Fix distinctness check for queries with grouping sets query_is_distinct_for() is intended to determine whether a query never returns duplicates of the specified columns. For queries using grouping sets, if there are no grouping expressions, the query may contain one or more empty grouping sets. The goal is to detect whether there is exactly one empty grouping set, in which case the query would return a single row and thus be distinct.
The previous logic in query_is_distinct_for() was incomplete because the check was insufficiently thorough and could return false when it could have returned true. It failed to consider cases where the DISTINCT clause is used on the GROUP BY, in which case duplicate empty grouping sets are removed, leaving only one. It also did not correctly handle all possible structures of GroupingSet nodes that represent a single empty grouping set. To fix, add a check for the groupDistinct flag, and expand the query's groupingSets tree into a flat list, then verify that the expanded list contains only one element. No backpatch as this could result in plan changes. Author: Richard Guo <[email protected]> Reviewed-by: David Rowley <[email protected]> Discussion: https://postgr.es/m/cambws480z04ntp8-o55uroq2zego309+h3hhazhz6ztmgwl...@mail.gmail.com Branch ------ master Details ------- https://git.postgresql.org/pg/commitdiff/f00484c170f56199c3eeacc82bd72f8c1e3baf6b Modified Files -------------- src/backend/optimizer/plan/analyzejoins.c | 19 ++++--- src/test/regress/expected/join.out | 83 +++++++++++++++++++++++++++++++ src/test/regress/sql/join.sql | 32 ++++++++++++ 3 files changed, 127 insertions(+), 7 deletions(-)
