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Without a few entries beyond old_snapshot_threshold, the lookup would often fail, resulting in the more aggressive pruning or vacuum being skipped often enough to matter. This was very clearly shown by a python test script posted by Ants Aasma, and was likely a factor in an earlier but somewhat less clear-cut test case posted by Jeff Janes. This patch makes no change to the logic, per se -- it just makes the array of mapping entries big enough to make lookup misses based on timing much less likely. An occasional miss is still possible if a thread stalls for more than 10 minutes, but that does not create any problem with correctness of behavior. Besides, if things are so busy that a thread is stalling for more than 10 minutes, it is probably OK to skip the more aggressive cleanup at that particular point in time.
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/.