Avoid useless truncation attempts during VACUUM.

VACUUM can skip heap pages altogether when there's a run of consecutive
pages that are all-visible according to the visibility map.  This causes it
to not update its nonempty_pages count, just as if those pages were empty,
which means that at the end we will think they are candidates for deletion.
Thus, we may take the table's AccessExclusive lock only to find that no
pages are really truncatable.  This usually causes no real problems on a
master server, thanks to the lock being acquired only conditionally; but on
hot-standby servers, the same lock must be acquired unconditionally which
can result in unnecessary query cancellations.

To improve matters, force examination of the table's last page whenever
we reach there with a nonempty_pages count that would allow a truncation
attempt.  If it's not empty, we'll advance nonempty_pages and thereby
prevent the truncation attempt.

If we are unable to acquire cleanup lock on that page, there's no need to
force it, unless we're doing an anti-wraparound vacuum.  We can just check
for tuples with a shared buffer lock and then give up.  (When we are doing
an anti-wraparound vacuum, and decide it's okay to skip the page because it
contains no freezable tuples, this patch still improves matters because
nonempty_pages is properly updated, which it was not before.)

Since only the last page is special-cased in this way, we might attempt a
truncation that will release many fewer pages than the normal heuristic
would suggest; at worst, only one page would be truncated.  But that seems
all right, because the situation won't repeat during the next vacuum.
The real problem with the old logic is that the useless truncation attempt
happens every time we vacuum, so long as the state of the last few dozen
pages doesn't change.

This is a longstanding deficiency, but since the consequences aren't very
severe in most scenarios, I'm not going to risk a back-patch.

Jeff Janes and Tom Lane

src/backend/commands/vacuumlazy.c

@@ -138,7 +138,7 @@ static BufferAccessStrategy vac_strategy;
 static void lazy_scan_heap(Relation onerel, LVRelStats *vacrelstats,
 			   Relation *Irel, int nindexes, bool scan_all);
 static void lazy_vacuum_heap(Relation onerel, LVRelStats *vacrelstats);
-static bool lazy_check_needs_freeze(Buffer buf);
+static bool lazy_check_needs_freeze(Buffer buf, bool *hastup);
 static void lazy_vacuum_index(Relation indrel,
 				  IndexBulkDeleteResult **stats,
 				  LVRelStats *vacrelstats);
@@ -147,6 +147,7 @@ static void lazy_cleanup_index(Relation indrel,
 				   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);
@@ -175,7 +176,6 @@ lazy_vacuum_rel(Relation onerel, int options, VacuumParams *params,
 	LVRelStats *vacrelstats;
 	Relation   *Irel;
 	int			nindexes;
-	BlockNumber possibly_freeable;
 	PGRUsage	ru0;
 	TimestampTz starttime = 0;
 	long		secs;
@@ -263,14 +263,8 @@ lazy_vacuum_rel(Relation onerel, int options, VacuumParams *params,
 
 	/*
 	 * Optionally truncate the relation.
-	 *
-	 * 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.
 	 */
-	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))
+	if (should_attempt_truncation(vacrelstats))
 		lazy_truncate_heap(onerel, vacrelstats);
 
 	/* Vacuum the Free Space Map */
@@ -510,6 +504,15 @@ lazy_scan_heap(Relation onerel, LVRelStats *vacrelstats,
 	 * scan_all is not set, so no great harm done; the next vacuum will find
 	 * them.  If we make the reverse mistake and vacuum a page unnecessarily,
 	 * it'll just be a no-op.
+	 *
+	 * 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.
 	 */
 	for (next_not_all_visible_block = 0;
 		 next_not_all_visible_block < nblocks;
@@ -540,6 +543,10 @@ lazy_scan_heap(Relation onerel, LVRelStats *vacrelstats,
 		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))
+
 		if (blkno == next_not_all_visible_block)
 		{
 			/* Time to advance next_not_all_visible_block */
@@ -567,7 +574,7 @@ lazy_scan_heap(Relation onerel, LVRelStats *vacrelstats,
 		else
 		{
 			/* Current block is all-visible */
-			if (skipping_all_visible_blocks && !scan_all)
+			if (skipping_all_visible_blocks && !scan_all && !FORCE_CHECK_PAGE())
 				continue;
 			all_visible_according_to_vm = true;
 		}
@@ -631,10 +638,11 @@ lazy_scan_heap(Relation onerel, LVRelStats *vacrelstats,
 		{
 			/*
 			 * If we're not scanning the whole relation to guard against XID
-			 * wraparound, it's OK to skip vacuuming a page.  The next vacuum
-			 * will clean it up.
+			 * 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 (!scan_all)
+			if (!scan_all && !FORCE_CHECK_PAGE())
 			{
 				ReleaseBuffer(buf);
 				vacrelstats->pinskipped_pages++;
@@ -642,22 +650,29 @@ lazy_scan_heap(Relation onerel, LVRelStats *vacrelstats,
 			}
 
 			/*
-			 * If this is a wraparound checking vacuum, then we read the page
-			 * with share lock to see if any xids need to be frozen. If the
-			 * page doesn't need attention we just skip and continue. If it
-			 * does, we wait for cleanup lock.
+			 * 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 scan_all 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))
+			if (!lazy_check_needs_freeze(buf, &hastup) || !scan_all)
 			{
 				UnlockReleaseBuffer(buf);
 				vacrelstats->scanned_pages++;
 				vacrelstats->pinskipped_pages++;
+				if (hastup)
+					vacrelstats->nonempty_pages = blkno + 1;
 				continue;
 			}
 			LockBuffer(buf, BUFFER_LOCK_UNLOCK);
@@ -1304,22 +1319,25 @@ lazy_vacuum_page(Relation onerel, BlockNumber blkno, Buffer buffer,
  *					 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)
+lazy_check_needs_freeze(Buffer buf, bool *hastup)
 {
-	Page		page;
+	Page		page = BufferGetPage(buf);
 	OffsetNumber offnum,
 				maxoff;
 	HeapTupleHeader tupleheader;
 
-	page = BufferGetPage(buf);
+	*hastup = false;
 
-	if (PageIsNew(page) || PageIsEmpty(page))
-	{
-		/* PageIsNew probably shouldn't happen... */
+	/* 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;
@@ -1330,6 +1348,11 @@ lazy_check_needs_freeze(Buffer buf)
 
 		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;
 
@@ -1432,6 +1455,30 @@ lazy_cleanup_index(Relation indrel,
 	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.
+ *
+ * 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))
+		return true;
+	else
+		return false;
+}
+
 /*
  * lazy_truncate_heap - try to truncate off any empty pages at the end
  */