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Here we adjust the costs for WindowAggs so that they properly take into account how much of their subnode they must read before outputting the first row. Without this, we always assumed that the startup cost for the WindowAgg was not much more expensive than the startup cost of its subnode, however, that's going to be completely wrong in many cases. The WindowAgg may have to read *all* of its subnode to output a single row with certain window bound options. Here we estimate how many rows we'll need to read from the WindowAgg's subnode and proportionally add more of the subnode's run costs onto the WindowAgg's startup costs according to how much of it we expect to have to read in order to produce the first WindowAgg row. The reason this is more important than we might have initially thought is that we may end up making use of a path from the lower planner that works well as a cheap startup plan when the query has a LIMIT clause, however, the WindowAgg might mean we need to read far more rows than what the LIMIT specifies. No backpatch on this so as not to cause plan changes in released versions. Bug: #17862 Reported-by: Tim Palmer Author: David Rowley Reviewed-by: Andy Fan Discussion: https://postgr.es/m/17862-1ab8f74b0f7b0611@postgresql.org Discussion: https://postgr.es/m/CAApHDvrB0S5BMv+0-wTTqWFE-BJ0noWqTnDu9QQfjZ2VSpLv_g@mail.gmail.com |
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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: 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/.