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Hash indexes are not WAL-logged, and so do not maintain the LSN of index pages. Since the "snapshot too old" feature counts on detecting error conditions using the LSN of a table and all indexes on it, this makes it impossible to safely do early vacuuming on any table with a hash index, so add this to the tests for whether the xid used to vacuum a table can be adjusted based on old_snapshot_threshold. While at it, add a paragraph to the docs for old_snapshot_threshold which specifically mentions this and other aspects of the feature which may otherwise surprise users. Problem reported and patch reviewed by Amit Kapila
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/.