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This patch introduces "pg_blocking_pids(int) returns int[]", which returns the PIDs of any sessions that are blocking the session with the given PID. Historically people have obtained such information using a self-join on the pg_locks view, but it's unreasonably tedious to do it that way with any modicum of correctness, and the addition of parallel queries has pretty much broken that approach altogether. (Given some more columns in the view than there are today, you could imagine handling parallel-query cases with a 4-way join; but ugh.) The new function has the following behaviors that are painful or impossible to get right via pg_locks: 1. Correctly understands which lock modes block which other ones. 2. In soft-block situations (two processes both waiting for conflicting lock modes), only the one that's in front in the wait queue is reported to block the other. 3. In parallel-query cases, reports all sessions blocking any member of the given PID's lock group, and reports a session by naming its leader process's PID, which will be the pg_backend_pid() value visible to clients. The motivation for doing this right now is mostly to fix the isolation tests. Commit 38f8bdcac4982215beb9f65a19debecaf22fd470 lobotomized isolationtester's is-it-waiting query by removing its ability to recognize nonconflicting lock modes, as a crude workaround for the inability to handle soft-block situations properly. But even without the lock mode tests, the old query was excessively slow, particularly in CLOBBER_CACHE_ALWAYS builds; some of our buildfarm animals fail the new deadlock-hard test because the deadlock timeout elapses before they can probe the waiting status of all eight sessions. Replacing the pg_locks self-join with use of pg_blocking_pids() is not only much more correct, but a lot faster: I measure it at about 9X faster in a typical dev build with Asserts, and 3X faster in CLOBBER_CACHE_ALWAYS builds. That should provide enough headroom for the slower CLOBBER_CACHE_ALWAYS animals to pass the test, without having to lengthen deadlock_timeout yet more and thus slow down the test for everyone else.
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/.