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697492434 added 3 new qsort specialization functions aimed to improve the performance of sorting many of the common pass-by-value data types when they're the leading or only sort key. Unfortunately, that has caused a performance regression when sorting datasets where many of the values being compared were equal. What was happening here was that we were falling back to the standard sort comparison function to handle tiebreaks. When the two given Datums compared equally we would incur both the overhead of an indirect function call to the standard comparer to perform the tiebreak and also the standard comparer function would go and compare the leading key needlessly all over again. Here improve the situation in the 3 new comparison functions. We now return 0 directly when the two Datums compare equally and we're performing a 1-key sort. Here we don't do anything to help the multi-key sort case where the leading key uses one of the sort specializations functions. On testing this case, even when the leading key's values are all equal, there appeared to be no performance regression. Let's leave it up to future work to optimize that case so that the tiebreak function no longer re-compares the leading key over again. Another possible fix for this would have been to add 3 additional sort specialization functions to handle single-key sorts for these pass-by-value types. The reason we didn't do that here is that we may deem some other sort specialization to be more useful than single-key sorts. It may be impractical to have sort specialization functions for every single combination of what may be useful and it was already decided that further analysis into which ones are the most useful would be delayed until the v16 cycle. Let's not let this regression force our hand into trying to make that decision for v15. Author: David Rowley Reviewed-by: John Naylor Discussion: https://postgr.es/m/CA+hUKGJRbzaAOUtBUcjF5hLtaSHnJUqXmtiaLEoi53zeWSizeA@mail.gmail.com
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/.