On Thu, Mar 30, 2006 at 02:23:53PM +0200, Peter Eisentraut wrote:
>>> EXPLAIN ANALYZE select activity_id from activity where state in (10000,
>>> 10001) order by activity_id limit 100;
>>>
>>> QUERY PLAN
>>>
>>> Limit (cost=0.00..622.72 rows=100 width=8) (actual
>>> time=207356.054..207356.876 rows=100 loops=1)
>>> -> Index Scan using activity_pk on activity (cost=0.00..40717259.91
>>> rows=6538650 width=8) (actual time=207356.050..207356.722 rows=100
>>> loops=1) Filter: ((state = 10000) OR (state = 10001))
>>> Total runtime: 207357.000 ms
>>>
>>> The table has seen VACUUM FULL and REINDEX before this.
>> The index scan is by activity_id, not by state. Do you have an index on
>> state at all?
> There is an index on state as well but the column is not selective enough.
Well, it's logical enough; it scans along activity_id until it finds one with
state=10000 or state=10001. You obviously have a _lot_ of records with low
activity_id and state none of these two, so Postgres needs to scan all those
records before it founds 100 it can output. This is the “startup cost” you're
seeing.
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