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Relations dropped in a single transaction are tracked in a list of unowned relations. With large number of dropped relations this resulted in poor performance at the end of a transaction, when the relations are removed from the singly linked list one by one. Commitb4166911attempted to address this issue (particularly when it happens during recovery) by removing the relations in a reverse order, resulting in O(1) lookups in the list of unowned relations. This did not work reliably, though, and it was possible to trigger the O(N^2) behavior in various ways. Instead of trying to remove the relations in a specific order with respect to the linked list, which seems rather fragile, switch to a regular doubly linked. That allows us to remove relations cheaply no matter where in the list they are. Asb4166911was a bugfix, backpatched to all supported versions, do the same thing here. Reviewed-by: Alvaro Herrera Discussion: https://www.postgresql.org/message-id/flat/80c27103-99e4-1d0c-642c-d9f3b94aaa0a%402ndquadrant.com Backpatch-through: 9.4
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/.