From: Manuel Reyes Bravo Subject: [PATCH v2] Reject index-only scans when the AM can return nothing Variant of Andrey Rachitskiy's patch for BUG #19638. His guard keys off bms_is_empty(index_canreturn_attrs). That bitmapset is also empty for an index whose columns are all expressions, because the loop that fills it skips them (attno == 0), so the guard rejects index-only scans over expression indexes as well -- for example count(*) over a table whose only index is on (a + b), which is a legitimate and useful plan. Test index->canreturn[] directly instead. plancat.c fills it per column from index_can_return(), expression columns included, so an expression btree has a true entry while an AM with amcanreturn == NULL has none. Measured on 18.6: the #19638 reproducer returns the correct answer, count(*) over an expression-only index keeps its Index Only Scan, and make check passes 231/231. Bug: 19638 Author: Andrey Rachitskiy Co-authored-by: Manuel Reyes Bravo Reported-by: Manuel Reyes Bravo Discussion: https://www.postgresql.org/message-id/19638-277d0f73dfaeaec8@postgresql.org --- src/backend/optimizer/path/indxpath.c | 25 ++++++++++++++++++++++++++ 1 file changed, 25 insertions(+) diff --git a/src/backend/optimizer/path/indxpath.c b/src/backend/optimizer/path/indxpath.c --- a/src/backend/optimizer/path/indxpath.c +++ b/src/backend/optimizer/path/indxpath.c @@ -2291,6 +2291,31 @@ /* Do we have all the necessary attributes? */ result = bms_is_subset(attrs_used, index_canreturn_attrs); + /* + * bms_is_subset() is true when attrs_used is empty, even if the index + * returns nothing. That would allow a broken index-only scan for AMs + * with amcanreturn == NULL. + * + * Test the AM's capability directly rather than the bitmapset, which is + * empty for expression-only indexes too (attno == 0 is skipped above) + * even though such an index can perfectly well feed an index-only scan. + */ + if (result) + { + bool any_canreturn = false; + + for (i = 0; i < index->ncolumns; i++) + { + if (index->canreturn[i]) + { + any_canreturn = true; + break; + } + } + if (!any_canreturn) + result = false; + } + bms_free(attrs_used); bms_free(index_canreturn_attrs);