Re: nested loop semijoin estimates

On 05/30/15 01:20, Tomas Vondra wrote:
>
> Notice the cost - it's way lover than the previous plan (9.2 vs
> ~111k), yet this plan was not chosen. So either the change broke
> something (e.g. by violating some optimizer assumption), or maybe
> there's a bug somewhere else ...

After a bit more investigation, what I think is happening here is 
add_path() does not realize this is a SEMI join and a single tuple is 
enough, and discards the simple indexscan path in favor of the bitmap 
index scan as that seems cheaper when scanning everything.

So not really a bug, but maybe 'consider_startup' would help?


--
Tomas Vondra                  http://www.2ndQuadrant.com
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