From a8ccf4e93a7eeaae66007bbf78cf9183ceb1b371 Mon Sep 17 00:00:00 2001 From: Richard Guo Date: Tue, 26 Nov 2024 09:25:18 +0900 Subject: Reordering DISTINCT keys to match input path's pathkeys The ordering of DISTINCT items is semantically insignificant, so we can reorder them as needed. In fact, in the parser, we absorb the sorting semantics of the sortClause as much as possible into the distinctClause, ensuring that one clause is a prefix of the other. This can help avoid a possible need to re-sort. In this commit, we attempt to adjust the DISTINCT keys to match the input path's pathkeys. This can likewise help avoid re-sorting, or allow us to use incremental-sort to save efforts. For DISTINCT ON expressions, the parser already ensures that they match the initial ORDER BY expressions. When reordering the DISTINCT keys, we must ensure that the resulting pathkey list matches the initial distinctClause pathkeys. This introduces a new GUC, enable_distinct_reordering, which allows the optimization to be disabled if needed. Author: Richard Guo Reviewed-by: Andrei Lepikhov Discussion: https://postgr.es/m/CAMbWs48dR26cCcX0f=8bja2JKQPcU64136kHk=xekHT9xschiQ@mail.gmail.com --- doc/src/sgml/config.sgml | 14 ++++++++++++++ 1 file changed, 14 insertions(+) (limited to 'doc/src') diff --git a/doc/src/sgml/config.sgml b/doc/src/sgml/config.sgml index a84e60c09b9..76ab72db964 100644 --- a/doc/src/sgml/config.sgml +++ b/doc/src/sgml/config.sgml @@ -5360,6 +5360,20 @@ ANY num_sync ( + enable_distinct_reordering (boolean) + + enable_distinct_reordering configuration parameter + + + + + Enables or disables the query planner's ability to reorder DISTINCT + keys to match the input path's pathkeys. The default is on. + + + + enable_gathermerge (boolean) -- cgit v1.2.3