I have a table with 165M records like this:
Performance id integer installs integer hour timestamp without time zone
I also have an index on hour:
CREATE INDEX hour_idx ON performance USING btree (hour DESC NULLS LAST);
However, select top 10 records ordered by hour takes 6! minutes
EXPLAIN ANALYZE select hour from performance order by hour desc limit 10
Limit (cost=7952135.23..7952135.25 rows=10 width=8) (actual time=376313.958..376313.964 rows=10 loops=1) -> Sort (cost=7952135.23..8368461.00 rows=166530310 width=8) (actual time=376313.957..376313.960 rows=10 loops=1) Sort Key: hour Sort Method: top-N heapsort Memory: 25kB -> Seq Scan on performance (cost=0.00..4353475.10 rows=166530310 width=8) (actual time=0.006..327149.828 rows=192330557 loops=1) Planning time: 0.070 ms Execution time: 376330.573 ms
Why is it taking so slow? if there is an index on date field desc – should it be super quick to retrieve data?
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