Re: [SQL] index usage ... strange !?

> Marten Feldtmann <marten(at)feki(dot)toppoint(dot)de> writes:
> > SELECT AO,AT,AV FROM P3AT
> > WHERE EXISTS( SELECT AO FROM P3AT WHERE AV='12' AND AT=12 AND CI=17)
>
> Um ... I dunno what you are trying to accomplish, but this query
> almost certainly doesn't do what you want. Since the inner query
> is independent of the outer, you will get back either all the rows
> of P3AT (if the inner query yields rows) or none (if it doesn't).
>
> The plan you quote is perfectly reasonable for this query...
> the machine is even bright enough to figure out that it only
> needs to evaluate the subquery once.
>
> regards, tom lane
>

Yes, this seems to be a wrong statement. Actually I would write it like:

select AO,AT,AV FROM P3AT WHERE AO IN (SELECT AO FROM P3AT WHERE ...)

but as PostgreSQL does not like "IN" very much I rewrote it to the wrong
statement above.

select P3AT.AO,P3AT.AT,P3AT.AV FROM P3AT,P3AT as B WHERE
(B.AO=P3AT.AO) AND (B.AV='12') AND (B.AT=12) AND (B.CI=17)

Therefore I have to rewrite the sql statement above to:

SELECT P3AT.AV,P3AT.AT,P3AT.AV FROM P3AT WHERE
EXISTS( SELECT B.AO FROM P3AT as B WHERE B.AO=P3AT.AO AND ..... )

Right ?

Marten Feldtmann