Subject: [PATCH 2/4] Write WAL for empty nbtree index build

From: Kyotaro Horiguchi <[email-masked]>
Date: Thu, 11 Oct 2018 10:03:08 +0900

After relation truncation indexes are also rebuild. It doesn't emit
WAL in minimal mode and if truncation happened within its creation
transaction, crash recovery leaves an empty index heap, which is
considered broken. This patch forces to emit WAL when an index_build
turns into empty nbtree index.
---
 src/backend/access/nbtree/nbtsort.c | 15 +++++++++++++--
 1 file changed, 13 insertions(+), 2 deletions(-)

diff --git a/src/backend/access/nbtree/nbtsort.c b/src/backend/access/nbtree/nbtsort.c
index 16f5755777..2c2647b530 100644
--- a/src/backend/access/nbtree/nbtsort.c
+++ b/src/backend/access/nbtree/nbtsort.c
@@ -610,8 +610,14 @@ _bt_blwritepage(BTWriteState *wstate, Page page, BlockNumber blkno)
 	/* Ensure rd_smgr is open (could have been closed by relcache flush!) */
 	RelationOpenSmgr(wstate->index);
 
-	/* XLOG stuff */
-	if (wstate->btws_use_wal)
+	/* XLOG stuff
+	 *
+	 * Even if minimal mode, WAL is required here if truncation happened after
+	 * being created in the same transaction. It is not needed otherwise but
+	 * we don't bother identifying the case precisely.
+	 */
+	if (wstate->btws_use_wal ||
+		(blkno == BTREE_METAPAGE && BTPageGetMeta(page)->btm_root == 0))
 	{
 		/* We use the heap NEWPAGE record type for this */
 		log_newpage(&wstate->index->rd_node, MAIN_FORKNUM, blkno, page, true);
@@ -1055,6 +1061,11 @@ _bt_uppershutdown(BTWriteState *wstate, BTPageState *state)
 	 * set to point to "P_NONE").  This changes the index to the "valid" state
 	 * by filling in a valid magic number in the metapage.
 	 */
+	/*
+	 * If no tuple was inserted, it's possible that we are truncating a
+	 * relation. We need to emit WAL for the metapage in the case. However it
+	 * is not required elsewise,
+	 */
 	metapage = (Page) palloc(BLCKSZ);
 	_bt_initmetapage(metapage, rootblkno, rootlevel);
 	_bt_blwritepage(wstate, metapage, BTREE_METAPAGE);
-- 
2.16.3

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