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In databases with many tables, re-fetching the statistics takes some time, so that this behavior seriously decreases the available concurrency for multiple autovac workers. There's discussion afoot about more complete fixes, but a simple and back-patchable amelioration is to claim the table and release the lock before rechecking stats. If we find out there's no longer a reason to process the table, re-taking the lock to un-claim the table is cheap enough. (This patch is quite old, but got lost amongst a discussion of more aggressive fixes. It's not clear when or if such a fix will be accepted, but in any case it'd be unlikely to get back-patched. Let's do this now so we have some improvement for the back branches.) In passing, make the normal un-claim step take AutovacuumScheduleLock not AutovacuumLock, since that is what is documented to protect the wi_tableoid field. This wasn't an actual bug in view of the fact that readers of that field hold both locks, but it creates some concurrency penalty against operations that need only AutovacuumLock. Back-patch to all supported versions. Jeff Janes Discussion: https://postgr.es/m/26118.1520865816@sss.pgh.pa.us |
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