From 9945ad6e904041be1567a89916ddc64087ed1edc Mon Sep 17 00:00:00 2001 From: Peter Geoghegan Date: Fri, 27 Mar 2020 16:44:52 -0700 Subject: [PATCH] Justify nbtree page split locking in code comment. Delaying unlocking the right child page until after the point that the left child's parent page has been refound is no longer truly necessary. Commit 40dae7ec made nbtree tolerant of interrupted page splits. VACUUM was taught to avoid deleting a page that happens to be the right half of an incomplete split. As long as page splits don't unlock the left child page until the end of the second/final phase, it should be safe to unlock the right child page earlier (at the end of the first phase). It probably isn't actually useful to release the right child's lock earlier like this (it probably won't improve performance). Even still, pointing out that it ought to be safe to do so should make it easier to understand the overall design. --- src/backend/access/nbtree/nbtinsert.c | 13 +++++++++++-- 1 file changed, 11 insertions(+), 2 deletions(-) diff --git a/src/backend/access/nbtree/nbtinsert.c b/src/backend/access/nbtree/nbtinsert.c index 00df0e1b88f..1675298f73d 100644 --- a/src/backend/access/nbtree/nbtinsert.c +++ b/src/backend/access/nbtree/nbtinsert.c @@ -2128,8 +2128,17 @@ _bt_insert_parent(Relation rel, pbuf = _bt_getstackbuf(rel, stack, bknum); /* - * Now we can unlock the right child. The left child will be unlocked - * by _bt_insertonpg(). + * Unlock the right child. The left child will be unlocked in + * _bt_insertonpg(). + * + * Unlocking the right child must be delayed until here to ensure that + * no concurrent VACUUM operation can become confused. Page deletion + * cannot be allowed to fail to re-find a downlink for the rbuf page. + * (Actually, this is just a vestige of how things used to work. The + * page deletion code is expected to check for the INCOMPLETE_SPLIT + * flag on the left child. It won't attempt deletion of the right + * child until the split is complete. Despite all this, we opt to + * conservatively delay unlocking the right child until here.) */ _bt_relbuf(rel, rbuf);