Re: [PERFORM] Poor Performance for large queries

[John Meinel <john(at)johnmeinel(dot)com>]

Richard Huxton wrote:
> John Meinel wrote:
>
> > So notice that when doing the actual select it is able to do the index 
> > query. But for some reason with a prepared statement, it is not able 
> > to do it.
> >
> > Any ideas?
>
>
> In the index-using example, PG knows the value you are comparing to. So, 
> it can make a better estimate of how many rows will be returned. With 
> the prepared/compiled version it has to come up with a plan that makes 
> sense for any value.
>
> If you look back at the explain output you'll see PG is guessing 181,923 
> rows will match with the prepared query but only 1 for the second query. 
> If in fact you returned that many rows, you wouldn't want to use the 
> index - it would mean fetching values twice.
>
> The only work-around if you are using plpgsql functions is to use 
> EXECUTE to make sure your queries are planned for each value provided.
I suppose that make sense. If the number was small (< 100) then there 
probably would be a lot of responses. Because the tproject table is all 
small integers.

But for a large number, it probably doesn't exist on that table at all.

Thanks for the heads up.

John
=:->

Attachment: signature.asc
Description: OpenPGP digital signature