> It currently convert to CNF so it can select the most restrictive
> restriction and join, and use those first. However, the CNF conversion
> is a memory exploder for some queries, and we certainly need to have
> another method to split up those queries into UNIONS. I think we need
> to code to identify those queries capable of being converted to UNIONS,
> and do that before the query gets to the CNF section. That would be
> great, and David Hartwig has implemented a limited capability of doing
> this, but we really need a general routine to do this with 100%
> reliability.
Well, if you're talking about a routine to generate a heuristic for CNF vs.
DNF, it is possible to precalculate the query sizes for CNF and DNF
rewrites...
For conversion to CNF:
At every node:
if nodeType = AND then f(node) = f(left) + f(right)
if nodeType = OR then f(node) = f(left) * f(right)
f(root) = a reasonably (but not wonderful) metric
For DNF just switch AND and OR in the above. You may want to compute both
metrics and compare... take the smaller one and use that path.
How to deal with other operators depends on their implementation...
Taral
