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While prior to this patch the user-visible effect on the database of any set of successfully committed serializable transactions was always consistent with some one-at-a-time order of execution of those transactions, the presence of declarative constraints could allow errors to occur which were not possible in any such ordering, and developers had no good workarounds to prevent user-facing errors where they were not necessary or desired. This patch adds a check for serialization failure ahead of duplicate key checking so that if a developer explicitly (redundantly) checks for the pre-existing value they will get the desired serialization failure where the problem is caused by a concurrent serializable transaction; otherwise they will get a duplicate key error. While it would be better if the reads performed by the constraints could count as part of the work of the transaction for serialization failure checking, and we will hopefully get there some day, this patch allows a clean and reliable way for developers to work around the issue. In many cases existing code will already be doing the right thing for this to "just work". Author: Thomas Munro, with minor editing of docs by me Reviewed-by: Marko Tiikkaja, Kevin Grittner |
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README
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/.