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on non-full-page-image WAL records, and quite arbitrarily, only if there's less than 20% free space on the page after the insert/update (not on HOT updates, though). The 20% cutoff should avoid most of the overhead, when replaying a bulk insertion, for example, while ensuring that pages that are full are marked as full in the FSM. This is mostly to avoid the nasty worst case scenario, where you replay from a PITR archive, and the FSM information in the base backup is really out of date. If there was a lot of pages that the outdated FSM claims to have free space, but don't actually have any, the first unlucky inserter after the recovery would traverse through all those pages, just to find out that they're full. We didn't have this problem with the old FSM implementation, because we simply threw the FSM information away on a non-clean shutdown.
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. Changes between all PostgreSQL releases are recorded in the file HISTORY. 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/.