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Attempt to clarify comments related to force_parallel_mode.

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Per discussion with Tom Lane.

Discussion: http://postgr.es/m/28589.1502902172@sss.pgh.pa.us
This commit is contained in:
Robert Haas 2017-08-17 14:04:15 -04:00
parent a2b70c89ca
commit 1e56883a52
1 changed files with 15 additions and 7 deletions
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22
src/backend/optimizer/plan/planner.c
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@ -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);