On Fri, Feb 19, 2021 at 9:38 PM Amit Kapila <amit.kapil...@gmail.com> wrote: > > On Thu, Feb 18, 2021 at 11:05 AM Amit Langote <amitlangot...@gmail.com> wrote: > > > > > > It also occurred to me that we can avoid pointless adding of > > > > partitions if the final plan won't use parallelism. For that, the > > > > patch adds checking glob->parallelModeNeeded, which seems to do the > > > > trick though I don't know if that's the correct way of doing that. > > > > > > > > > > I'm not sure if's pointless adding partitions even in the case of NOT > > > using parallelism, because we may be relying on the result of > > > parallel-safety checks on partitions in both cases. > > > For example, insert_parallel.sql currently includes a test (that you > > > originally provided in a previous post) that checks a non-parallel > > > plan is generated after a parallel-unsafe trigger is created on a > > > partition involved in the INSERT. > > > If I further add to that test by then dropping that trigger and then > > > again using EXPLAIN to see what plan is generated, then I'd expect a > > > parallel-plan to be generated, but with the setrefs-v3.patch it still > > > generates a non-parallel plan. So I think the "&& > > > glob->parallelModeNeeded" part of test needs to be removed. > > > > Ah, okay, I didn't retest my case after making that change. > > > > Greg has point here but I feel something on previous lines (having a > test of glob->parallelModeNeeded) is better. We only want to > invalidate the plan if the prepared plan is unsafe to execute next > time. It is quite possible that there are unsafe triggers on different > partitions and only one of them is dropped, so next time planning will > again yield to the same non-parallel plan. If we agree with that I > think it is better to add this dependency in set_plan_refs (along with > Gather node handling). >
I think we should try to be consistent with current plan-cache functionality, rather than possibly inventing new rules for partitions. I'm finding that with current Postgres functionality (master branch), if there are two parallel-unsafe triggers defined on a normal table and one is dropped, it results in replanning and it yields the same (non-parallel) plan. This would seem to go against what you are suggesting. Regards, Greg Nancarrow Fujitsu Australia
