Re: [HACKERS] Idea for speeding up uncorrelated subqueries

"Ansley, Michael" wrote:
> 
> I thought that all correlated and uncorrelated sub-queries could be
> rewritten as a join, simplifying the query tree.  It should be a mechanical
> process which can probably be performed in the rewriter.

IN can't be rewritten as a join! Subquery may return duplicates
and join would return tuple for all of them. 

And how about WHERE x = (select max(y) from ...) ?

And even for WHERE x = (select y from ...) we have to check
that subquery returns exactly ONE row, or abort.

Vadim