Dror Matalon <[EMAIL PROTECTED]> writes: > explain analyze select count(*) from items where channel < 5000; > QUERY PLAN > -------------------------------------------------------------------------------------------------------------------------- > Aggregate (cost=249141.54..249141.54 rows=1 width=0) (actual > time=26224.603..26224.608 rows=1 loops=1) > -> Seq Scan on items (cost=0.00..245377.52 rows=1505605 width=0) (actual > time=7.599..17686.869 rows=1632057 loops=1) > Filter: (channel < 5000) > Total runtime: 26224.703 ms > > > How can it do a sequential scan and apply a filter to it in less time > than the full sequential scan? Is it actually using an index without > really telling me?
It's not using the index and not telling you. It's possible the count(*) operator itself is taking some time. Postgres doesn't have to call it on the rows that don't match the where clause. How long does "explain analyze select 1 from items" with and without the where clause take? What version of postgres is this?. In 7.4 (and maybe 7.3?) count() uses an int8 to store its count so it's not limited to 4 billion records. Unfortunately int8 is somewhat inefficient as it has to be dynamically allocated repeatedly. It's possible it's making a noticeable difference, especially with all the pages in cache, though I'm a bit surprised. There's some thought about optimizing this in 7.5. -- greg ---------------------------(end of broadcast)--------------------------- TIP 7: don't forget to increase your free space map settings
