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Serializable Snapshot Isolation used for serializable transactions depends on acquiring SIRead locks on all heap relation tuples which are used to generate the query result, so that a later delete or update of any of the tuples can flag a read-write conflict between transactions. This is normally handled in heapam.c, with tuple level locking. Since an index-only scan avoids heap access in many cases, building the result from the index tuple, the necessary predicate locks were not being acquired for all tuples in an index-only scan. To prevent problems with tuple IDs which are vacuumed and re-used while the transaction still matters, the xmin of the tuple is part of the tag for the tuple lock. Since xmin is not available to the index-only scan for result rows generated from the index tuples, it is not possible to acquire a tuple-level predicate lock in such cases, in spite of having the tid. If we went to the heap to get the xmin value, it would no longer be an index-only scan. Rather than prohibit index-only scans under serializable transaction isolation, we acquire an SIRead lock on the page containing the tuple, when it was not necessary to visit the heap for other reasons. Backpatch to 9.2. Kevin Grittner and Tom Lane |
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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. 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/.