On Wed, 28 Apr 2021 at 00:39, Amit Khandekar <amitdkhan...@gmail.com> wrote: > If planned parallel workers do not get launched, the Result Cache plan > node shows all-0 stats for each of those workers:
Thanks for reporting this and for the patch. You're right that there is a problem here. I did in fact have code to skip workers that didn't have any cache misses right up until v18 of the patch [1], but I removed it because I was getting some test instability in the partition_prune regression tests. That was highlighted by the CFbot machines. I mentioned about that in the final paragraph of [2]. I didn't mention the exact test there, but I was talking about the test in partition_prune.sql. By the time it came to b6002a796, I did end up changing the partition_prune tests. However, I had to back that version out again because of some problems with force_parallel_mode = regress buildfarm animals. By the time I re-committed Result Cache in 9eacee2e6, I had changed the partition_prune tests so they did SET enable_resultcache = 0 before running that parallel test. I'd basically decided that the test was never going to be stable on the buildfarm. The problem there was that the results would vary depending on if the parallel worker managed to do anything before the main process did all the work. Because the tests are pretty small scale, many machines wouldn't manage to get their worker(s) in gear and running before the main process had finished the test. This was the reason I removed the cache_misses == 0 test in explain.c. I'd thought that I could make that test stable by just always outputting the cache stats line from workers, even if they didn't assist during execution. So, given that I removed the parallel test in partition_prune.sql, and don't have any EXPLAIN ANALYZE output for parallel tests in resultcache.sql, it should be safe enough to put that cache_misses == 0 test back into explain.c I've attached a patch to do this. The explain.c part is pretty similar to your patch, I just took my original code and comment. The patch also removes the SET force_parallel_mode = off in resultcache.sql. That's no longer needed due to adding this check in explain.c again. I also removed the changes I made to the explain_parallel_append function in partition_prune.sql. I shouldn't have included those in 9eacee2e6. I plan to push this in the next 24 hours or so. David [1] https://postgr.es/m/caaphdvoomttnpof-+q1daoma8vwfsfbyqb_a0iukts5nf2d...@mail.gmail.com [2] https://postgr.es/m/CAApHDvrz4f+i1wu-8hyqJ=pxydroga5okgo5rwpoj47rz6q...@mail.gmail.com
resultcache_explain_fix.patch
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