mirror of
https://github.com/postgres/postgres.git
synced 2025-05-03 22:24:49 +03:00
ca2d6a6cef
In bug #7626, Brian Dunavant exposes a performance problem created by commit 3b8968f25232ad09001bf35ab4cc59f5a501193e: that commit attempted to consider *all* possible combinations of indexable join clauses, but if said clauses join to enough different relations, there's an exponential increase in the number of outer-relation sets considered. In Brian's example, all the clauses come from the same equivalence class, which means it's redundant to use more than one of them in an indexscan anyway. So we can prevent the problem in this class of cases (which is probably the majority of real examples) by rejecting combinations that would only serve to add a known-redundant clause. But that still leaves us exposed to exponential growth of planning time when the query has a lot of non-equivalence join clauses that are usable with the same index. I chose to prevent such cases by setting an upper limit on the number of relation sets considered, equal to ten times the number of index clauses considered so far. (This sliding limit still allows new relsets to be added on as we move to additional index columns, which is probably more important than considering even more combinations of clauses for the previous column.) This should keep the amount of work done roughly linear rather than exponential in the apparent query complexity. This part of the fix is pretty ad-hoc; but without a clearer idea of real-world cases for which this would result in markedly inferior plans, it's hard to see how to do better.
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. Changes between all PostgreSQL releases are recorded in the file HISTORY. 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/.