On Wed, Apr 4, 2012 at 6:47 AM, Kim Hansen <kim@rthansen.dk> wrote:
> Hi All
>
> I have a query where the planner makes a wrong cost estimate, it looks
> like it underestimates the cost of a "Bitmap Heap Scan" compared to an
> "Index Scan".
>
> This it the two plans, I have also pasted them below:
> Slow (189ms): http://explain.depesz.com/s/2Wq
> Fast (21ms): http://explain.depesz.com/s/ThQ
Could you do explain (analyze, buffers)?
Did you run these queries multiple times in both orders? If you just
ran them once each, in the order indicated, then the bitmap scan may
have done the hard work of reading all the needed buffers into cache,
and the index scan then got to enjoy that cache.
Cheers,
Jeff