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We were trying to acquire the lock even when we were subsequently not sleeping in some other transaction, which opens us up unnecessarily to deadlocks. In particular, this is troublesome if an update tries to lock an updated version of a tuple and finds itself doing EvalPlanQual update chain walking; more than two sessions doing this concurrently will find themselves sleeping on each other because the HW tuple lock acquisition in heap_lock_tuple called from EvalPlanQualFetch races with the same tuple lock being acquired in heap_update -- one of these sessions sleeps on the other one to finish while holding the tuple lock, and the other one sleeps on the tuple lock. Per trouble report from Andrew Sackville-West in http://www.postgresql.org/message-id/20140731233051.GN17765@andrew-ThinkPad-X230 His scenario can be simplified down to a relatively simple isolationtester spec file which I don't include in this commit; the reason is that the current isolationtester is not able to deal with more than one blocked session concurrently and it blocks instead of raising the expected deadlock. In the future, if we improve isolationtester, it would be good to include the spec file in the isolation schedule. I posted it in http://www.postgresql.org/message-id/20141212205254.GC1768@alvh.no-ip.org Hat tip to Mark Kirkwood, who helped diagnose the trouble. |
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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. 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/.