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There's a heuristic in estimate_rel_size() to clamp the minimum size estimate for a table to 10 pages, unless we can see that vacuum or analyze has been run (and set relpages to something nonzero, so this will always happen for a table that's actually empty). However, it would be better not to do this for inheritance parent tables, which very commonly are really empty and can be expected to stay that way. Per discussion of a recent pgsql-performance report from Anish Kejariwal. Also prevent it from happening for indexes (although this is more in the nature of documentation, since CREATE INDEX normally initializes relpages to something nonzero anyway). Back-patch to 9.0, because the ability to collect statistics across a whole inheritance tree has improved the planner's estimates to the point where this relatively small error makes a significant difference. In the referenced report, merge or hash joins were incorrectly estimated as cheaper than a nestloop with inner indexscan on the inherited table. That was less likely before 9.0 because the lack of inherited stats would have resulted in a default (and rather pessimistic) estimate of the cost of a merge or hash join. |
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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/.