Re: Boolean and Non-Boolean Terms

From: Brian Selzer <brian_at_selzer-software.com>
Date: Wed, 30 Jan 2008 13:48:55 GMT
Message-ID: <bf%nj.4758$J41.383_at_newssvr14.news.prodigy.net>


"Roy Hann" <specially_at_processed.almost.meat> wrote in message news:LMOdnYIpi_PI0T3anZ2dnUVZ8vGdnZ2d_at_pipex.net...
>I am looking at some DB2 performance-tuning notes where I am seeing a
>phrase I've not seen before. The notes speak of a test in a WHERE clause
>that allows a row to be definitely exluded being a "Boolean Term". The
>converse is referred to a non-Boolean term.
>
> A quick Google tells me this terminology is not widely used outside DB2.
> Is there a more widely understood term meaning the same thing?
>
> Roy
>

Disjunctive predicates are those that have an 'OR' in them. They are a bit more difficult to optimize than simple and conjunctive predicates because whenever any term in a conjunctive predicate is false, the row is excluded, and therefore processing need not continue; whereas for disjunctive predicates, both sides of the 'OR' must be false in order to exclude a row.

Some other predicates are really disjunctive predicates, IN predicates, for example. They can cause performance on some systems to bottom out because often the conjunctive part of the query is executed many, many times--once for each element in the IN list. Received on Wed Jan 30 2008 - 14:48:55 CET

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