Bruce Momjian <br...@momjian.us> wrote: > I talked to Robert Haas and he said that index-only scans do not > optimize COUNT(*). Is this something we can do for PG 9.2? Is > anyone working on this? Well, it's not that it doesn't optimize COUNT(*) -- it's that it doesn't yet cost the index scan as cheaper than a table scan when you're accessing every row. create table t (id int not null primary key); insert into t select generate_series(1, 1000000); vacuum freeze analyze; explain analyze select count(*) from t where id between 500000 and 500010; That gives you an index-only scan; but without the WHERE clause it uses a seq scan. I think it's mainly a matter of doing enough benchmarks to figure out how best to model the costs of the index scan so that it can be picked for that case. -Kevin
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