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If a sub-select's output column is a simple Var, recursively look for statistics applying to that Var, and use them if available. The need for this was foreseen ages ago, but we didn't have enough infrastructure to do it with reasonable speed until just now. We punt and stick with default estimates if the subquery uses set operations, GROUP BY, or DISTINCT, since those operations would change the underlying column statistics (particularly, the relative frequencies of different values) beyond recognition. This means that the types of sub-selects for which this improvement applies are fairly limited, since most subqueries satisfying those restrictions would have gotten flattened into the parent query anyway. But it does help for some cases, such as subqueries with ORDER BY or LIMIT. |
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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/.