Truncate line pointer array during VACUUM.

Teach VACUUM to truncate the line pointer array of each heap page when a
contiguous group of LP_UNUSED line pointers appear at the end of the
array -- these unused and unreferenced items are excluded.  This process
occurs during VACUUM's second pass over the heap, right after LP_DEAD
line pointers on the page (those encountered/pruned during the first
pass) are marked LP_UNUSED.

Truncation avoids line pointer bloat with certain workloads,
particularly those involving continual range DELETEs and bulk INSERTs
against the same table.

Also harden heapam code to check for an out-of-range page offset number
in places where we weren't already doing so.

Author: Matthias van de Meent <boekewurm+postgres@gmail.com>
Author: Peter Geoghegan <pg@bowt.ie>
Reviewed-By: Masahiko Sawada <sawada.mshk@gmail.com>
Reviewed-By: Peter Geoghegan <pg@bowt.ie>
Discussion: https://postgr.es/m/CAEze2WjgaQc55Y5f5CQd3L=eS5CZcff2Obxp=O6pto8-f0hC4w@mail.gmail.com
Discussion: https://postgr.es/m/CAH2-Wzn6a64PJM1Ggzm=uvx2otsopJMhFQj_g1rAj4GWr3ZSzw@mail.gmail.com

1 files changed, 17 insertions, 5 deletions

diff --git a/src/backend/access/heap/heapam.c b/src/backend/access/heap/heapam.c
index 9cbc161d7a9..03d4abc938b 100644
--- a/src/backend/access/heap/heapam.c
+++ b/src/backend/access/heap/heapam.c
@@ -635,8 +635,15 @@ heapgettup(HeapScanDesc scan,
 		}
 		else
 		{
+			/*
+			 * The previous returned tuple may have been vacuumed since the
+			 * previous scan when we use a non-MVCC snapshot, so we must
+			 * re-establish the lineoff <= PageGetMaxOffsetNumber(dp)
+			 * invariant
+			 */
 			lineoff =			/* previous offnum */
-				OffsetNumberPrev(ItemPointerGetOffsetNumber(&(tuple->t_self)));
+				Min(lines,
+					OffsetNumberPrev(ItemPointerGetOffsetNumber(&(tuple->t_self))));
 		}
 		/* page and lineoff now reference the physically previous tid */
 
@@ -678,6 +685,13 @@ heapgettup(HeapScanDesc scan,
 	lpp = PageGetItemId(dp, lineoff);
 	for (;;)
 	{
+		/*
+		 * Only continue scanning the page while we have lines left.
+		 *
+		 * Note that this protects us from accessing line pointers past
+		 * PageGetMaxOffsetNumber(); both for forward scans when we resume the
+		 * table scan, and for when we start scanning a new page.
+		 */
 		while (linesleft > 0)
 		{
 			if (ItemIdIsNormal(lpp))
@@ -8556,10 +8570,8 @@ heap_xlog_vacuum(XLogReaderState *record)
 			ItemIdSetUnused(lp);
 		}
 
-		/*
-		 * Update the page's hint bit about whether it has free pointers
-		 */
-		PageSetHasFreeLinePointers(page);
+		/* Attempt to truncate line pointer array now */
+		PageTruncateLinePointerArray(page);
 
 		PageSetLSN(page, lsn);
 		MarkBufferDirty(buffer);