0002-Remove-btree-page-items-after-page-unlink.patch

From 32080536e312020054ed33f3246d58ab99e9efc0 Mon Sep 17 00:00:00 2001
From: Alexander Korotkov <akorotkov@postgresql.org>
Date: Wed, 6 May 2020 15:55:33 +0300
Subject: [PATCH 2/2] Remove btree page items after page unlink

Currently, page unlink leaves remaining items "as is", but replay of
corresponding WAL-record re-initializes page leaving it with no items.
For the sake of consistency, this commit makes primary delete all the items
during page unlink as well.

Discussion: https://postgr.es/m/CAPpHfdt_OTyQpXaPJcWzV2N-LNeNJseNB-K_A66qG%3DL518VTFw%40mail.gmail.com
---
 contrib/amcheck/verify_nbtree.c     |  7 ++-----
 src/backend/access/nbtree/nbtpage.c | 10 ++++++++++
 2 files changed, 12 insertions(+), 5 deletions(-)

diff --git a/contrib/amcheck/verify_nbtree.c b/contrib/amcheck/verify_nbtree.c
index e4d501a85d1..c9f9e755dcc 100644
--- a/contrib/amcheck/verify_nbtree.c
+++ b/contrib/amcheck/verify_nbtree.c
@@ -2864,11 +2864,8 @@ palloc_btree_page(BtreeCheckState *state, BlockNumber blocknum)
 	 * As noted at the beginning of _bt_binsrch(), an internal page must have
 	 * children, since there must always be a negative infinity downlink
 	 * (there may also be a highkey).  In the case of non-rightmost leaf
-	 * pages, there must be at least a highkey.  Deleted pages on replica
-	 * might contain no items, because page unlink re-initializes
-	 * page-to-be-deleted.  Deleted pages with no items might be on primary
-	 * too due to preceding recovery, but on primary new deletions can't
-	 * happen concurrently to amcheck.
+	 * pages, there must be at least a highkey.  The exceptions are deleted
+	 * pages, which contain no items.
 	 *
 	 * This is correct when pages are half-dead, since internal pages are
 	 * never half-dead, and leaf pages must have a high key when half-dead
diff --git a/src/backend/access/nbtree/nbtpage.c b/src/backend/access/nbtree/nbtpage.c
index e146cc326c9..4963d8b0fc4 100644
--- a/src/backend/access/nbtree/nbtpage.c
+++ b/src/backend/access/nbtree/nbtpage.c
@@ -2019,6 +2019,7 @@ _bt_unlink_halfdead_page(Relation rel, Buffer leafbuf, BlockNumber scanblkno,
 	BTMetaPageData *metad = NULL;
 	ItemId		itemid;
 	Page		page;
+	PageHeader	header;
 	BTPageOpaque opaque;
 	bool		rightsib_is_rightmost;
 	int			targetlevel;
@@ -2293,6 +2294,15 @@ _bt_unlink_halfdead_page(Relation rel, Buffer leafbuf, BlockNumber scanblkno,
 	opaque->btpo_flags |= BTP_DELETED;
 	opaque->btpo.xact = ReadNewTransactionId();
 
+	/*
+	 * Replaying page unlink on replica will re-initialize the target page
+	 * leaving it with no items.  For consistency, we remove all the items on
+	 * primary too.
+	 */
+	header = (PageHeader) page;
+	header->pd_lower = SizeOfPageHeaderData;
+	header->pd_upper = header->pd_special;
+
 	/* And update the metapage, if needed */
 	if (BufferIsValid(metabuf))
 	{
-- 
2.14.3