Constraint exclusion on UNION ALL subqueries with WHERE conditions

Gunnlaugur Þór Briem <gunnlaugur@xxxxxxxxx> · Wed, 14 Sep 2011 10:53:22 -0700 (PDT)

Hi,

I did this:

CREATE VIEW unionview AS
  SELECT col, otherstuff FROM (heavy subquery)
  WHERE col BETWEEN 1 AND 3
  UNION ALL
  SELECT col, otherstuff FROM (another heavy subquery)
  WHERE col BETWEEN 4 AND 6;

hoping that the planner could use the WHERE conditions (like it would use check constraints on tables) to exclude one of the subqueries, for a query like:

SELECT * FROM unionview WHERE col=2;

But it doesn't. (In PostgreSQL 8.4.5, at least.)

Is there a way (currently) to get the planner to use these conditions to exclude subqueries in the UNION ALL? Or is this a case of “sounds nice, but too rare to merit implementing”?

Thanks,

- Gulli

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