Clear empty space in a page.

[Yura Sokolov]

Good day.

Long time ago I've been played with proprietary "compressed storage"
patch on heavily updated table, and found empty pages (ie cleaned by
vacuum) are not compressed enough.

When table is stress-updated, page for new row versions are allocated
in round-robin kind, therefore some 1GB segments contains almost
no live tuples. Vacuum removes dead tuples, but segments remains large
after compression (>400MB) as if they are still full.

After some investigation I found it is because PageRepairFragmentation,
PageIndex*Delete* don't clear space that just became empty therefore it
still contains garbage data. Clearing it with memset greatly increase
compression ratio: some compressed relation segments become 30-60MB just
after vacuum remove tuples in them.

While this result is not directly applied to stock PostgreSQL, I believe
page compression is important for full_page_writes with wal_compression
enabled. And probably when PostgreSQL is used on filesystem with
compression enabled (ZFS?).

Therefore I propose clearing page's empty space with zero in
PageRepairFragmentation, PageIndexMultiDelete, PageIndexTupleDelete and
PageIndexTupleDeleteNoCompact.

Sorry, didn't measure impact on raw performance yet.

regards,
Yura Sokolov aka funny_falcon
commit 6abfcaeb87fcb396c5e2dccd434ce2511314ff76
Author: Yura Sokolov <y.soko...@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;