Re: Optimisation of INTERSECT expressions

On Tue, 23 Mar 2004, Phil Endecott wrote:

> Dear PostgresQL Experts,
>
> I am trying to get to the bottom of some efficiency problems and hope that
> you can help. The difficulty seems to be with INTERSECT expressions.
>
> I have a query of the form
> select A from T where C1 intersect select A from T where C2;
> It runs in about 100 ms.
>
> But it is equivalent to this query
> select A from T where C1 and C2;
> which runs in less than 10 ms.
>
> Looking at the output of "explain analyse" on the first query, it seems
> that PostgresQL always computes the two sub-expressions and then computes
> an explicit intersection on the results. I had hoped that it would notice
> that both subexpressions are scanning the same input table T and convert
> the expression to the second form.
>
> Is there a reason why it can't do this transformation?

Probably because noone's bothered to try to prove under what conditions
it's the same.

For example, given a non-unique A, the two queries can give different
answers (if say the same two A values match both C1 and C2 in different
rows how many output rows does each give? *), also given a non-stable A
(for example random) the two queries are not necessarily equivalent.