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Currently, a backend will reset it's PGXACT->xmin value when it doesn't have any registered snapshots left. That covered the common case that a transaction in read committed mode runs several queries, one after each other, as there would be no snapshots active between those queries. However, if you hold cursors across each of the query, we didn't get a chance to reset xmin. To make that better, keep all the registered snapshots in a pairing heap, ordered by xmin so that it's always quick to find the snapshot with the smallest xmin. That allows us to advance PGXACT->xmin whenever the oldest snapshot is deregistered, even if there are others still active. Per discussion originally started by Jeff Davis back in 2009 and more recently by Robert Haas. |
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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. 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/.