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> > 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...
[Moved to Hackers list.]
This is interesting. Check CNF size and DNF size. Choose smallest.
CNF uses existing code, DNF converts to UNIONs. How do you return the
proper rows with/without proper duplicates?
i.e.
SELECT * FROM tab1 WHERE x > 1 or x > 2
We need to return all rows where x > 1, even if some there are indentical
rows in tab1.
What I do in the index OR code is to test that rows in index matches
found in 2nd and 3rd index scans are false in earlier index scans. I am
not sure how to do that with a UNION query, but it may be possible.
We currently have UNION and UNION ALL, and I think we may need a new
UNION type internally to prevent 2nd and 3rd queries from returning rows
returned by earlier UNION queries.
This is interesting.
--
Bruce Momjian | http://www.op.net/~candle
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