Re: [HACKERS] Recognizing range constraints (was Re: Plan for relatively simple query seems to be very inefficient)

On Wed, Apr 06, 2005 at 06:09:37PM -0400, Tom Lane wrote:
> Can anyone suggest a more general rule? Do we need for example to
> consider whether the relation membership is the same in two clauses
> that might be opposite sides of a range restriction? It seems like
>
> a.x > b.y AND a.x < b.z

In a case like this, you could actually look at the data in b and see
what the average range size is. If you wanted to get really fancy, the
optimizer could decide how best to access a based on each row of b.

> probably can be treated as a range restriction on a.x for this purpose,
> but I'm much less sure that the same is true of
>
> a.x > b.y AND a.x < c.z

Well, this could end up being much trickier, since who knows how b and c
are related. Though thinking about it, although I threw out the
row-by-row analysis idea to be glib, that would actually work in this
case; you could take a look at what b and c look like each time 'through
the loop'.
--
Jim C. Nasby, Database Consultant decibel(at)decibel(dot)org
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