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If a sub-select-in-FROM gets flattened into the upper query, then we naturally get rid of any output columns that are defined in the sub-select text but not actually used in the upper query. However, this doesn't happen when it's not possible to flatten the subquery, for example because it contains GROUP BY, LIMIT, etc. Allowing the subquery to compute useless output columns is often fairly harmless, but sometimes it has significant performance cost: the unused output might be an expensive expression, or it might be a Var from a relation that we could remove entirely (via the join-removal logic) if only we realized that we didn't really need that Var. Situations like this are common when expanding views, so it seems worth taking the trouble to detect and remove unused outputs. Because the upper query's Var numbering for subquery references depends on positions in the subquery targetlist, we don't want to renumber the items we leave behind. Instead, we can implement "removal" by replacing the unwanted expressions with simple NULL constants. This wastes a few cycles at runtime, but not enough to justify more work in the planner. |
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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. 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/.