From 4c1f89d0c866001740642ef77a69a33b318db430 Mon Sep 17 00:00:00 2001 From: Thomas Munro Date: Tue, 4 Feb 2020 18:20:07 +1300 Subject: [PATCH 1/2] Handle lack of DSM slots in parallel btree build, take 2. Commit 74618e77 added a new check intended to fix a bug, but put it in the wrong place so that parallel btree build was always disabled. Only do that after we've actually tried to create a DSM segment. Back-patch to 11, like the earlier commit. Discussion: https://postgr.es/m/CAH2-WzmDABkJzrNnvf%2BOULK-_A_j9gkYg_Dz-H62jzNv4eKQTw%40mail.gmail.com --- src/backend/access/nbtree/nbtsort.c | 18 ++++++++++-------- 1 file changed, 10 insertions(+), 8 deletions(-) diff --git a/src/backend/access/nbtree/nbtsort.c b/src/backend/access/nbtree/nbtsort.c index e6be7bba59..baec5de999 100644 --- a/src/backend/access/nbtree/nbtsort.c +++ b/src/backend/access/nbtree/nbtsort.c @@ -1332,14 +1332,6 @@ _bt_begin_parallel(BTBuildState *buildstate, bool isconcurrent, int request) pcxt = CreateParallelContext("postgres", "_bt_parallel_build_main", request); - /* If no DSM segment was available, back out (do serial build) */ - if (pcxt->seg == NULL) - { - DestroyParallelContext(pcxt); - ExitParallelMode(); - return; - } - scantuplesortstates = leaderparticipates ? request + 1 : request; /* @@ -1383,6 +1375,16 @@ _bt_begin_parallel(BTBuildState *buildstate, bool isconcurrent, int request) /* Everyone's had a chance to ask for space, so now create the DSM */ InitializeParallelDSM(pcxt); + /* If no DSM segment was available, back out (do serial build) */ + if (pcxt->seg == NULL) + { + if (IsMVCCSnapshot(snapshot)) + UnregisterSnapshot(snapshot); + DestroyParallelContext(pcxt); + ExitParallelMode(); + return; + } + /* Store shared build state, for which we reserved space */ btshared = (BTShared *) shm_toc_allocate(pcxt->toc, estbtshared); /* Initialize immutable state */ -- 2.23.0