Pavel Stehule <pavel.stehule@gmail.com> writes: > -- I was surprised, so following query can use index > postgres=# explain select a from test2 where a at time zone > 'America/Santiago' >= now() at time zone 'America/Santiago' ; > QUERY > PLAN > ══════════════════════════════════════════════════════════════════════════════════════════════ > Index Only Scan using test2_a_idx on test2 (cost=0.13..12.18 rows=1 > width=8) > Filter: (timezone('America/Santiago'::text, a) >= > timezone('America/Santiago'::text, now())) > (2 rows)
This plan isn't actually "using" the index in any meaningful way; it's applying the where condition as a filter. It happens to be sane to use the index as a dumb data source, because it can be an index-only scan, and that might (if you're lucky and don't hit too many recheckable rows) be cheaper than a seqscan. But we don't consider plain indexscans as worth the trouble to consider in such cases, because a full-table plain indexscan can never beat a seqscan, either in the planner's cost model or in reality. > why, the index isn't used in this case? > postgres=# explain select a,b from test2 where a at time zone > 'America/Santiago' >= now() at time zone 'America/Santiago' ;
Can't be an index-only scan because of the use of b, so there's no possible way that this can be better than a seqscan.
