Re: Recheck condition

>Some indexes are inexact, i.e. they may sometimes return tuples that
>don't actually match the index condition.

What causes an index to be inexact. When you create an index and vacuum it
regularly, it is suppose to be correct....right??

>This also happens with bitmap
>scans, because it'll return anything in the bitmap which will probably
>be more than what you asked for. The recheck just means that the
>planner retests the index condition on the result to make sure you only
>get the rows you wanted

So does recheck condition affect the performance of the queries since it
basically rechecks the condition?
Also does it goes to the heap to retest ?

For example for this query
explain analyze select count(*) from foo where foo_id=1 I get the following
plan

QUERY PLAN

--------------------------------------------------------------------------------------------------------------------------------

Aggregate (cost=1949.84..1949.85 rows=1 width=0) (actual
time=7.996..7.996rows=1 loops=1)
-> Bitmap Heap Scan on foo (cost=277.45..1924.94 rows=9959 width=0)
(actual time=1.903..5.270 rows=10020 loops=1)
Recheck Cond: (foo_id =
1::numeric)

-> Bitmap Index Scan on foo_pk (cost=0.00..274.96 rows=9959
width=0) (actual time=1.864..1.864 rows=10020 loops=1)
Index Cond: (foo_id =
1::numeric)

Total runtime: 8.062 ms

Can you please explain to me with respect to this example what is happening
here? This is a small table but for big tables the performance is not very
good. Does recheck condition brings down the query performance?

Thanks
josh