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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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PostgreSQL Database Management System ===================================== This directory contains the source code distribution of the PostgreSQL database management system. PostgreSQL is an advanced object-relational database management system that supports an extended subset of the SQL standard, including transactions, foreign keys, subqueries, triggers, user-defined types and functions. This distribution also contains C language bindings. PostgreSQL has many language interfaces, many of which are listed here: https://www.postgresql.org/download See the file INSTALL for instructions on how to build and install PostgreSQL. That file also lists supported operating systems and hardware platforms and contains information regarding any other software packages that are required to build or run the PostgreSQL system. Copyright and license information can be found in the file COPYRIGHT. A comprehensive documentation set is included in this distribution; it can be read as described in the installation instructions. The latest version of this software may be obtained at https://www.postgresql.org/download/. For more information look at our web site located at https://www.postgresql.org/.