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Re: Errors and Optimization: re-ordering predicates

From: Michael Kay <mike@saxonica.com>
Date: Sun, 3 Jan 2016 20:56:30 +0000
Cc: Public Joint XSLT XQuery XPath <public-xsl-query@w3.org>
Message-Id: <44011B87-8EFC-44A8-A9D5-88027DF210DC@saxonica.com>
To: Adam Retter <adam@exist-db.org>
> eXist does not do much in the way of predicate reordering, currently
> we suggest to users to put the most selective predicates first as we
> evaluate them from left-to-right. In future, we could do more
> intelligent reordering based on index statistics.

Aside: The optimum ordering, of course, is based on some combination of evaluation cost and selectivity. Saxon at the moment is doing a very rough - but still useful - estimate of evaluation cost, and is not attempting to take selectivity into account.

Michael Kay
Received on Sunday, 3 January 2016 20:57:05 UTC

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