Re: factoring problem with view in 7.3.3

Rajesh Kumar Mallah <mallah@trade-india.com> writes:
> I have a view which is a union of select of certain feilds from
> indentical tables. The problem is when we query a column on
> which index exists exists foreach of the tables does not use the
> indexes.

Hard to be certain since you didn't show us the table definitions,
but I suspect the culprit is a datatype mismatch.  Here are the
comments for 7.3's subquery_is_pushdown_safe, which determines whether
it's okay to push down a qualifier:

 * Conditions checked here:
 *
 * 1. If the subquery has a LIMIT clause or a DISTINCT ON clause, we must
 * not push down any quals, since that could change the set of rows
 * returned.  (Actually, we could push down quals into a DISTINCT ON
 * subquery if they refer only to DISTINCT-ed output columns, but
 * checking that seems more work than it's worth.  In any case, a
 * plain DISTINCT is safe to push down past.)
 *
 * 2. If the subquery has any functions returning sets in its target list,
 * we do not push down any quals, since the quals
 * might refer to those tlist items, which would mean we'd introduce
 * functions-returning-sets into the subquery's WHERE/HAVING quals.
 * (It'd be sufficient to not push down quals that refer to those
 * particular tlist items, but that's much clumsier to check.)
 *
 * 3. If the subquery contains EXCEPT or EXCEPT ALL set ops we cannot push
 * quals into it, because that would change the results.  For subqueries
 * using UNION/UNION ALL/INTERSECT/INTERSECT ALL, we can push the quals
 * into each component query, so long as all the component queries share
 * identical output types.  (That restriction could probably be relaxed,
 * but it would take much more code to include type coercion code into
 * the quals, and I'm also concerned about possible semantic gotchas.)

1 and 2 don't seem to apply to your problem, which leaves 3 ...

(BTW, 7.4 has addressed all of the possible improvements noted in the
parenthetical remarks here.)

            regards, tom lane