Hello, PostgreSQL doesn't use 'Index Only Scan' if there is an _expression_ in index. The documentation says that PostgreSQL's planner considers a query to be potentially executable by index-only scan only when all columns needed by the query are available from the index. I think an example on https://www.postgresql.org/docs/16/indexes-index-only-scans.html : SELECT f(x) FROM tab WHERE f(x) < 1; is a bit confusing. Even the following query does not use 'Index Only Scan' SELECT 1 FROM tab WHERE f(x) < 1; Demonstration: --------------------------- drop table if exists test; create table test(s text); create index ix_test_upper on test (upper(s)); create index ix_test_normal on test (s); insert into test (s) select 'Item' || t.i from pg_catalog.generate_series(1, 100000, 1) t(i); analyze verbose "test"; explain select 1 from test where s = 'Item123'; explain select 1 from test where upper(s) = upper('Item123'); -------------------------- Query plan 1: Index Only Scan using ix_test_normal on test (cost=0.42..8.44 rows=1 width=4) Index Cond: (s = 'Item123'::text) Query plan 2 (SHOULD BE 'Index Only Scan'): Index Scan using ix_test_upper on test (cost=0.42..8.44 rows=1 width=4) Index Cond: (upper(s) = 'ITEM123'::text) ------------------------ If I add 's' as included column to ix_test_upper the plan does use 'Index Only Scan'. That looks strange to me: there is no 's' in SELECT-clause, only in WHERE-clause in the form of 'upper(s)' and this is why ix_test_upper is choosen by the planner. Thanks, Pavel |
