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The original coding in tuplestore_trim() was only meant to work efficiently in cases where each trim call deleted most of the tuples in the store. Which, in fact, was the pattern of the original usage with a Material node supporting mark/restore operations underneath a MergeJoin. However, WindowAgg now uses tuplestores and it has considerably less friendly trimming behavior. In particular it can attempt to trim one tuple at a time off a large tuplestore. tuplestore_trim() had O(N^2) runtime in this situation because of repeatedly shifting its tuple pointer array. Fix by avoiding shifting the array until a reasonably large number of tuples have been deleted. This can waste some pointer space, but we do still reclaim the tuples themselves, so the percentage wastage should be pretty small. Per Jie Li's report of slow percent_rank() evaluation. cume_dist() and ntile() would certainly be affected as well, along with any other window function that has a moving frame start and requires reading substantially ahead of the current row. Back-patch to 8.4, where window functions were introduced. There's no need to tweak it before that. |
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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/.