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Formerly, if the system clock went backwards, the stats collector would fail to update the stats file any more until the clock reading again exceeds whatever timestamp was last written into the stats file. Such glitches in the clock's behavior are not terribly unlikely on machines not using NTP. Such a scenario has been observed to cause regression test failures in the buildfarm, and it could have bad effects on the behavior of autovacuum, so it seems prudent to install some defenses. We could directly detect the clock going backwards by adding GetCurrentTimestamp calls in the stats collector's main loop, but that would hurt performance on platforms where GetCurrentTimestamp is expensive. To minimize the performance hit in normal cases, adopt a more complicated scheme wherein backends check for clock skew when reading the stats file, and if they see it, signal the stats collector by sending an extra stats inquiry message. The stats collector does an extra GetCurrentTimestamp only when it receives an inquiry with an apparently out-of-order timestamp. To avoid unnecessary GetCurrentTimestamp calls, expand the inquiry messages to carry the backend's current clock reading as well as its stats cutoff time. The latter, being intentionally slightly in-the-past, would trigger more clock rechecks than we need if it were used for this purpose. We might want to backpatch this change at some point, but let's let it shake out in the buildfarm for awhile first. |
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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/.