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In 1753b1b027035029c2a2a1649065762fafbf63f3, the pg_sequence system catalog was introduced. This made sequence metadata changes transactional, while the actual sequence values are still behaving nontransactionally. This requires some refinement in how ALTER SEQUENCE, which operates on both, locks the sequence and the catalog. The main problems were: - Concurrent ALTER SEQUENCE causes "tuple concurrently updated" error, caused by updates to pg_sequence catalog. - Sequence WAL writes and catalog updates are not protected by same lock, which could lead to inconsistent recovery order. - nextval() disregarding uncommitted ALTER SEQUENCE changes. To fix, nextval() and friends now lock the sequence using RowExclusiveLock instead of AccessShareLock. ALTER SEQUENCE locks the sequence using ShareRowExclusiveLock. This means that nextval() and ALTER SEQUENCE block each other, and ALTER SEQUENCE on the same sequence blocks itself. (This was already the case previously for the OWNER TO, RENAME, and SET SCHEMA variants.) Also, rearrange some code so that the entire AlterSequence is protected by the lock on the sequence. As an exception, use reduced locking for ALTER SEQUENCE ... RESTART. Since that is basically a setval(), it does not require the full locking of other ALTER SEQUENCE actions. So check whether we are only running a RESTART and run with less locking if so. Reviewed-by: Michael Paquier <michael.paquier@gmail.com> Reported-by: Jason Petersen <jason@citusdata.com> Reported-by: Andres Freund <andres@anarazel.de>
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/.