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no-longer-needed pages at the end of a table. We thought we could throw away pages containing HEAPTUPLE_DEAD tuples; but this is not so, because such tuples very likely have index entries pointing at them, and we wouldn't have removed the index entries. The problem only emerges in a somewhat unlikely race condition: the dead tuples have to have been inserted by a transaction that later aborted, and this has to have happened between VACUUM's initial scan of the page and then rechecking it for empty in count_nondeletable_pages. But that timespan will include an index-cleaning pass, so it's not all that hard to hit. This seems to explain a couple of previously unsolved bug reports.
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 several language bindings, including C, Perl, Python, and Tcl, as well as a JDBC driver. The ODBC and C++ interfaces have been moved to the PostgreSQL Projects Web Site at http://gborg.postgresql.org for separate maintenance. 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 ftp://ftp.postgresql.org/pub/. For more information look at our web site located at http://www.postgresql.org/.