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=xekht9xsc...@mail.gmail.com Branch ------ master Details ------- https://git.postgresql.org/pg/commitdiff/a8ccf4e93a7eeaae66007bbf78cf9183ceb1b371 Modified Files -------------- doc/src/sgml/config.sgml | 14 + src/backend/optimizer/path/pathkeys.c | 38 ++ src/backend/optimizer/plan/planner.c | 456 +++++++++++++---------- src/backend/utils/misc/guc_tables.c | 10 + src/backend/utils/misc/postgresql.conf.sample | 1 + src/include/optimizer/optimizer.h | 1 + src/test/regress/expected/select_distinct.out | 132 +++++++ src/test/regress/expected/select_distinct_on.out | 125 +++++++ src/test/regress/expected/sysviews.out | 3 +- src/test/regress/sql/select_distinct.sql | 53 +++ src/test/regress/sql/select_distinct_on.sql | 40 ++ 11 files changed, 672 insertions(+), 201 deletions(-)
