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Previously it was possible that a tuple was not pruned during vacuum, even though its update xmax (i.e. the updating xid in a multixact with both key share lockers and an updater) was below the cutoff horizon. As the freezing code assumed, rightly so, that that's not supposed to happen, xmax would be preserved (as a member of a new multixact or xmax directly). That causes two problems: For one the tuple is below the xmin horizon, which can cause problems if the clog is truncated or once there's an xid wraparound. The bigger problem is that that will break HOT chains, which in turn can lead two to breakages: First, failing index lookups, which in turn can e.g lead to constraints being violated. Second, future hot prunes / vacuums can end up making invisible tuples visible again. There's other harmful scenarios. Fix the problem by recognizing that tuples can be DEAD instead of RECENTLY_DEAD, even if the multixactid has alive members, if the update_xid is below the xmin horizon. That's safe because newer versions of the tuple will contain the locking xids. A followup commit will harden the code somewhat against future similar bugs and already corrupted data. Author: Andres Freund, with changes by Alvaro Herrera Reported-By: Daniel Wood Analyzed-By: Andres Freund, Alvaro Herrera, Robert Haas, Peter Geoghegan, Daniel Wood, Yi Wen Wong, Michael Paquier Reviewed-By: Alvaro Herrera, Robert Haas, Michael Paquier Discussion: https://postgr.es/m/E5711E62-8FDF-4DCA-A888-C200BF6B5742@amazon.com https://postgr.es/m/20171102112019.33wb7g5wp4zpjelu@alap3.anarazel.de Backpatch: 9.3-
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: https://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 https://www.postgresql.org/download/. For more information look at our web site located at https://www.postgresql.org/.
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