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There is a rare race condition, when a transaction that inserted a tuple aborts while vacuum is processing the page containing the inserted tuple. Vacuum prunes the page first, which normally removes any dead tuples, but if the inserting transaction aborts right after that, the loop after pruning will see a dead tuple and remove it instead. That's OK, but if the page is on a table with no indexes, and the page becomes completely empty after removing the dead tuple (or tuples) on it, it will be immediately marked as all-visible. That's OK, but the sanity check in vacuum would throw a warning because it thinks that the page contains dead tuples and was nevertheless marked as all-visible, even though it just vacuumed away the dead tuples and so it doesn't actually contain any. Spotted this while reading the code. It's difficult to hit the race condition otherwise, but can be done by putting a breakpoint after the heap_page_prune() call. Backpatch all the way to 8.4, where this code first appeared. |
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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/.