From: Peter Geoghegan <pg@bowt.ie>
Date: Wed, 8 May 2019 19:47:20 +0000 (-0700)
Subject: Remove obsolete nbtree split REDO routine comment.
X-Git-Tag: REL_12_BETA1~104
X-Git-Url: https://granicus.if.org/sourcecode?a=commitdiff_plain;h=d95e36dc384e3068ae2db909c228b1800737b18d;p=postgresql

Remove obsolete nbtree split REDO routine comment.

Commit dd299df8189, which added suffix truncation to nbtree, simplified
the WAL record format used by page splits.  It became necessary to
explicitly WAL-log the new high key for the left half of a split in all
cases, which relieved the REDO routine from having to reconstruct a new
high key for the left page by copying the first item from the right
page.  Remove a comment that referred to the previous practice.
---

diff --git a/src/backend/access/nbtree/nbtxlog.c b/src/backend/access/nbtree/nbtxlog.c
index 0a85d8b535..6532a25d3d 100644
--- a/src/backend/access/nbtree/nbtxlog.c
+++ b/src/backend/access/nbtree/nbtxlog.c
@@ -248,8 +248,6 @@ btree_xlog_split(bool onleft, XLogReaderState *record)
 	PageSetLSN(rpage, lsn);
 	MarkBufferDirty(rbuf);
 
-	/* don't release the buffer yet; we touch right page's first item below */
-
 	/* Now reconstruct left (original) sibling page */
 	if (XLogReadBufferForRedo(record, 0, &lbuf) == BLK_NEEDS_REDO)
 	{
@@ -257,10 +255,9 @@ btree_xlog_split(bool onleft, XLogReaderState *record)
 		 * To retain the same physical order of the tuples that they had, we
 		 * initialize a temporary empty page for the left page and add all the
 		 * items to that in item number order.  This mirrors how _bt_split()
-		 * works.  It's not strictly required to retain the same physical
-		 * order, as long as the items are in the correct item number order,
-		 * but it helps debugging.  See also _bt_restore_page(), which does
-		 * the same for the right page.
+		 * works.  Retaining the same physical order makes WAL consistency
+		 * checking possible.  See also _bt_restore_page(), which does the
+		 * same for the right page.
 		 */
 		Page		lpage = (Page) BufferGetPage(lbuf);
 		BTPageOpaque lopaque = (BTPageOpaque) PageGetSpecialPointer(lpage);
@@ -345,7 +342,10 @@ btree_xlog_split(bool onleft, XLogReaderState *record)
 		MarkBufferDirty(lbuf);
 	}
 
-	/* We no longer need the buffers */
+	/*
+	 * We no longer need the buffers.  They must be released together, so that
+	 * readers cannot observe two inconsistent halves.
+	 */
 	if (BufferIsValid(lbuf))
 		UnlockReleaseBuffer(lbuf);
 	UnlockReleaseBuffer(rbuf);