Re: [GENERAL] Speed of joins using sparse indexes

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> This mail tries to explain the solution that I've found to address the
> problem of the joins that uses tables with very sparse indexes.
>
> The exact problem was :
>
> How can I manage the problem of select a few rows with a boolean atribute
> when they are 5 rows with flag='Y' in a table of 100000 rows?
> I't must to be an index, but
> the optimizer asumes that a Seq Scan is more cheap... yes, yes... I know :
> if I ask for the 100000 rows with flag='N' then Seq Scan is the solution,
> but the interesting query is the other : to extract the 5 rows with
> flag='Y' from whitin the 100000 rows with the flag='N'.
>
> A possible solution to optimize this kind of query is to create an auxiliar
> table with the id's of the 5 rows with flag='Y', maintained by rules watching
> the attribute flag in the target table. In this manner, I never do a
> update/insert in the flag table and I replace the "flag='Y'" in the query in
> favour of "TABLE.id=FLAG_TABLE.id" (another join).
>
> It's a kind of tell to Postgres  "Hey, I'm very interested in the rows with
> flag='Y'" ... :) and the results in speed-up are amazing.

This sounds like a very good solution.  Rules can help.

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