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VACUUM normally ends by running vac_update_datfrozenxid(), which requires a scan of pg_class. Therefore, if one attempts to vacuum a database one table at a time --- as vacuumdb has done since v12 --- we will spend O(N^2) time in vac_update_datfrozenxid(). That causes serious performance problems in databases with tens of thousands of tables, and indeed the effect is measurable with only a few hundred. To add insult to injury, only one process can run vac_update_datfrozenxid at the same time per DB, so this behavior largely defeats vacuumdb's -j option. Hence, invent options SKIP_DATABASE_STATS and ONLY_DATABASE_STATS to allow applications to postpone vac_update_datfrozenxid() until the end of a series of VACUUM requests, and teach vacuumdb to use them. Per bug #17717 from Gunnar L. Sadly, this answer doesn't seem like something we'd consider back-patching, so the performance problem will remain in v12-v15. Tom Lane and Nathan Bossart Discussion: https://postgr.es/m/17717-6c50eb1c7d23a886@postgresql.org
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/.