From 1ad0df67c7904ff64166d7a453c53943f069ee52 Mon Sep 17 00:00:00 2001 From: Amit Kapila Date: Tue, 26 Nov 2019 09:41:41 +0530 Subject: [PATCH] Don't shut down Gather[Merge] early under Limit. Revert part of commit 19df1702f5. Early shutdown was added by that commit so that we could collect statistics from workers, but unfortunately, it interacted badly with rescans. The problem is that we ended up destroying the parallel context which is required for rescans. This leads to rescans of a Limit node over a Gather node to produce unpredictable results as it tries to access destroyed parallel context. By reverting the early shutdown code, we might lose statistics in some cases of Limit over Gather [Merge], but that will require further study to fix. Reported-by: Jerry Sievers Diagnosed-by: Thomas Munro Author: Amit Kapila Backpatch-through: 9.6 Discussion: https://postgr.es/m/87ims2amh6.fsf@jsievers.enova.com --- src/backend/executor/nodeLimit.c | 14 ++++++-------- 1 file changed, 6 insertions(+), 8 deletions(-) diff --git a/src/backend/executor/nodeLimit.c b/src/backend/executor/nodeLimit.c index 9d78b827520..2b5e9c7d19a 100644 --- a/src/backend/executor/nodeLimit.c +++ b/src/backend/executor/nodeLimit.c @@ -125,19 +125,17 @@ ExecLimit(LimitState *node) * we are at the end of the window, return NULL without * advancing the subplan or the position variable; but change * the state machine state to record having done so. + * + * Once at the end, ideally, we can shut down parallel + * resources but that would destroy the parallel context which + * would be required for rescans. To do that, we need to find + * a way to pass down more information about whether rescans + * are possible. */ if (!node->noCount && node->position - node->offset >= node->count) { node->lstate = LIMIT_WINDOWEND; - - /* - * If we know we won't need to back up, we can release - * resources at this point. - */ - if (!(node->ps.state->es_top_eflags & EXEC_FLAG_BACKWARD)) - (void) ExecShutdownNode(outerPlan); - return NULL; }