Re: Recheck condition

On Fri, Nov 30, 2007 at 08:21:18AM -0500, Josh Harrison wrote:
> > > *Query1*
> > > SELECT person_id FROM person WHERE (column1=1 AND column2='62')
> > > INTERSECT
> > > SELECT person_id FROM person WHERE (column1=1 AND column2='189')

> I get the same plan(see below) with 'sort' for 'intersect all' operation
> too. Why is intersect not an effecient way? Is there any other way this
> query/index can be written/created so that I can get the intersect results
> in an efficient way?

Set operations are rather inefficient. To find the intersection of two
arbitrary sets you need to sort them and compare. A query like you
write would be better expressed as a join, something like:

SELECT a.person_id
FROM (SELECT person_id FROM person WHERE (column1=1 AND column2='62') a,
(SELECT person_id FROM person WHERE (column1=1 AND column2='189') b
WHERE a.person_id = b.person_id;

or perhaps:

SELECT a.person_id
FROM person a, person b
WHERE a.column1=1 AND a.column2='62'
AND b.column1=1 AND b.column2='189'
AND a.person_id = b.person_id;

Which will probably generate a merge join...

Have a nice day,
--
Martijn van Oosterhout <kleptog(at)svana(dot)org> http://svana.org/kleptog/
> Those who make peaceful revolution impossible will make violent revolution inevitable.
> -- John F Kennedy