min_age,
+						  params->multixact_freeze_table_age,
+						  &OldestXmin, &FreezeLimit, &xidFullScanLimit,
+						  &MultiXactCutoff, &mxactFullScanLimit);
+
+	/*
+	 * We request an aggressive scan if the table's frozen Xid is now older
+	 * than or equal to the requested Xid full-table scan limit; or if the
+	 * table's minimum MultiXactId is older than or equal to the requested
+	 * mxid full-table scan limit; or if DISABLE_PAGE_SKIPPING was specified.
+	 */
+	aggressive = TransactionIdPrecedesOrEquals(onerel->rd_rel->relfrozenxid,
+											   xidFullScanLimit);
+	aggressive |= MultiXactIdPrecedesOrEquals(onerel->rd_rel->relminmxid,
+											  mxactFullScanLimit);
+	if (options & VACOPT_DISABLE_PAGE_SKIPPING)
+		aggressive = true;
+
+	vacrelstats = (LVRelStats *) palloc0(sizeof(LVRelStats));
+
+	vacrelstats->old_rel_pages = onerel->rd_rel->relpages;
+	vacrelstats->old_live_tuples = onerel->rd_rel->reltuples;
+	vacrelstats->num_index_scans = 0;
+	vacrelstats->pages_removed = 0;
+	vacrelstats->lock_waiter_detected = false;
+
+	/* Open all indexes of the relation */
+	vac_open_indexes(onerel, RowExclusiveLock, &nindexes, &Irel);
+	vacrelstats->hasindex = (nindexes > 0);
+
+	/* Do the vacuuming */
+	lazy_scan_heap(onerel, options, vacrelstats, Irel, nindexes, aggressive);
+
+	/* Done with indexes */
+	vac_close_indexes(nindexes, Irel, NoLock);
+
+	/*
+	 * Compute whether we actually scanned the all unfrozen pages. If we did,
+	 * we can adjust relfrozenxid and relminmxid.
+	 *
+	 * NB: We need to check this before truncating the relation, because that
+	 * will change ->rel_pages.
+	 */
+	if ((vacrelstats->scanned_pages + vacrelstats->frozenskipped_pages)
+		< vacrelstats->rel_pages)
+	{
+		Assert(!aggressive);
+		scanned_all_unfrozen = false;
+	}
+	else
+		scanned_all_unfrozen = true;
+
+	/*
+	 * Optionally truncate the relation.
+	 */
+	if (should_attempt_truncation(vacrelstats))
+		lazy_truncate_heap(onerel, vacrelstats);
+
+	/* Report that we are now doing final cleanup */
+	pgstat_progress_update_param(PROGRESS_VACUUM_PHASE,
+								 PROGRESS_VACUUM_PHASE_FINAL_CLEANUP);
+
+	/*
+	 * Update statistics in pg_class.
+	 *
+	 * A corner case here is that if we scanned no pages at all because every
+	 * page is all-visible, we should not update relpages/reltuples, because
+	 * we have no new information to contribute.  In particular this keeps us
+	 * from replacing relpages=reltuples=0 (which means "unknown tuple
+	 * density") with nonzero relpages and reltuples=0 (which means "zero
+	 * tuple density") unless there's some actual evidence for the latter.
+	 *
+	 * It's important that we use tupcount_pages and not scanned_pages for the
+	 * check described above; scanned_pages counts pages where we could not
+	 * get cleanup lock, and which were processed only for frozenxid purposes.
+	 *
+	 * We do update relallvisible even in the corner case, since if the table
+	 * is all-visible we'd definitely like to know that.  But clamp the value
+	 * to be not more than what we're setting relpages to.
+	 *
+	 * Also, don't change relfrozenxid/relminmxid if we skipped any pages,
+	 * since then we don't know for certain that all tuples have a newer xmin.
+	 */
+	new_rel_pages = vacrelstats->rel_pages;
+	new_live_tuples = vacrelstats->new_live_tuples;
+	if (vacrelstats->tupcount_pages == 0 && new_rel_pages > 0)
+	{
+		new_rel_pages = vacrelstats->old_rel_pages;
+		new_live_tuples = vacrelstats->old_live_tuples;
+	}
+
+	visibilitymap_count(onerel, &new_rel_allvisible, NULL);
+	if (new_rel_allvisible > new_rel_pages)
+		new_rel_allvisible = new_rel_pages;
+
+	new_frozen_xid = scanned_all_unfrozen ? FreezeLimit : InvalidTransactionId;
+	new_min_multi = scanned_all_unfrozen ? MultiXactCutoff : InvalidMultiXactId;
+
+	vac_update_relstats(onerel,
+						new_rel_pages,
+						new_live_tuples,
+						new_rel_allvisible,
+						vacrelstats->hasindex,
+						new_frozen_xid,
+						new_min_multi,
+						false);
+
+	/* report results to the stats collector, too */
+	pgstat_report_vacuum(RelationGetRelid(onerel),
+						 onerel->rd_rel->relisshared,
+						 new_live_tuples,
+						 vacrelstats->new_dead_tuples);
+	pgstat_progress_end_command();
+
+	/* and log the action if appropriate */
+	if (IsAutoVacuumWorkerProcess() && params->log_min_duration >= 0)
+	{
+		TimestampTz endtime = GetCurrentTimestamp();
+
+		if (params->log_min_duration == 0 ||
+			TimestampDifferenceExceeds(starttime, endtime,
+									   params->log_min_duration))
+		{
+			StringInfoData buf;
+			char	   *msgfmt;
+
+			TimestampDifference(starttime, endtime, &secs, &usecs);
+
+			read_rate = 0;
+			write_rate = 0;
+			if ((secs > 0) || (usecs > 0))
+			{
+				read_rate = (double) BLCKSZ * VacuumPageMiss / (1024 * 1024) /
+					(secs + usecs / 1000000.0);
+				write_rate = (double) BLCKSZ * VacuumPageDirty / (1024 * 1024) /
+					(secs + usecs / 1000000.0);
+			}
+
+			/*
+			 * This is pretty messy, but we split it up so that we can skip
+			 * emitting individual parts of the message when not applicable.
+			 */
+			initStringInfo(&buf);
+			if (params->is_wraparound)
+			{
+				if (aggressive)
+					msgfmt = _("automatic aggressive vacuum to prevent wraparound of table \"%s.%s.%s\": index scans: %d\n");
+				else
+					msgfmt = _("automatic vacuum to prevent wraparound of table \"%s.%s.%s\": index scans: %d\n");
+			}
+			else
+			{
+				if (aggressive)
+					msgfmt = _("automatic aggressive vacuum of table \"%s.%s.%s\": index scans: %d\n");
+				else
+					msgfmt = _("automatic vacuum of table \"%s.%s.%s\": index scans: %d\n");
+			}
+			appendStringInfo(&buf, msgfmt,
+							 get_database_name(MyDatabaseId),
+							 get_namespace_name(RelationGetNamespace(onerel)),
+							 RelationGetRelationName(onerel),
+							 vacrelstats->num_index_scans);
+			appendStringInfo(&buf, _("pages: %u removed, %u remain, %u skipped due to pins, %u skipped frozen\n"),
+							 vacrelstats->pages_removed,
+							 vacrelstats->rel_pages,
+							 vacrelstats->pinskipped_pages,
+							 vacrelstats->frozenskipped_pages);
+			appendStringInfo(&buf,
+							 _("tuples: %.0f removed, %.0f remain, %.0f are dead but not yet removable, oldest xmin: %u\n"),
+							 vacrelstats->tuples_deleted,
+							 vacrelstats->new_rel_tuples,
+							 vacrelstats->new_dead_tuples,
+							 OldestXmin);
+			appendStringInfo(&buf,
+							 _("buffer usage: %d hits, %d misses, %d dirtied\n"),
+							 VacuumPageHit,
+							 VacuumPageMiss,
+							 VacuumPageDirty);
+			appendStringInfo(&buf, _("avg read rate: %.3f MB/s, avg write rate: %.3f MB/s\n"),
+							 read_rate, write_rate);
+			appendStringInfo(&buf, _("system usage: %s"), pg_rusage_show(&ru0));
+
+			ereport(LOG,
+					(errmsg_internal("%s", buf.data)));
+			pfree(buf.data);
+		}
+	}
+}
+
+/*
+ * For Hot Standby we need to know the highest transaction id that will
+ * be removed by any change. VACUUM proceeds in a number of passes so
+ * we need to consider how each pass operates. The first phase runs
+ * heap_page_prune(), which can issue XLOG_HEAP2_CLEAN records as it
+ * progresses - these will have a latestRemovedXid on each record.
+ * In some cases this removes all of the tuples to be removed, though
+ * often we have dead tuples with index pointers so we must remember them
+ * for removal in phase 3. Index records for those rows are removed
+ * in phase 2 and index blocks do not have MVCC information attached.
+ * So before we can allow removal of any index tuples we need to issue
+ * a WAL record containing the latestRemovedXid of rows that will be
+ * removed in phase three. This allows recovery queries to block at the
+ * correct place, i.e. before phase two, rather than during phase three
+ * which would be after the rows have become inaccessible.
+ */
+static void
+vacuum_log_cleanup_info(Relation rel, LVRelStats *vacrelstats)
+{
+	/*
+	 * Skip this for relations for which no WAL is to be written, or if we're
+	 * not trying to support archive recovery.
+	 */
+	if (!RelationNeedsWAL(rel) || !XLogIsNeeded())
+		return;
+
+	/*
+	 * No need to write the record at all unless it contains a valid value
+	 */
+	if (TransactionIdIsValid(vacrelstats->latestRemovedXid))
+		(void) log_heap_cleanup_info(rel->rd_node, vacrelstats->latestRemovedXid);
+}
+
+/*
+ *	lazy_scan_heap() -- scan an open heap relation
+ *
+ *		This routine prunes each page in the heap, which will among other
+ *		things truncate dead tuples to dead line pointers, defragment the
+ *		page, and set commit status bits (see heap_page_prune).  It also builds
+ *		lists of dead tuples and pages with free space, calculates statistics
+ *		on the number of live tuples in the heap, and marks pages as
+ *		all-visible if appropriate.  When done, or when we run low on space for
+ *		dead-tuple TIDs, invoke vacuuming of indexes and call lazy_vacuum_heap
+ *		to reclaim dead line pointers.
+ *
+ *		If there are no indexes then we can reclaim line pointers on the fly;
+ *		dead line pointers need only be retained until all index pointers that
+ *		reference them have been killed.
+ */
+static void
+lazy_scan_heap(Relation onerel, int options, LVRelStats *vacrelstats,
+			   Relation *Irel, int nindexes, bool aggressive)
+{
+	BlockNumber nblocks,
+				blkno;
+	HeapTupleData tuple;
+	char	   *relname;
+	TransactionId relfrozenxid = onerel->rd_rel->relfrozenxid;
+	TransactionId relminmxid = onerel->rd_rel->relminmxid;
+	BlockNumber empty_pages,
+				vacuumed_pages,
+				next_fsm_block_to_vacuum;
+	double		num_tuples,		/* total number of nonremovable tuples */
+				live_tuples,	/* live tuples (reltuples estimate) */
+				tups_vacuumed,	/* tuples cleaned up by vacuum */
+				nkeep,			/* dead-but-not-removable tuples */
+				nunused;		/* unused item pointers */
+	IndexBulkDeleteResult **indstats;
+	int			i;
+	PGRUsage	ru0;
+	Buffer		vmbuffer = InvalidBuffer;
+	BlockNumber next_unskippable_block;
+	bool		skipping_blocks;
+	xl_heap_freeze_tuple *frozen;
+	StringInfoData buf;
+	const int	initprog_index[] = {
+		PROGRESS_VACUUM_PHASE,
+		PROGRESS_VACUUM_TOTAL_HEAP_BLKS,
+		PROGRESS_VACUUM_MAX_DEAD_TUPLES
+	};
+	int64		initprog_val[3];
+
+	pg_rusage_init(&ru0);
+
+	relname = RelationGetRelationName(onerel);
+	if (aggressive)
+		ereport(elevel,
+				(errmsg("aggressively vacuuming \"%s.%s\"",
+						get_namespace_name(RelationGetNamespace(onerel)),
+						relname)));
+	else
+		ereport(elevel,
+				(errmsg("vacuuming \"%s.%s\"",
+						get_namespace_name(RelationGetNamespace(onerel)),
+						relname)));
+
+	empty_pages = vacuumed_pages = 0;
+	next_fsm_block_to_vacuum = (BlockNumber) 0;
+	num_tuples = live_tuples = tups_vacuumed = nkeep = nunused = 0;
+
+	indstats = (IndexBulkDeleteResult **)
+		palloc0(nindexes * sizeof(IndexBulkDeleteResult *));
+
+	nblocks = RelationGetNumberOfBlocks(onerel);
+	vacrelstats->rel_pages = nblocks;
+	vacrelstats->scanned_pages = 0;
+	vacrelstats->tupcount_pages = 0;
+	vacrelstats->nonempty_pages = 0;
+	vacrelstats->latestRemovedXid = InvalidTransactionId;
+
+	lazy_space_alloc(vacrelstats, nblocks);
+	frozen = palloc(sizeof(xl_heap_freeze_tuple) * MaxHeapTuplesPerPage);
+
+	/* Report that we're scanning the heap, advertising total # of blocks */
+	initprog_val[0] = PROGRESS_VACUUM_PHASE_SCAN_HEAP;
+	initprog_val[1] = nblocks;
+	initprog_val[2] = vacrelstats->max_dead_tuples;
+	pgstat_progress_update_multi_param(3, initprog_index, initprog_val);
+
+	/*
+	 * Except when aggressive is set, we want to skip pages that are
+	 * all-visible according to the visibility map, but only when we can skip
+	 * at least SKIP_PAGES_THRESHOLD consecutive pages.  Since we're reading
+	 * sequentially, the OS should be doing readahead for us, so there's no
+	 * gain in skipping a page now and then; that's likely to disable
+	 * readahead and so be counterproductive. Also, skipping even a single
+	 * page means that we can't update relfrozenxid, so we only want to do it
+	 * if we can skip a goodly number of pages.
+	 *
+	 * When aggressive is set, we can't skip pages just because they are
+	 * all-visible, but we can still skip pages that are all-frozen, since
+	 * such pages do not need freezing and do not affect the value that we can
+	 * safely set for relfrozenxid or relminmxid.
+	 *
+	 * Before entering the main loop, establish the invariant that
+	 * next_unskippable_block is the next block number >= blkno that we can't
+	 * skip based on the visibility map, either all-visible for a regular scan
+	 * or all-frozen for an aggressive scan.  We set it to nblocks if there's
+	 * no such block.  We also set up the skipping_blocks flag correctly at
+	 * this stage.
+	 *
+	 * Note: The value returned by visibilitymap_get_status could be slightly
+	 * out-of-date, since we make this test before reading the corresponding
+	 * heap page or locking the buffer.  This is OK.  If we mistakenly think
+	 * that the page is all-visible or all-frozen when in fact the flag's just
+	 * been cleared, we might fail to vacuum the page.  It's easy to see that
+	 * skipping a page when aggressive is not set is not a very big deal; we
+	 * might leave some dead tuples lying around, but the next vacuum will
+	 * find them.  But even when aggressive *is* set, it's still OK if we miss
+	 * a page whose all-frozen marking has just been cleared.  Any new XIDs
+	 * just added to that page are necessarily newer than the GlobalXmin we
+	 * computed, so they'll have no effect on the value to which we can safely
+	 * set relfrozenxid.  A similar argument applies for MXIDs and relminmxid.
+	 *
+	 * We will scan the table's last page, at least to the extent of
+	 * determining whether it has tuples or not, even if it should be skipped
+	 * according to the above rules; except when we've already determined that
+	 * it's not worth trying to truncate the table.  This avoids having
+	 * lazy_truncate_heap() take access-exclusive lock on the table to attempt
+	 * a truncation that just fails immediately because there are tuples in
+	 * the last page.  This is worth avoiding mainly because such a lock must
+	 * be replayed on any hot standby, where it can be disruptive.
+	 */
+	next_unskippable_block = 0;
+	if ((options & VACOPT_DISABLE_PAGE_SKIPPING) == 0)
+	{
+		while (next_unskippable_block < nblocks)
+		{
+			uint8		vmstatus;
+
+			vmstatus = visibilitymap_get_status(onerel, next_unskippable_block,
+												&vmbuffer);
+			if (aggressive)
+			{
+				if ((vmstatus & VISIBILITYMAP_ALL_FROZEN) == 0)
+					break;
+			}
+			else
+			{
+				if ((vmstatus & VISIBILITYMAP_ALL_VISIBLE) == 0)
+					break;
+			}
+			vacuum_delay_point();
+			next_unskippable_block++;
+		}
+	}
+
+	if (next_unskippable_block >= SKIP_PAGES_THRESHOLD)
+		skipping_blocks = true;
+	else
+		skipping_blocks = false;
+
+	for (blkno = 0; blkno < nblocks; blkno++)
+	{
+		Buffer		buf;
+		Page		page;
+		OffsetNumber offnum,
+					maxoff;
+		bool		tupgone,
+					hastup;
+		int			prev_dead_count;
+		int			nfrozen;
+		Size		freespace;
+		bool		all_visible_according_to_vm = false;
+		bool		all_visible;
+		bool		all_frozen = true;	/* provided all_visible is also true */
+		bool		has_dead_tuples;
+		TransactionId visibility_cutoff_xid = InvalidTransactionId;
+
+		/* see note above about forcing scanning of last page */
+#define FORCE_CHECK_PAGE() \
+		(blkno == nblocks - 1 && should_attempt_truncation(vacrelstats))
+
+		pgstat_progress_update_param(PROGRESS_VACUUM_HEAP_BLKS_SCANNED, blkno);
+
+		if (blkno == next_unskippable_block)
+		{
+			/* Time to advance next_unskippable_block */
+			next_unskippable_block++;
+			if ((options & VACOPT_DISABLE_PAGE_SKIPPING) == 0)
+			{
+				while (next_unskippable_block < nblocks)
+				{
+					uint8		vmskipflags;
+
+					vmskipflags = visibilitymap_get_status(onerel,
+														   next_unskippable_block,
+														   &vmbuffer);
+					if (aggressive)
+					{
+						if ((vmskipflags & VISIBILITYMAP_ALL_FROZEN) == 0)
+							break;
+					}
+					else
+					{
+						if ((vmskipflags & VISIBILITYMAP_ALL_VISIBLE) == 0)
+							break;
+					}
+					vacuum_delay_point();
+					next_unskippable_block++;
+				}
+			}
+
+			/*
+			 * We know we can't skip the current block.  But set up
+			 * skipping_blocks to do the right thing at the following blocks.
+			 */
+			if (next_unskippable_block - blkno > SKIP_PAGES_THRESHOLD)
+				skipping_blocks = true;
+			else
+				skipping_blocks = false;
+
+			/*
+			 * Normally, the fact that we can't skip this block must mean that
+			 * it's not all-visible.  But in an aggressive vacuum we know only
+			 * that it's not all-frozen, so it might still be all-visible.
+			 */
+			if (aggressive && VM_ALL_VISIBLE(onerel, blkno, &vmbuffer))
+				all_visible_according_to_vm = true;
+		}
+		else
+		{
+			/*
+			 * The current block is potentially skippable; if we've seen a
+			 * long enough run of skippable blocks to justify skipping it, and
+			 * we're not forced to check it, then go ahead and skip.
+			 * Otherwise, the page must be at least all-visible if not
+			 * all-frozen, so we can set all_visible_according_to_vm = true.
+			 */
+			if (skipping_blocks && !FORCE_CHECK_PAGE())
+			{
+				/*
+				 * Tricky, tricky.  If this is in aggressive vacuum, the page
+				 * must have been all-frozen at the time we checked whether it
+				 * was skippable, but it might not be any more.  We must be
+				 * careful to count it as a skipped all-frozen page in that
+				 * case, or else we'll think we can't update relfrozenxid and
+				 * relminmxid.  If it's not an aggressive vacuum, we don't
+				 * know whether it was all-frozen, so we have to recheck; but
+				 * in this case an approximate answer is OK.
+				 */
+				if (aggressive || VM_ALL_FROZEN(onerel, blkno, &vmbuffer))
+					vacrelstats->frozenskipped_pages++;
+				continue;
+			}
+			all_visible_according_to_vm = true;
+		}
+
+		vacuum_delay_point();
+
+		/*
+		 * If we are close to overrunning the available space for dead-tuple
+		 * TIDs, pause and do a cycle of vacuuming before we tackle this page.
+		 */
+		if ((vacrelstats->max_dead_tuples - vacrelstats->num_dead_tuples) < MaxHeapTuplesPerPage &&
+			vacrelstats->num_dead_tuples > 0)
+		{
+			const int	hvp_index[] = {
+				PROGRESS_VACUUM_PHASE,
+				PROGRESS_VACUUM_NUM_INDEX_VACUUMS
+			};
+			int64		hvp_val[2];
+
+			/*
+			 * Before beginning index vacuuming, we release any pin we may
+			 * hold on the visibility map page.  This isn't necessary for
+			 * correctness, but we do it anyway to avoid holding the pin
+			 * across a lengthy, unrelated operation.
+			 */
+			if (BufferIsValid(vmbuffer))
+			{
+				ReleaseBuffer(vmbuffer);
+				vmbuffer = InvalidBuffer;
+			}
+
+			/* Log cleanup info before we touch indexes */
+			vacuum_log_cleanup_info(onerel, vacrelstats);
+
+			/* Report that we are now vacuuming indexes */
+			pgstat_progress_update_param(PROGRESS_VACUUM_PHASE,
+										 PROGRESS_VACUUM_PHASE_VACUUM_INDEX);
+
+			/* Remove index entries */
+			for (i = 0; i < nindexes; i++)
+				lazy_vacuum_index(Irel[i],
+								  &indstats[i],
+								  vacrelstats);
+
+			/*
+			 * Report that we are now vacuuming the heap.  We also increase
+			 * the number of index scans here; note that by using
+			 * pgstat_progress_update_multi_param we can update both
+			 * parameters atomically.
+			 */
+			hvp_val[0] = PROGRESS_VACUUM_PHASE_VACUUM_HEAP;
+			hvp_val[1] = vacrelstats->num_index_scans + 1;
+			pgstat_progress_update_multi_param(2, hvp_index, hvp_val);
+
+			/* Remove tuples from heap */
+			lazy_vacuum_heap(onerel, vacrelstats);
+
+			/*
+			 * Forget the now-vacuumed tuples, and press on, but be careful
+			 * not to reset latestRemovedXid since we want that value to be
+			 * valid.
+			 */
+			vacrelstats->num_dead_tuples = 0;
+			vacrelstats->num_index_scans++;
+
+			/*
+			 * Vacuum the Free Space Map to make newly-freed space visible on
+			 * upper-level FSM pages.  Note we have not yet processed blkno.
+			 */
+			FreeSpaceMapVacuumRange(onerel, next_fsm_block_to_vacuum, blkno);
+			next_fsm_block_to_vacuum = blkno;
+
+			/* Report that we are once again scanning the heap */
+			pgstat_progress_update_param(PROGRESS_VACUUM_PHASE,
+										 PROGRESS_VACUUM_PHASE_SCAN_HEAP);
+		}
+
+		/*
+		 * Pin the visibility map page in case we need to mark the page
+		 * all-visible.  In most cases this will be very cheap, because we'll
+		 * already have the correct page pinned anyway.  However, it's
+		 * possible that (a) next_unskippable_block is covered by a different
+		 * VM page than the current block or (b) we released our pin and did a
+		 * cycle of index vacuuming.
+		 *
+		 */
+		visibilitymap_pin(onerel, blkno, &vmbuffer);
+
+		buf = ReadBufferExtended(onerel, MAIN_FORKNUM, blkno,
+								 RBM_NORMAL, vac_strategy);
+
+		/* We need buffer cleanup lock so that we can prune HOT chains. */
+		if (!ConditionalLockBufferForCleanup(buf))
+		{
+			/*
+			 * If we're not performing an aggressive scan to guard against XID
+			 * wraparound, and we don't want to forcibly check the page, then
+			 * it's OK to skip vacuuming pages we get a lock conflict on. They
+			 * will be dealt with in some future vacuum.
+			 */
+			if (!aggressive && !FORCE_CHECK_PAGE())
+			{
+				ReleaseBuffer(buf);
+				vacrelstats->pinskipped_pages++;
+				continue;
+			}
+
+			/*
+			 * Read the page with share lock to see if any xids on it need to
+			 * be frozen.  If not we just skip the page, after updating our
+			 * scan statistics.  If there are some, we wait for cleanup lock.
+			 *
+			 * We could defer the lock request further by remembering the page
+			 * and coming back to it later, or we could even register
+			 * ourselves for multiple buffers and then service whichever one
+			 * is received first.  For now, this seems good enough.
+			 *
+			 * If we get here with aggressive false, then we're just forcibly
+			 * checking the page, and so we don't want to insist on getting
+			 * the lock; we only need to know if the page contains tuples, so
+			 * that we can update nonempty_pages correctly.  It's convenient
+			 * to use lazy_check_needs_freeze() for both situations, though.
+			 */
+			LockBuffer(buf, BUFFER_LOCK_SHARE);
+			if (!lazy_check_needs_freeze(buf, &hastup))
+			{
+				UnlockReleaseBuffer(buf);
+				vacrelstats->scanned_pages++;
+				vacrelstats->pinskipped_pages++;
+				if (hastup)
+					vacrelstats->nonempty_pages = blkno + 1;
+				continue;
+			}
+			if (!aggressive)
+			{
+				/*
+				 * Here, we must not advance scanned_pages; that would amount
+				 * to claiming that the page contains no freezable tuples.
+				 */
+				UnlockReleaseBuffer(buf);
+				vacrelstats->pinskipped_pages++;
+				if (hastup)
+					vacrelstats->nonempty_pages = blkno + 1;
+				continue;
+			}
+			LockBuffer(buf, BUFFER_LOCK_UNLOCK);
+			LockBufferForCleanup(buf);
+			/* drop through to normal processing */
+		}
+
+		vacrelstats->scanned_pages++;
+		vacrelstats->tupcount_pages++;
+
+		page = BufferGetPage(buf);
+
+		if (PageIsNew(page))
+		{
+			/*
+			 * An all-zeroes page could be left over if a backend extends the
+			 * relation but crashes before initializing the page. Reclaim such
+			 * pages for use.
+			 *
+			 * We have to be careful here because we could be looking at a
+			 * page that someone has just added to the relation and not yet
+			 * been able to initialize (see RelationGetBufferForTuple). To
+			 * protect against that, release the buffer lock, grab the
+			 * relation extension lock momentarily, and re-lock the buffer. If
+			 * the page is still uninitialized by then, it must be left over
+			 * from a crashed backend, and we can initialize it.
+			 *
+			 * We don't really need the relation lock when this is a new or
+			 * temp relation, but it's probably not worth the code space to
+			 * check that, since this surely isn't a critical path.
+			 *
+			 * Note: the comparable code in vacuum.c need not worry because
+			 * it's got exclusive lock on the whole relation.
+			 */
+			LockBuffer(buf, BUFFER_LOCK_UNLOCK);
+			LockRelationForExtension(onerel, ExclusiveLock);
+			UnlockRelationForExtension(onerel, ExclusiveLock);
+			LockBufferForCleanup(buf);
+			if (PageIsNew(page))
+			{
+				ereport(WARNING,
+						(errmsg("relation \"%s\" page %u is uninitialized --- fixing",
+								relname, blkno)));
+				PageInit(page, BufferGetPageSize(buf), 0);
+				empty_pages++;
+			}
+			freespace = PageGetHeapFreeSpace(page);
+			MarkBufferDirty(buf);
+			UnlockReleaseBuffer(buf);
+
+			RecordPageWithFreeSpace(onerel, blkno, freespace);
+			continue;
+		}
+
+		if (PageIsEmpty(page))
+		{
+			empty_pages++;
+			freespace = PageGetHeapFreeSpace(page);
+
+			/* empty pages are always all-visible and all-frozen */
+			if (!PageIsAllVisible(page))
+			{
+				START_CRIT_SECTION();
+
+				/* mark buffer dirty before writing a WAL record */
+				MarkBufferDirty(buf);
+
+				/*
+				 * It's possible that another backend has extended the heap,
+				 * initialized the page, and then failed to WAL-log the page
+				 * due to an ERROR.  Since heap extension is not WAL-logged,
+				 * recovery might try to replay our record setting the page
+				 * all-visible and find that the page isn't initialized, which
+				 * will cause a PANIC.  To prevent that, check whether the
+				 * page has been previously WAL-logged, and if not, do that
+				 * now.
+				 */
+				if (RelationNeedsWAL(onerel) &&
+					PageGetLSN(page) == InvalidXLogRecPtr)
+					log_newpage_buffer(buf, true);
+
+				PageSetAllVisible(page);
+				visibilitymap_set(onerel, blkno, buf, InvalidXLogRecPtr,
+								  vmbuffer, InvalidTransactionId,
+								  VISIBILITYMAP_ALL_VISIBLE | VISIBILITYMAP_ALL_FROZEN);
+				END_CRIT_SECTION();
+			}
+
+			UnlockReleaseBuffer(buf);
+			RecordPageWithFreeSpace(onerel, blkno, freespace);
+			continue;
+		}
+
+		/*
+		 * Prune all HOT-update chains in this page.
+		 *
+		 * We count tuples removed by the pruning step as removed by VACUUM.
+		 */
+		tups_vacuumed += heap_page_prune(onerel, buf, OldestXmin, false,
+										 &vacrelstats->latestRemovedXid);
+
+		/*
+		 * Now scan the page to collect vacuumable items and check for tuples
+		 * requiring freezing.
+		 */
+		all_visible = true;
+		has_dead_tuples = false;
+		nfrozen = 0;
+		hastup = false;
+		prev_dead_count = vacrelstats->num_dead_tuples;
+		maxoff = PageGetMaxOffsetNumber(page);
+
+		/*
+		 * Note: If you change anything in the loop below, also look at
+		 * heap_page_is_all_visible to see if that needs to be changed.
+		 */
+		for (offnum = FirstOffsetNumber;
+			 offnum <= maxoff;
+			 offnum = OffsetNumberNext(offnum))
+		{
+			ItemId		itemid;
+
+			itemid = PageGetItemId(page, offnum);
+
+			/* Unused items require no processing, but we count 'em */
+			if (!ItemIdIsUsed(itemid))
+			{
+				nunused += 1;
+				continue;
+			}
+
+			/* Redirect items mustn't be touched */
+			if (ItemIdIsRedirected(itemid))
+			{
+				hastup = true;	/* this page won't be truncatable */
+				continue;
+			}
+
+			ItemPointerSet(&(tuple.t_self), blkno, offnum);
+
+			/*
+			 * DEAD item pointers are to be vacuumed normally; but we don't
+			 * count them in tups_vacuumed, else we'd be double-counting (at
+			 * least in the common case where heap_page_prune() just freed up
+			 * a non-HOT tuple).
+			 */
+			if (ItemIdIsDead(itemid))
+			{
+				lazy_record_dead_tuple(vacrelstats, &(tuple.t_self));
+				all_visible = false;
+				continue;
+			}
+
+			Assert(ItemIdIsNormal(itemid));
+
+			tuple.t_data = (HeapTupleHeader) PageGetItem(page, itemid);
+			tuple.t_len = ItemIdGetLength(itemid);
+			tuple.t_tableOid = RelationGetRelid(onerel);
+
+			tupgone = false;
+
+			/*
+			 * 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).
+			 */
+			switch (HeapTupleSatisfiesVacuum(&tuple, OldestXmin, buf))
+			{
+				case HEAPTUPLE_DEAD:
+
+					/*
+					 * Ordinarily, DEAD tuples would have been removed by
+					 * heap_page_prune(), but it's possible that the tuple
+					 * state changed since heap_page_prune() looked.  In
+					 * particular an INSERT_IN_PROGRESS tuple could have
+					 * changed to DEAD if the inserter aborted.  So this
+					 * cannot be considered an error condition.
+					 *
+					 * If the tuple is HOT-updated then it must only be
+					 * removed by a prune operation; so we keep it just as if
+					 * it were RECENTLY_DEAD.  Also, if it's a heap-only
+					 * tuple, we choose to keep it, because it'll be a lot
+					 * cheaper to get rid of it in the next pruning pass than
+					 * to treat it like an indexed tuple.
+					 *
+					 * If this were to happen for a tuple that actually needed
+					 * to be deleted, we'd be in trouble, because it'd
+					 * possibly leave a tuple below the relation's xmin
+					 * horizon alive.  heap_prepare_freeze_tuple() is prepared
+					 * to detect that case and abort the transaction,
+					 * preventing corruption.
+					 */
+					if (HeapTupleIsHotUpdated(&tuple) ||
+						HeapTupleIsHeapOnly(&tuple))
+						nkeep += 1;
+					else
+						tupgone = true; /* we can delete the tuple */
+					all_visible = false;
+					break;
+				case HEAPTUPLE_LIVE:
+					/*
+					 * Count it as live.  Not only is this natural, but it's
+					 * also what acquire_sample_rows() does.
+					 */
+					live_tuples += 1;
+
+					/*
+					 * 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 (all_visible)
+					{
+						TransactionId xmin;
+
+						if (!HeapTupleHeaderXminCommitted(tuple.t_data))
+						{
+							all_visible = false;
+							break;
+						}
+
+						/*
+						 * The inserter definitely committed. But is it old
+						 * enough that everyone sees it as committed?
+						 */
+						xmin = HeapTupleHeaderGetXmin(tuple.t_data);
+						if (!TransactionIdPrecedes(xmin, OldestXmin))
+						{
+							all_visible = false;
+							break;
+						}
+
+						/* Track newest xmin on page. */
+						if (TransactionIdFollows(xmin, visibility_cutoff_xid))
+							visibility_cutoff_xid = xmin;
+					}
+					break;
+				case HEAPTUPLE_RECENTLY_DEAD:
+
+					/*
+					 * If tuple is recently deleted then we must not remove it
+					 * from relation.
+					 */
+					nkeep += 1;
+					all_visible = false;
+					break;
+				case HEAPTUPLE_INSERT_IN_PROGRESS:
+
+					/*
+					 * This is an expected case during concurrent vacuum.
+					 *
+					 * 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.
+					 */
+					all_visible = false;
+					break;
+				case HEAPTUPLE_DELETE_IN_PROGRESS:
+					/* This is an expected case during concurrent vacuum */
+					all_visible = false;
+
+					/*
+					 * Count such rows as live.  As above, we assume the
+					 * deleting transaction will commit and update the
+					 * counters after we report.
+					 */
+					live_tuples += 1;
+					break;
+				default:
+					elog(ERROR, "unexpected HeapTupleSatisfiesVacuum result");
+					break;
+			}
+
+			if (tupgone)
+			{
+				lazy_record_dead_tuple(vacrelstats, &(tuple.t_self));
+				HeapTupleHeaderAdvanceLatestRemovedXid(tuple.t_data,
+													   &vacrelstats->latestRemovedXid);
+				tups_vacuumed += 1;
+				has_dead_tuples = true;
+			}
+			else
+			{
+				bool		tuple_totally_frozen;
+
+				num_tuples += 1;
+				hastup = true;
+
+				/*
+				 * Each non-removable tuple must be checked to see if it needs
+				 * freezing.  Note we already have exclusive buffer lock.
+				 */
+				if (heap_prepare_freeze_tuple(tuple.t_data,
+											  relfrozenxid, relminmxid,
+											  FreezeLimit, MultiXactCutoff,
+											  &frozen[nfrozen],
+											  &tuple_totally_frozen))
+					frozen[nfrozen++].offset = offnum;
+
+				if (!tuple_totally_frozen)
+					all_frozen = false;
+			}
+		}						/* scan along page */
+
+		/*
+		 * If we froze any tuples, mark the buffer dirty, and write a WAL
+		 * record recording the changes.  We must log the changes to be
+		 * crash-safe against future truncation of CLOG.
+		 */
+		if (nfrozen > 0)
+		{
+			START_CRIT_SECTION();
+
+			MarkBufferDirty(buf);
+
+			/* execute collected freezes */
+			for (i = 0; i < nfrozen; i++)
+			{
+				ItemId		itemid;
+				HeapTupleHeader htup;
+
+				itemid = PageGetItemId(page, frozen[i].offset);
+				htup = (HeapTupleHeader) PageGetItem(page, itemid);
+
+				heap_execute_freeze_tuple(htup, &frozen[i]);
+			}
+
+			/* Now WAL-log freezing if necessary */
+			if (RelationNeedsWAL(onerel))
+			{
+				XLogRecPtr	recptr;
+
+				recptr = log_heap_freeze(onerel, buf, FreezeLimit,
+										 frozen, nfrozen);
+				PageSetLSN(page, recptr);
+			}
+
+			END_CRIT_SECTION();
+		}
+
+		/*
+		 * If there are no indexes then we can vacuum the page right now
+		 * instead of doing a second scan.
+		 */
+		if (nindexes == 0 &&
+			vacrelstats->num_dead_tuples > 0)
+		{
+			/* Remove tuples from heap */
+			lazy_vacuum_page(onerel, blkno, buf, 0, vacrelstats, &vmbuffer);
+			has_dead_tuples = false;
+
+			/*
+			 * Forget the now-vacuumed tuples, and press on, but be careful
+			 * not to reset latestRemovedXid since we want that value to be
+			 * valid.
+			 */
+			vacrelstats->num_dead_tuples = 0;
+			vacuumed_pages++;
+
+			/*
+			 * Periodically do incremental FSM vacuuming to make newly-freed
+			 * space visible on upper FSM pages.  Note: although we've cleaned
+			 * the current block, we haven't yet updated its FSM entry (that
+			 * happens further down), so passing end == blkno is correct.
+			 */
+			if (blkno - next_fsm_block_to_vacuum >= VACUUM_FSM_EVERY_PAGES)
+			{
+				FreeSpaceMapVacuumRange(onerel, next_fsm_block_to_vacuum,
+										blkno);
+				next_fsm_block_to_vacuum = blkno;
+			}
+		}
+
+		freespace = PageGetHeapFreeSpace(page);
+
+		/* mark page all-visible, if appropriate */
+		if (all_visible && !all_visible_according_to_vm)
+		{
+			uint8		flags = VISIBILITYMAP_ALL_VISIBLE;
+
+			if (all_frozen)
+				flags |= VISIBILITYMAP_ALL_FROZEN;
+
+			/*
+			 * It should never be the case that the visibility map page is set
+			 * while the page-level bit is clear, but the reverse is allowed
+			 * (if checksums are not enabled).  Regardless, set the both bits
+			 * so that we get back in sync.
+			 *
+			 * NB: If the heap page is all-visible but the VM bit is not set,
+			 * we don't need to dirty the heap page.  However, if checksums
+			 * are enabled, we do need to make sure that the heap page is
+			 * dirtied before passing it to visibilitymap_set(), because it
+			 * may be logged.  Given that this situation should only happen in
+			 * rare cases after a crash, it is not worth optimizing.
+			 */
+			PageSetAllVisible(page);
+			MarkBufferDirty(buf);
+			visibilitymap_set(onerel, blkno, buf, InvalidXLogRecPtr,
+							  vmbuffer, visibility_cutoff_xid, flags);
+		}
+
+		/*
+		 * As of PostgreSQL 9.2, the visibility map bit should never be set if
+		 * the page-level bit is clear.  However, it's possible that the bit
+		 * got cleared after we checked it and before we took the buffer
+		 * content lock, so we must recheck before jumping to the conclusion
+		 * that something bad has happened.
+		 */
+		else if (all_visible_according_to_vm && !PageIsAllVisible(page)
+				 && VM_ALL_VISIBLE(onerel, blkno, &vmbuffer))
+		{
+			elog(WARNING, "page is not marked all-visible but visibility map bit is set in relation \"%s\" page %u",
+				 relname, blkno);
+			visibilitymap_clear(onerel, blkno, vmbuffer,
+								VISIBILITYMAP_VALID_BITS);
+		}
+
+		/*
+		 * It's possible for the value returned by GetOldestXmin() to move
+		 * backwards, so it's not wrong for us to see tuples that appear to
+		 * not be visible to everyone yet, while PD_ALL_VISIBLE is already
+		 * set. The real safe xmin value never moves backwards, but
+		 * GetOldestXmin() is conservative and sometimes returns a value
+		 * that's unnecessarily small, so if we see that contradiction it just
+		 * means that the tuples that we think are not visible to everyone yet
+		 * actually are, and the PD_ALL_VISIBLE flag is correct.
+		 *
+		 * There should never be dead tuples on a page with PD_ALL_VISIBLE
+		 * set, however.
+		 */
+		else if (PageIsAllVisible(page) && has_dead_tuples)
+		{
+			elog(WARNING, "page containing dead tuples is marked as all-visible in relation \"%s\" page %u",
+				 relname, blkno);
+			PageClearAllVisible(page);
+			MarkBufferDirty(buf);
+			visibilitymap_clear(onerel, blkno, vmbuffer,
+								VISIBILITYMAP_VALID_BITS);
+		}
+
+		/*
+		 * If the all-visible page is turned out to be all-frozen but not
+		 * marked, we should so mark it.  Note that all_frozen is only valid
+		 * if all_visible is true, so we must check both.
+		 */
+		else if (all_visible_according_to_vm && all_visible && all_frozen &&
+				 !VM_ALL_FROZEN(onerel, blkno, &vmbuffer))
+		{
+			/*
+			 * We can pass InvalidTransactionId as the cutoff XID here,
+			 * because setting the all-frozen bit doesn't cause recovery
+			 * conflicts.
+			 */
+			visibilitymap_set(onerel, blkno, buf, InvalidXLogRecPtr,
+							  vmbuffer, InvalidTransactionId,
+							  VISIBILITYMAP_ALL_FROZEN);
+		}
+
+		UnlockReleaseBuffer(buf);
+
+		/* Remember the location of the last page with nonremovable tuples */
+		if (hastup)
+			vacrelstats->nonempty_pages = blkno + 1;
+
+		/*
+		 * If we remembered any tuples for deletion, then the page will be
+		 * visited again by lazy_vacuum_heap, which will compute and record
+		 * its post-compaction free space.  If not, then we're done with this
+		 * page, so remember its free space as-is.  (This path will always be
+		 * taken if there are no indexes.)
+		 */
+		if (vacrelstats->num_dead_tuples == prev_dead_count)
+			RecordPageWithFreeSpace(onerel, blkno, freespace);
+	}
+
+	/* report that everything is scanned and vacuumed */
+	pgstat_progress_update_param(PROGRESS_VACUUM_HEAP_BLKS_SCANNED, blkno);
+
+	pfree(frozen);
+
+	/* save stats for use later */
+	vacrelstats->tuples_deleted = tups_vacuumed;
+	vacrelstats->new_dead_tuples = nkeep;
+
+	/* now we can compute the new value for pg_class.reltuples */
+	vacrelstats->new_live_tuples = vac_estimate_reltuples(onerel,
+														  nblocks,
+														  vacrelstats->tupcount_pages,
+														  live_tuples);
+
+	/* also compute total number of surviving heap entries */
+	vacrelstats->new_rel_tuples =
+		vacrelstats->new_live_tuples + vacrelstats->new_dead_tuples;
+
+	/*
+	 * Release any remaining pin on visibility map page.
+	 */
+	if (BufferIsValid(vmbuffer))
+	{
+		ReleaseBuffer(vmbuffer);
+		vmbuffer = InvalidBuffer;
+	}
+
+	/* If any tuples need to be deleted, perform final vacuum cycle */
+	/* XXX put a threshold on min number of tuples here? */
+	if (vacrelstats->num_dead_tuples > 0)
+	{
+		const int	hvp_index[] = {
+			PROGRESS_VACUUM_PHASE,
+			PROGRESS_VACUUM_NUM_INDEX_VACUUMS
+		};
+		int64		hvp_val[2];
+
+		/* Log cleanup info before we touch indexes */
+		vacuum_log_cleanup_info(onerel, vacrelstats);
+
+		/* Report that we are now vacuuming indexes */
+		pgstat_progress_update_param(PROGRESS_VACUUM_PHASE,
+									 PROGRESS_VACUUM_PHASE_VACUUM_INDEX);
+
+		/* Remove index entries */
+		for (i = 0; i < nindexes; i++)
+			lazy_vacuum_index(Irel[i],
+							  &indstats[i],
+							  vacrelstats);
+
+		/* Report that we are now vacuuming the heap */
+		hvp_val[0] = PROGRESS_VACUUM_PHASE_VACUUM_HEAP;
+		hvp_val[1] = vacrelstats->num_index_scans + 1;
+		pgstat_progress_update_multi_param(2, hvp_index, hvp_val);
+
+		/* Remove tuples from heap */
+		pgstat_progress_update_param(PROGRESS_VACUUM_PHASE,
+									 PROGRESS_VACUUM_PHASE_VACUUM_HEAP);
+		lazy_vacuum_heap(onerel, vacrelstats);
+		vacrelstats->num_index_scans++;
+	}
+
+	/*
+	 * Vacuum the remainder of the Free Space Map.  We must do this whether or
+	 * not there were indexes.
+	 */
+	if (blkno > next_fsm_block_to_vacuum)
+		FreeSpaceMapVacuumRange(onerel, next_fsm_block_to_vacuum, blkno);
+
+	/* report all blocks vacuumed; and that we're cleaning up */
+	pgstat_progress_update_param(PROGRESS_VACUUM_HEAP_BLKS_VACUUMED, blkno);
+	pgstat_progress_update_param(PROGRESS_VACUUM_PHASE,
+								 PROGRESS_VACUUM_PHASE_INDEX_CLEANUP);
+
+	/* Do post-vacuum cleanup and statistics update for each index */
+	for (i = 0; i < nindexes; i++)
+		lazy_cleanup_index(Irel[i], indstats[i], vacrelstats);
+
+	/* If no indexes, make log report that lazy_vacuum_heap would've made */
+	if (vacuumed_pages)
+		ereport(elevel,
+				(errmsg("\"%s\": removed %.0f row versions in %u pages",
+						RelationGetRelationName(onerel),
+						tups_vacuumed, vacuumed_pages)));
+
+	/*
+	 * This is pretty messy, but we split it up so that we can skip emitting
+	 * individual parts of the message when not applicable.
+	 */
+	initStringInfo(&buf);
+	appendStringInfo(&buf,
+					 _("%.0f dead row versions cannot be removed yet, oldest xmin: %u\n"),
+					 nkeep, OldestXmin);
+	appendStringInfo(&buf, _("There were %.0f unused item pointers.\n"),
+					 nunused);
+	appendStringInfo(&buf, ngettext("Skipped %u page due to buffer pins, ",
+									"Skipped %u pages due to buffer pins, ",
+									vacrelstats->pinskipped_pages),
+					 vacrelstats->pinskipped_pages);
+	appendStringInfo(&buf, ngettext("%u frozen page.\n",
+									"%u frozen pages.\n",
+									vacrelstats->frozenskipped_pages),
+					 vacrelstats->frozenskipped_pages);
+	appendStringInfo(&buf, ngettext("%u page is entirely empty.\n",
+									"%u pages are entirely empty.\n",
+									empty_pages),
+					 empty_pages);
+	appendStringInfo(&buf, _("%s."), pg_rusage_show(&ru0));
+
+	ereport(elevel,
+			(errmsg("\"%s\": found %.0f removable, %.0f nonremovable row versions in %u out of %u pages",
+					RelationGetRelationName(onerel),
+					tups_vacuumed, num_tuples,
+					vacrelstats->scanned_pages, nblocks),
+			 errdetail_internal("%s", buf.data)));
+	pfree(buf.data);
+}
+
+
+/*
+ *	lazy_vacuum_heap() -- second pass over the heap
+ *
+ *		This routine marks dead tuples as unused and compacts out free
+ *		space on their pages.  Pages not having dead tuples recorded from
+ *		lazy_scan_heap are not visited at all.
+ *
+ * Note: the reason for doing this as a second pass is we cannot remove
+ * the tuples until we've removed their index entries, and we want to
+ * process index entry removal in batches as large as possible.
+ */
+static void
+lazy_vacuum_heap(Relation onerel, LVRelStats *vacrelstats)
+{
+	int			tupindex;
+	int			npages;
+	PGRUsage	ru0;
+	Buffer		vmbuffer = InvalidBuffer;
+
+	pg_rusage_init(&ru0);
+	npages = 0;
+
+	tupindex = 0;
+	while (tupindex < vacrelstats->num_dead_tuples)
+	{
+		BlockNumber tblk;
+		Buffer		buf;
+		Page		page;
+		Size		freespace;
+
+		vacuum_delay_point();
+
+		tblk = ItemPointerGetBlockNumber(&vacrelstats->dead_tuples[tupindex]);
+		buf = ReadBufferExtended(onerel, MAIN_FORKNUM, tblk, RBM_NORMAL,
+								 vac_strategy);
+		if (!ConditionalLockBufferForCleanup(buf))
+		{
+			ReleaseBuffer(buf);
+			++tupindex;
+			continue;
+		}
+		tupindex = lazy_vacuum_page(onerel, tblk, buf, tupindex, vacrelstats,
+									&vmbuffer);
+
+		/* Now that we've compacted the page, record its available space */
+		page = BufferGetPage(buf);
+		freespace = PageGetHeapFreeSpace(page);
+
+		UnlockReleaseBuffer(buf);
+		RecordPageWithFreeSpace(onerel, tblk, freespace);
+		npages++;
+	}
+
+	if (BufferIsValid(vmbuffer))
+	{
+		ReleaseBuffer(vmbuffer);
+		vmbuffer = InvalidBuffer;
+	}
+
+	ereport(elevel,
+			(errmsg("\"%s\": removed %d row versions in %d pages",
+					RelationGetRelationName(onerel),
+					tupindex, npages),
+			 errdetail_internal("%s", pg_rusage_show(&ru0))));
+}
+
+/*
+ *	lazy_vacuum_page() -- free dead tuples on a page
+ *					 and repair its fragmentation.
+ *
+ * Caller must hold pin and buffer cleanup lock on the buffer.
+ *
+ * tupindex is the index in vacrelstats->dead_tuples of the first dead
+ * tuple for this page.  We assume the rest follow sequentially.
+ * The return value is the first tupindex after the tuples of this page.
+ */
+static int
+lazy_vacuum_page(Relation onerel, BlockNumber blkno, Buffer buffer,
+				 int tupindex, LVRelStats *vacrelstats, Buffer *vmbuffer)
+{
+	Page		page = BufferGetPage(buffer);
+	OffsetNumber unused[MaxOffsetNumber];
+	int			uncnt = 0;
+	TransactionId visibility_cutoff_xid;
+	bool		all_frozen;
+
+	pgstat_progress_update_param(PROGRESS_VACUUM_HEAP_BLKS_VACUUMED, blkno);
+
+	START_CRIT_SECTION();
+
+	for (; tupindex < vacrelstats->num_dead_tuples; tupindex++)
+	{
+		BlockNumber tblk;
+		OffsetNumber toff;
+		ItemId		itemid;
+
+		tblk = ItemPointerGetBlockNumber(&vacrelstats->dead_tuples[tupindex]);
+		if (tblk != blkno)
+			break;				/* past end of tuples for this block */
+		toff = ItemPointerGetOffsetNumber(&vacrelstats->dead_tuples[tupindex]);
+		itemid = PageGetItemId(page, toff);
+		ItemIdSetUnused(itemid);
+		unused[uncnt++] = toff;
+	}
+
+	PageRepairFragmentation(page);
+
+	/*
+	 * Mark buffer dirty before we write WAL.
+	 */
+	MarkBufferDirty(buffer);
+
+	/* XLOG stuff */
+	if (RelationNeedsWAL(onerel))
+	{
+		XLogRecPtr	recptr;
+
+		recptr = log_heap_clean(onerel, buffer,
+								NULL, 0, NULL, 0,
+								unused, uncnt,
+								vacrelstats->latestRemovedXid);
+		PageSetLSN(page, recptr);
+	}
+
+	/*
+	 * End critical section, so we safely can do visibility tests (which
+	 * possibly need to perform IO and allocate memory!). If we crash now the
+	 * page (including the corresponding vm bit) might not be marked all
+	 * visible, but that's fine. A later vacuum will fix that.
+	 */
+	END_CRIT_SECTION();
+
+	/*
+	 * Now that we have removed the dead tuples from the page, once again
+	 * check if the page has become all-visible.  The page is already marked
+	 * dirty, exclusively locked, and, if needed, a full page image has been
+	 * emitted in the log_heap_clean() above.
+	 */
+	if (heap_page_is_all_visible(onerel, buffer, &visibility_cutoff_xid,
+								 &all_frozen))
+		PageSetAllVisible(page);
+
+	/*
+	 * All the changes to the heap page have been done. If the all-visible
+	 * flag is now set, also set the VM all-visible bit (and, if possible, the
+	 * all-frozen bit) unless this has already been done previously.
+	 */
+	if (PageIsAllVisible(page))
+	{
+		uint8		vm_status = visibilitymap_get_status(onerel, blkno, vmbuffer);
+		uint8		flags = 0;
+
+		/* Set the VM all-frozen bit to flag, if needed */
+		if ((vm_status & VISIBILITYMAP_ALL_VISIBLE) == 0)
+			flags |= VISIBILITYMAP_ALL_VISIBLE;
+		if ((vm_status & VISIBILITYMAP_ALL_FROZEN) == 0 && all_frozen)
+			flags |= VISIBILITYMAP_ALL_FROZEN;
+
+		Assert(BufferIsValid(*vmbuffer));
+		if (flags != 0)
+			visibilitymap_set(onerel, blkno, buffer, InvalidXLogRecPtr,
+							  *vmbuffer, visibility_cutoff_xid, flags);
+	}
+
+	return tupindex;
+}
+
+/*
+ *	lazy_check_needs_freeze() -- scan page to see if any tuples
+ *					 need to be cleaned to avoid wraparound
+ *
+ * Returns true if the page needs to be vacuumed using cleanup lock.
+ * Also returns a flag indicating whether page contains any tuples at all.
+ */
+static bool
+lazy_check_needs_freeze(Buffer buf, bool *hastup)
+{
+	Page		page = BufferGetPage(buf);
+	OffsetNumber offnum,
+				maxoff;
+	HeapTupleHeader tupleheader;
+
+	*hastup = false;
+
+	/* If we hit an uninitialized page, we want to force vacuuming it. */
+	if (PageIsNew(page))
+		return true;
+
+	/* Quick out for ordinary empty page. */
+	if (PageIsEmpty(page))
+		return false;
+
+	maxoff = PageGetMaxOffsetNumber(page);
+	for (offnum = FirstOffsetNumber;
+		 offnum <= maxoff;
+		 offnum = OffsetNumberNext(offnum))
+	{
+		ItemId		itemid;
+
+		itemid = PageGetItemId(page, offnum);
+
+		/* this should match hastup test in count_nondeletable_pages() */
+		if (ItemIdIsUsed(itemid))
+			*hastup = true;
+
+		/* dead and redirect items never need freezing */
+		if (!ItemIdIsNormal(itemid))
+			continue;
+
+		tupleheader = (HeapTupleHeader) PageGetItem(page, itemid);
+
+		if (heap_tuple_needs_freeze(tupleheader, FreezeLimit,
+									MultiXactCutoff, buf))
+			return true;
+	}							/* scan along page */
+
+	return false;
+}
+
+
+/*
+ *	lazy_vacuum_index() -- vacuum one index relation.
+ *
+ *		Delete all the index entries pointing to tuples listed in
+ *		vacrelstats->dead_tuples, and update running statistics.
+ */
+static void
+lazy_vacuum_index(Relation indrel,
+				  IndexBulkDeleteResult **stats,
+				  LVRelStats *vacrelstats)
+{
+	IndexVacuumInfo ivinfo;
+	PGRUsage	ru0;
+
+	pg_rusage_init(&ru0);
+
+	ivinfo.index = indrel;
+	ivinfo.analyze_only = false;
+	ivinfo.estimated_count = true;
+	ivinfo.message_level = elevel;
+	/* We can only provide an approximate value of num_heap_tuples here */
+	ivinfo.num_heap_tuples = vacrelstats->old_live_tuples;
+	ivinfo.strategy = vac_strategy;
+
+	/* Do bulk deletion */
+	*stats = index_bulk_delete(&ivinfo, *stats,
+							   lazy_tid_reaped, (void *) vacrelstats);
+
+	ereport(elevel,
+			(errmsg("scanned index \"%s\" to remove %d row versions",
+					RelationGetRelationName(indrel),
+					vacrelstats->num_dead_tuples),
+			 errdetail_internal("%s", pg_rusage_show(&ru0))));
+}
+
+/*
+ *	lazy_cleanup_index() -- do post-vacuum cleanup for one index relation.
+ */
+static void
+lazy_cleanup_index(Relation indrel,
+				   IndexBulkDeleteResult *stats,
+				   LVRelStats *vacrelstats)
+{
+	IndexVacuumInfo ivinfo;
+	PGRUsage	ru0;
+
+	pg_rusage_init(&ru0);
+
+	ivinfo.index = indrel;
+	ivinfo.analyze_only = false;
+	ivinfo.estimated_count = (vacrelstats->tupcount_pages < vacrelstats->rel_pages);
+	ivinfo.message_level = elevel;
+
+	/*
+	 * Now we can provide a better estimate of total number of surviving
+	 * tuples (we assume indexes are more interested in that than in the
+	 * number of nominally live tuples).
+	 */
+	ivinfo.num_heap_tuples = vacrelstats->new_rel_tuples;
+	ivinfo.strategy = vac_strategy;
+
+	stats = index_vacuum_cleanup(&ivinfo, stats);
+
+	if (!stats)
+		return;
+
+	/*
+	 * Now update statistics in pg_class, but only if the index says the count
+	 * is accurate.
+	 */
+	if (!stats->estimated_count)
+		vac_update_relstats(indrel,
+							stats->num_pages,
+							stats->num_index_tuples,
+							0,
+							false,
+							InvalidTransactionId,
+							InvalidMultiXactId,
+							false);
+
+	ereport(elevel,
+			(errmsg("index \"%s\" now contains %.0f row versions in %u pages",
+					RelationGetRelationName(indrel),
+					stats->num_index_tuples,
+					stats->num_pages),
+			 errdetail("%.0f index row versions were removed.\n"
+					   "%u index pages have been deleted, %u are currently reusable.\n"
+					   "%s.",
+					   stats->tuples_removed,
+					   stats->pages_deleted, stats->pages_free,
+					   pg_rusage_show(&ru0))));
+
+	pfree(stats);
+}
+
+/*
+ * should_attempt_truncation - should we attempt to truncate the heap?
+ *
+ * Don't even think about it unless we have a shot at releasing a goodly
+ * number of pages.  Otherwise, the time taken isn't worth it.
+ *
+ * Also don't attempt it if we are doing early pruning/vacuuming, because a
+ * scan which cannot find a truncated heap page cannot determine that the
+ * snapshot is too old to read that page.  We might be able to get away with
+ * truncating all except one of the pages, setting its LSN to (at least) the
+ * maximum of the truncated range if we also treated an index leaf tuple
+ * pointing to a missing heap page as something to trigger the "snapshot too
+ * old" error, but that seems fragile and seems like it deserves its own patch
+ * if we consider it.
+ *
+ * This is split out so that we can test whether truncation is going to be
+ * called for before we actually do it.  If you change the logic here, be
+ * careful to depend only on fields that lazy_scan_heap updates on-the-fly.
+ */
+static bool
+should_attempt_truncation(LVRelStats *vacrelstats)
+{
+	BlockNumber possibly_freeable;
+
+	possibly_freeable = vacrelstats->rel_pages - vacrelstats->nonempty_pages;
+	if (possibly_freeable > 0 &&
+		(possibly_freeable >= REL_TRUNCATE_MINIMUM ||
+		 possibly_freeable >= vacrelstats->rel_pages / REL_TRUNCATE_FRACTION) &&
+		old_snapshot_threshold < 0)
+		return true;
+	else
+		return false;
+}
+
+/*
+ * lazy_truncate_heap - try to truncate off any empty pages at the end
+ */
+static void
+lazy_truncate_heap(Relation onerel, LVRelStats *vacrelstats)
+{
+	BlockNumber old_rel_pages = vacrelstats->rel_pages;
+	BlockNumber new_rel_pages;
+	PGRUsage	ru0;
+	int			lock_retry;
+
+	pg_rusage_init(&ru0);
+
+	/* Report that we are now truncating */
+	pgstat_progress_update_param(PROGRESS_VACUUM_PHASE,
+								 PROGRESS_VACUUM_PHASE_TRUNCATE);
+
+	/*
+	 * Loop until no more truncating can be done.
+	 */
+	do
+	{
+		/*
+		 * We need full exclusive lock on the relation in order to do
+		 * truncation. If we can't get it, give up rather than waiting --- we
+		 * don't want to block other backends, and we don't want to deadlock
+		 * (which is quite possible considering we already hold a lower-grade
+		 * lock).
+		 */
+		vacrelstats->lock_waiter_detected = false;
+		lock_retry = 0;
+		while (true)
+		{
+			if (ConditionalLockRelation(onerel, AccessExclusiveLock))
+				break;
+
+			/*
+			 * Check for interrupts while trying to (re-)acquire the exclusive
+			 * lock.
+			 */
+			CHECK_FOR_INTERRUPTS();
+
+			if (++lock_retry > (VACUUM_TRUNCATE_LOCK_TIMEOUT /
+								VACUUM_TRUNCATE_LOCK_WAIT_INTERVAL))
+			{
+				/*
+				 * We failed to establish the lock in the specified number of
+				 * retries. This means we give up truncating.
+				 */
+				vacrelstats->lock_waiter_detected = true;
+				ereport(elevel,
+						(errmsg("\"%s\": stopping truncate due to conflicting lock request",
+								RelationGetRelationName(onerel))));
+				return;
+			}
+
+			pg_usleep(VACUUM_TRUNCATE_LOCK_WAIT_INTERVAL * 1000L);
+		}
+
+		/*
+		 * Now that we have exclusive lock, look to see if the rel has grown
+		 * whilst we were vacuuming with non-exclusive lock.  If so, give up;
+		 * the newly added pages presumably contain non-deletable tuples.
+		 */
+		new_rel_pages = RelationGetNumberOfBlocks(onerel);
+		if (new_rel_pages != old_rel_pages)
+		{
+			/*
+			 * Note: we intentionally don't update vacrelstats->rel_pages with
+			 * the new rel size here.  If we did, it would amount to assuming
+			 * that the new pages are empty, which is unlikely. Leaving the
+			 * numbers alone amounts to assuming that the new pages have the
+			 * same tuple density as existing ones, which is less unlikely.
+			 */
+			UnlockRelation(onerel, AccessExclusiveLock);
+			return;
+		}
+
+		/*
+		 * Scan backwards from the end to verify that the end pages actually
+		 * contain no tuples.  This is *necessary*, not optional, because
+		 * other backends could have added tuples to these pages whilst we
+		 * were vacuuming.
+		 */
+		new_rel_pages = count_nondeletable_pages(onerel, vacrelstats);
+
+		if (new_rel_pages >= old_rel_pages)
+		{
+			/* can't do anything after all */
+			UnlockRelation(onerel, AccessExclusiveLock);
+			return;
+		}
+
+		/*
+		 * Okay to truncate.
+		 */
+		RelationTruncate(onerel, new_rel_pages);
+
+		/*
+		 * We can release the exclusive lock as soon as we have truncated.
+		 * Other backends can't safely access the relation until they have
+		 * processed the smgr invalidation that smgrtruncate sent out ... but
+		 * that should happen as part of standard invalidation processing once
+		 * they acquire lock on the relation.
+		 */
+		UnlockRelation(onerel, AccessExclusiveLock);
+
+		/*
+		 * Update statistics.  Here, it *is* correct to adjust rel_pages
+		 * without also touching reltuples, since the tuple count wasn't
+		 * changed by the truncation.
+		 */
+		vacrelstats->pages_removed += old_rel_pages - new_rel_pages;
+		vacrelstats->rel_pages = new_rel_pages;
+
+		ereport(elevel,
+				(errmsg("\"%s\": truncated %u to %u pages",
+						RelationGetRelationName(onerel),
+						old_rel_pages, new_rel_pages),
+				 errdetail_internal("%s",
+									pg_rusage_show(&ru0))));
+		old_rel_pages = new_rel_pages;
+	} while (new_rel_pages > vacrelstats->nonempty_pages &&
+			 vacrelstats->lock_waiter_detected);
+}
+
+/*
+ * Rescan end pages to verify that they are (still) empty of tuples.
+ *
+ * Returns number of nondeletable pages (last nonempty page + 1).
+ */
+static BlockNumber
+count_nondeletable_pages(Relation onerel, LVRelStats *vacrelstats)
+{
+	BlockNumber blkno;
+	BlockNumber prefetchedUntil;
+	instr_time	starttime;
+
+	/* Initialize the starttime if we check for conflicting lock requests */
+	INSTR_TIME_SET_CURRENT(starttime);
+
+	/*
+	 * Start checking blocks at what we believe relation end to be and move
+	 * backwards.  (Strange coding of loop control is needed because blkno is
+	 * unsigned.)  To make the scan faster, we prefetch a few blocks at a time
+	 * in forward direction, so that OS-level readahead can kick in.
+	 */
+	blkno = vacrelstats->rel_pages;
+	StaticAssertStmt((PREFETCH_SIZE & (PREFETCH_SIZE - 1)) == 0,
+					 "prefetch size must be power of 2");
+	prefetchedUntil = InvalidBlockNumber;
+	while (blkno > vacrelstats->nonempty_pages)
+	{
+		Buffer		buf;
+		Page		page;
+		OffsetNumber offnum,
+					maxoff;
+		bool		hastup;
+
+		/*
+		 * Check if another process requests a lock on our relation. We are
+		 * holding an AccessExclusiveLock here, so they will be waiting. We
+		 * only do this once per VACUUM_TRUNCATE_LOCK_CHECK_INTERVAL, and we
+		 * only check if that interval has elapsed once every 32 blocks to
+		 * keep the number of system calls and actual shared lock table
+		 * lookups to a minimum.
+		 */
+		if ((blkno % 32) == 0)
+		{
+			instr_time	currenttime;
+			instr_time	elapsed;
+
+			INSTR_TIME_SET_CURRENT(currenttime);
+			elapsed = currenttime;
+			INSTR_TIME_SUBTRACT(elapsed, starttime);
+			if ((INSTR_TIME_GET_MICROSEC(elapsed) / 1000)
+				>= VACUUM_TRUNCATE_LOCK_CHECK_INTERVAL)
+			{
+				if (LockHasWaitersRelation(onerel, AccessExclusiveLock))
+				{
+					ereport(elevel,
+							(errmsg("\"%s\": suspending truncate due to conflicting lock request",
+									RelationGetRelationName(onerel))));
+
+					vacrelstats->lock_waiter_detected = true;
+					return blkno;
+				}
+				starttime = currenttime;
+			}
+		}
+
+		/*
+		 * We don't insert a vacuum delay point here, because we have an
+		 * exclusive lock on the table which we want to hold for as short a
+		 * time as possible.  We still need to check for interrupts however.
+		 */
+		CHECK_FOR_INTERRUPTS();
+
+		blkno--;
+
+		/* If we haven't prefetched this lot yet, do so now. */
+		if (prefetchedUntil > blkno)
+		{
+			BlockNumber prefetchStart;
+			BlockNumber pblkno;
+
+			prefetchStart = blkno & ~(PREFETCH_SIZE - 1);
+			for (pblkno = prefetchStart; pblkno <= blkno; pblkno++)
+			{
+				PrefetchBuffer(onerel, MAIN_FORKNUM, pblkno);
+				CHECK_FOR_INTERRUPTS();
+			}
+			prefetchedUntil = prefetchStart;
+		}
+
+		buf = ReadBufferExtended(onerel, MAIN_FORKNUM, blkno,
+								 RBM_NORMAL, vac_strategy);
+
+		/* In this phase we only need shared access to the buffer */
+		LockBuffer(buf, BUFFER_LOCK_SHARE);
+
+		page = BufferGetPage(buf);
+
+		if (PageIsNew(page) || PageIsEmpty(page))
+		{
+			/* PageIsNew probably shouldn't happen... */
+			UnlockReleaseBuffer(buf);
+			continue;
+		}
+
+		hastup = false;
+		maxoff = PageGetMaxOffsetNumber(page);
+		for (offnum = FirstOffsetNumber;
+			 offnum <= maxoff;
+			 offnum = OffsetNumberNext(offnum))
+		{
+			ItemId		itemid;
+
+			itemid = PageGetItemId(page, offnum);
+
+			/*
+			 * Note: any non-unused item should be taken as a reason to keep
+			 * this page.  We formerly thought that DEAD tuples could be
+			 * thrown away, but that's not so, because we'd not have cleaned
+			 * out their index entries.
+			 */
+			if (ItemIdIsUsed(itemid))
+			{
+				hastup = true;
+				break;			/* can stop scanning */
+			}
+		}						/* scan along page */
+
+		UnlockReleaseBuffer(buf);
+
+		/* Done scanning if we found a tuple here */
+		if (hastup)
+			return blkno + 1;
+	}
+
+	/*
+	 * If we fall out of the loop, all the previously-thought-to-be-empty
+	 * pages still are; we need not bother to look at the last known-nonempty
+	 * page.
+	 */
+	return vacrelstats->nonempty_pages;
+}
+
+/*
+ * lazy_space_alloc - space allocation decisions for lazy vacuum
+ *
+ * See the comments at the head of this file for rationale.
+ */
+static void
+lazy_space_alloc(LVRelStats *vacrelstats, BlockNumber relblocks)
+{
+	long		maxtuples;
+	int			vac_work_mem = IsAutoVacuumWorkerProcess() &&
+	autovacuum_work_mem != -1 ?
+	autovacuum_work_mem : maintenance_work_mem;
+
+	if (vacrelstats->hasindex)
+	{
+		maxtuples = (vac_work_mem * 1024L) / sizeof(ItemPointerData);
+		maxtuples = Min(maxtuples, INT_MAX);
+		maxtuples = Min(maxtuples, MaxAllocSize / sizeof(ItemPointerData));
+
+		/* curious coding here to ensure the multiplication can't overflow */
+		if ((BlockNumber) (maxtuples / LAZY_ALLOC_TUPLES) > relblocks)
+			maxtuples = relblocks * LAZY_ALLOC_TUPLES;
+
+		/* stay sane if small maintenance_work_mem */
+		maxtuples = Max(maxtuples, MaxHeapTuplesPerPage);
+	}
+	else
+	{
+		maxtuples = MaxHeapTuplesPerPage;
+	}
+
+	vacrelstats->num_dead_tuples = 0;
+	vacrelstats->max_dead_tuples = (int) maxtuples;
+	vacrelstats->dead_tuples = (ItemPointer)
+		palloc(maxtuples * sizeof(ItemPointerData));
+}
+
+/*
+ * lazy_record_dead_tuple - remember one deletable tuple
+ */
+static void
+lazy_record_dead_tuple(LVRelStats *vacrelstats,
+					   ItemPointer itemptr)
+{
+	/*
+	 * The array shouldn't overflow under normal behavior, but perhaps it
+	 * could if we are given a really small maintenance_work_mem. In that
+	 * case, just forget the last few tuples (we'll get 'em next time).
+	 */
+	if (vacrelstats->num_dead_tuples < vacrelstats->max_dead_tuples)
+	{
+		vacrelstats->dead_tuples[vacrelstats->num_dead_tuples] = *itemptr;
+		vacrelstats->num_dead_tuples++;
+		pgstat_progress_update_param(PROGRESS_VACUUM_NUM_DEAD_TUPLES,
+									 vacrelstats->num_dead_tuples);
+	}
+}
+
+/*
+ *	lazy_tid_reaped() -- is a particular tid deletable?
+ *
+ *		This has the right signature to be an IndexBulkDeleteCallback.
+ *
+ *		Assumes dead_tuples array is in sorted order.
+ */
+static bool
+lazy_tid_reaped(ItemPointer itemptr, void *state)
+{
+	LVRelStats *vacrelstats = (LVRelStats *) state;
+	ItemPointer res;
+
+	res = (ItemPointer) bsearch((void *) itemptr,
+								(void *) vacrelstats->dead_tuples,
+								vacrelstats->num_dead_tuples,
+								sizeof(ItemPointerData),
+								vac_cmp_itemptr);
+
+	return (res != NULL);
+}
+
+/*
+ * Comparator routines for use with qsort() and bsearch().
+ */
+static int
+vac_cmp_itemptr(const void *left, const void *right)
+{
+	BlockNumber lblk,
+				rblk;
+	OffsetNumber loff,
+				roff;
+
+	lblk = ItemPointerGetBlockNumber((ItemPointer) left);
+	rblk = ItemPointerGetBlockNumber((ItemPointer) right);
+
+	if (lblk < rblk)
+		return -1;
+	if (lblk > rblk)
+		return 1;
+
+	loff = ItemPointerGetOffsetNumber((ItemPointer) left);
+	roff = ItemPointerGetOffsetNumber((ItemPointer) right);
+
+	if (loff < roff)
+		return -1;
+	if (loff > roff)
+		return 1;
+
+	return 0;
+}
+
+/*
+ * Check if every tuple in the given page is visible to all current and future
+ * transactions. Also return the visibility_cutoff_xid which is the highest
+ * xmin amongst the visible tuples.  Set *all_frozen to true if every tuple
+ * on this page is frozen.
+ */
+static bool
+heap_page_is_all_visible(Relation rel, Buffer buf,
+						 TransactionId *visibility_cutoff_xid,
+						 bool *all_frozen)
+{
+	Page		page = BufferGetPage(buf);
+	BlockNumber blockno = BufferGetBlockNumber(buf);
+	OffsetNumber offnum,
+				maxoff;
+	bool		all_visible = true;
+
+	*visibility_cutoff_xid = InvalidTransactionId;
+	*all_frozen = true;
+
+	/*
+	 * This is a stripped down version of the line pointer scan in
+	 * lazy_scan_heap(). So if you change anything here, also check that code.
+	 */
+	maxoff = PageGetMaxOffsetNumber(page);
+	for (offnum = FirstOffsetNumber;
+		 offnum <= maxoff && all_visible;
+		 offnum = OffsetNumberNext(offnum))
+	{
+		ItemId		itemid;
+		HeapTupleData tuple;
+
+		itemid = PageGetItemId(page, offnum);
+
+		/* Unused or redirect line pointers are of no interest */
+		if (!ItemIdIsUsed(itemid) || ItemIdIsRedirected(itemid))
+			continue;
+
+		ItemPointerSet(&(tuple.t_self), blockno, offnum);
+
+		/*
+		 * Dead line pointers can have index pointers pointing to them. So
+		 * they can't be treated as visible
+		 */
+		if (ItemIdIsDead(itemid))
+		{
+			all_visible = false;
+			*all_frozen = false;
+			break;
+		}
+
+		Assert(ItemIdIsNormal(itemid));
+
+		tuple.t_data = (HeapTupleHeader) PageGetItem(page, itemid);
+		tuple.t_len = ItemIdGetLength(itemid);
+		tuple.t_tableOid = RelationGetRelid(rel);
+
+		switch (HeapTupleSatisfiesVacuum(&tuple, OldestXmin, buf))
+		{
+			case HEAPTUPLE_LIVE:
+				{
+					TransactionId xmin;
+
+					/* Check comments in lazy_scan_heap. */
+					if (!HeapTupleHeaderXminCommitted(tuple.t_data))
+					{
+						all_visible = false;
+						*all_frozen = false;
+						break;
+					}
+
+					/*
+					 * The inserter definitely committed. But is it old enough
+					 * that everyone sees it as committed?
+					 */
+					xmin = HeapTupleHeaderGetXmin(tuple.t_data);
+					if (!TransactionIdPrecedes(xmin, OldestXmin))
+					{
+						all_visible = false;
+						*all_frozen = false;
+						break;
+					}
+
+					/* Track newest xmin on page. */
+					if (TransactionIdFollows(xmin, *visibility_cutoff_xid))
+						*visibility_cutoff_xid = xmin;
+
+					/* Check whether this tuple is already frozen or not */
+					if (all_visible && *all_frozen &&
+						heap_tuple_needs_eventual_freeze(tuple.t_data))
+						*all_frozen = false;
+				}
+				break;
+
+			case HEAPTUPLE_DEAD:
+			case HEAPTUPLE_RECENTLY_DEAD:
+			case HEAPTUPLE_INSERT_IN_PROGRESS:
+			case HEAPTUPLE_DELETE_IN_PROGRESS:
+				{
+					all_visible = false;
+					*all_frozen = false;
+					break;
+				}
+			default:
+				elog(ERROR, "unexpected HeapTupleSatisfiesVacuum result");
+				break;
+		}
+	}							/* scan along page */
+
+	return all_visible;
+}