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 --- src/backend/utils/misc/guc_tables.c | 10 ++++++++++ 1 file changed, 10 insertions(+) (limited to 'src/backend/utils/misc/guc_tables.c') diff --git a/src/backend/utils/misc/guc_tables.c b/src/backend/utils/misc/guc_tables.c index 8a67f01200c..9845abd6932 100644 --- a/src/backend/utils/misc/guc_tables.c +++ b/src/backend/utils/misc/guc_tables.c @@ -996,6 +996,16 @@ struct config_bool ConfigureNamesBool[] = true, NULL, NULL, NULL }, + { + {"enable_distinct_reordering", PGC_USERSET, QUERY_TUNING_METHOD, + gettext_noop("Enables reordering of DISTINCT pathkeys."), + NULL, + GUC_EXPLAIN + }, + &enable_distinct_reordering, + true, + NULL, NULL, NULL + }, { {"geqo", PGC_USERSET, QUERY_TUNING_GEQO, gettext_noop("Enables genetic query optimization."), -- cgit v1.2.3