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Instead of truncating pg_multixact at vacuum time, do it only at checkpoint time. The reason for doing it this way is twofold: first, we want it to delete only segments that we're certain will not be required if there's a crash immediately after the removal; and second, we want to do it relatively often so that older files are not left behind if there's an untimely crash. Per my proposal in http://www.postgresql.org/message-id/20140626044519.GJ7340@eldon.alvh.no-ip.org we now execute the truncation in the checkpointer process rather than as part of vacuum. Vacuum is in only charge of maintaining in shared memory the value to which it's possible to truncate the files; that value is stored as part of checkpoints also, and so upon recovery we can reuse the same value to re-execute truncate and reset the oldest-value-still-safe-to-use to one known to remain after truncation. Per bug reported by Jeff Janes in the course of his tests involving bug #8673. While at it, update some comments that hadn't been updated since multixacts were changed. Backpatch to 9.3, where persistency of pg_multixact files was introduced by commit 0ac5ad5134f2.
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: http://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 http://www.postgresql.org/download/. For more information look at our web site located at http://www.postgresql.org/.