Re: [HACKERS] RE: [GENERAL] Long update query ? (also Re: [GENERAL] CNF vs. DNF) - Mailing list pgsql-hackers

I have another idea.

When we cnfify, this:

    (A AND B) OR (C AND D)

becomes

    (A OR C) AND (A OR D) AND (B OR C) AND (B OR D)

however if A and C are identical, this could become:

    (A OR A) AND (A OR D) AND (B OR A) AND (B OR D)

and A OR A is A:

    A AND (A OR D) AND (B OR A) AND (B OR D)

and since we are now saying A has to be true, we can remove OR's with A:

    A AND (B OR D)

Much smaller, and a big win for queries like:

    SELECT *
    FROM tab
    WHERE    (a=1 AND b=2) OR
        (a=1 AND b=3)

This becomes:

        (a=1) AND (b=2 OR b=3)

which is accurate, and uses our OR indexing.

Seems I could code cnfify() to look for identical qualifications in two
joined OR clauses and remove the duplicates.

Sound like big win, and fairly easy and inexpensive in processing time.

Comments?

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