commit 6abfcaeb87fcb396c5e2dccd434ce2511314ff76
Author: Yura Sokolov <y.sokolov@postgrespro.ru>
Date:   Sun May 30 02:39:17 2021 +0300

    Clear empty space in a page
    
    Write zeroes to just cleared space in PageRepairFragmentation,
    PageIndexTupleDelete, PageIndexMultiDelete and PageIndexDeleteNoCompact.
    
    It helps increase compression ration on compression enabled filesystems
    and with full_page_write and wal_compression enabled.

diff --git a/src/backend/storage/page/bufpage.c b/src/backend/storage/page/bufpage.c
index 82ca91f5977..7deb6cc71a4 100644
--- a/src/backend/storage/page/bufpage.c
+++ b/src/backend/storage/page/bufpage.c
@@ -681,6 +681,17 @@ compactify_tuples(itemIdCompact itemidbase, int nitems, Page page, bool presorte
 	phdr->pd_upper = upper;
 }
 
+/*
+ * Clean up space between pd_lower and pd_upper for better page compression.
+ */
+static void
+memset_hole(Page page, LocationIndex old_pd_upper)
+{
+	PageHeader	phdr = (PageHeader) page;
+	if (phdr->pd_upper > old_pd_upper)
+		MemSet((char *)page + old_pd_upper, 0, phdr->pd_upper - old_pd_upper);
+}
+
 /*
  * PageRepairFragmentation
  *
@@ -797,6 +808,7 @@ PageRepairFragmentation(Page page)
 
 		compactify_tuples(itemidbase, nstorage, page, presorted);
 	}
+	memset_hole(page, pd_upper);
 
 	/* Set hint bit for PageAddItemExtended */
 	if (nunused > 0)
@@ -1114,6 +1126,7 @@ PageIndexTupleDelete(Page page, OffsetNumber offnum)
 
 	if (offset > phdr->pd_upper)
 		memmove(addr + size, addr, offset - phdr->pd_upper);
+	MemSet(addr, 0, size);
 
 	/* adjust free space boundary pointers */
 	phdr->pd_upper += size;
@@ -1271,6 +1284,7 @@ PageIndexMultiDelete(Page page, OffsetNumber *itemnos, int nitems)
 		compactify_tuples(itemidbase, nused, page, presorted);
 	else
 		phdr->pd_upper = pd_special;
+	memset_hole(page, pd_upper);
 }
 
 
@@ -1351,6 +1365,7 @@ PageIndexTupleDeleteNoCompact(Page page, OffsetNumber offnum)
 
 	if (offset > phdr->pd_upper)
 		memmove(addr + size, addr, offset - phdr->pd_upper);
+	MemSet(addr, 0, size);
 
 	/* adjust free space boundary pointer */
 	phdr->pd_upper += size;