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Even with old_snapshot_threshold = -1 (which disables the "snapshot too old" feature), performance regressions were seen at moderate to high concurrency. For example, a one-socket, four-core system running 200 connections at saturation could see up to a 2.3% regression, with larger regressions possible on NUMA machines. By inlining the early (smaller, faster) tests in the TestForOldSnapshot() function, the i7 case dropped to a 0.2% regression, which could easily just be noise, and is clearly an improvement. Further testing will show whether more is needed.
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/.