From: Robert Haas Date: Thu, 17 Aug 2017 18:04:15 +0000 (-0400) Subject: Attempt to clarify comments related to force_parallel_mode. X-Git-Tag: REL_11_BETA1~1767 X-Git-Url: https://granicus.if.org/sourcecode?a=commitdiff_plain;h=1e56883a528eb623c9a55ec7e43b4eee3722014b;p=postgresql Attempt to clarify comments related to force_parallel_mode. Per discussion with Tom Lane. Discussion: http://postgr.es/m/28589.1502902172@sss.pgh.pa.us --- diff --git a/src/backend/optimizer/plan/planner.c b/src/backend/optimizer/plan/planner.c index 407df9ae79..fdef00ab39 100644 --- a/src/backend/optimizer/plan/planner.c +++ b/src/backend/optimizer/plan/planner.c @@ -291,13 +291,21 @@ standard_planner(Query *parse, int cursorOptions, ParamListInfo boundParams) } /* - * glob->parallelModeNeeded should tell us whether it's necessary to - * impose the parallel mode restrictions, but we don't actually want to - * impose them unless we choose a parallel plan, so it is normally set - * only if a parallel plan is chosen (see create_gather_plan). That way, - * people who mislabel their functions but don't use parallelism anyway - * aren't harmed. But when force_parallel_mode is set, we enable the - * restrictions whenever possible for testing purposes. + * glob->parallelModeNeeded is normally set to false here and changed to + * true during plan creation if a Gather or Gather Merge plan is actually + * created (cf. create_gather_plan, create_gather_merge_plan). + * + * However, if force_parallel_mode = on or force_parallel_mode = regress, + * then we impose parallel mode whenever it's safe to do so, even if the + * final plan doesn't use parallelism. It's not safe to do so if the + * query contains anything parallel-unsafe; parallelModeOK will be false + * in that case. Note that parallelModeOK can't change after this point. + * Otherwise, everything in the query is either parallel-safe or + * parallel-restricted, and in either case it should be OK to impose + * parallel-mode restrictions. If that ends up breaking something, then + * either some function the user included in the query is incorrectly + * labelled as parallel-safe or parallel-restricted when in reality it's + * parallel-unsafe, or else the query planner itself has a bug. */ glob->parallelModeNeeded = glob->parallelModeOK && (force_parallel_mode != FORCE_PARALLEL_OFF);