From 762a3bb081e466475d37d1c5eae6d92e689f8946 Mon Sep 17 00:00:00 2001
From: Peter Geoghegan <pg@bowt.ie>
Date: Tue, 30 Mar 2021 19:43:06 -0700
Subject: [PATCH v4] Truncate a pages' line pointer array when it has trailing
 unused ItemIds.

This will allow reuse of what is effectively free space for data as well as
new line pointers, instead of keeping it reserved for line pointers only.

An additional benefit is that the HasFreeLinePointers hint-bit optimization
now doesn't hint for free line pointers at the end of the array, slightly
increasing the specificity of where the free lines are; and saving us from
needing to search to the end of the array if all other entries are already
filled.
---
 src/backend/access/heap/heapam.c    |  9 ++++++++-
 src/backend/access/heap/pruneheap.c |  1 +
 src/backend/storage/page/bufpage.c  | 15 ++++++++++++++-
 3 files changed, 23 insertions(+), 2 deletions(-)

diff --git a/src/backend/access/heap/heapam.c b/src/backend/access/heap/heapam.c
index 9cbc161d7a..c2d71fb964 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 */
 
diff --git a/src/backend/access/heap/pruneheap.c b/src/backend/access/heap/pruneheap.c
index f75502ca2c..3c8dc0af18 100644
--- a/src/backend/access/heap/pruneheap.c
+++ b/src/backend/access/heap/pruneheap.c
@@ -962,6 +962,7 @@ heap_get_root_tuples(Page page, OffsetNumber *root_offsets)
 		 */
 		for (;;)
 		{
+			Assert(OffsetNumberIsValid(nextoffnum) && nextoffnum <= maxoff);
 			lp = PageGetItemId(page, nextoffnum);
 
 			/* Check for broken chains */
diff --git a/src/backend/storage/page/bufpage.c b/src/backend/storage/page/bufpage.c
index 5d5989c2f5..214cb5f417 100644
--- a/src/backend/storage/page/bufpage.c
+++ b/src/backend/storage/page/bufpage.c
@@ -676,7 +676,11 @@ compactify_tuples(itemIdCompact itemidbase, int nitems, Page page, bool presorte
  * PageRepairFragmentation
  *
  * Frees fragmented space on a page.
- * It doesn't remove unused line pointers! Please don't change this.
+ * It doesn't remove intermediate unused line pointers (that would mean
+ * moving ItemIds, and that would imply invalidating indexed values), but it
+ * does truncate the page->pd_linp array to the last unused line pointer, so
+ * that this space may also be reused for data, instead of only for line
+ * pointers.
  *
  * This routine is usable for heap pages only, but see PageIndexMultiDelete.
  *
@@ -697,6 +701,7 @@ PageRepairFragmentation(Page page)
 	int			nline,
 				nstorage,
 				nunused;
+	OffsetNumber lastUsed = InvalidOffsetNumber;
 	int			i;
 	Size		totallen;
 	bool		presorted = true;	/* For now */
@@ -730,6 +735,7 @@ PageRepairFragmentation(Page page)
 		lp = PageGetItemId(page, i);
 		if (ItemIdIsUsed(lp))
 		{
+			lastUsed = i;
 			if (ItemIdHasStorage(lp))
 			{
 				itemidptr->offsetindex = i - 1;
@@ -777,6 +783,13 @@ PageRepairFragmentation(Page page)
 		compactify_tuples(itemidbase, nstorage, page, presorted);
 	}
 
+	if (lastUsed != nline)
+	{
+		((PageHeader) page)->pd_lower =
+				SizeOfPageHeaderData + (sizeof(ItemIdData) * lastUsed);
+		nunused = nunused - (nline - lastUsed);
+	}
+
 	/* Set hint bit for PageAddItemExtended */
 	if (nunused > 0)
 		PageSetHasFreeLinePointers(page);
-- 
2.27.0