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an old transaction running in the master, and a lot of transactions have started and finished since, and a WAL-record is written in the gap between the creating the running-xacts snapshot and WAL-logging it, recovery will fail with "too many KnownAssignedXids" error. This bug was reported by Joachim Wieland on Nov 19th. In the same scenario, when fewer transactions have started so that all the xids fit in KnownAssignedXids despite the first bug, a more serious bug arises. We incorrectly initialize the clog code with the oldest still running transaction, and when we see the WAL record belonging to a transaction with an XID larger than one that committed already before the checkpoint we're recovering from, we zero the clog page containing the already committed transaction, leading to data loss. In hindsight, trying to track xids in the known-assigned-xids array before seeing the running-xacts record was too complicated. To fix that, hold XidGenLock while the running-xacts snapshot is taken and WAL-logged. That ensures that no transaction can begin or end in that gap, so that in recvoery we know that the snapshot contains all transactions running at that point in WAL. |
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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/.