Re: add_path optimization

Did this ever get applied/resolved?

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Robert Haas wrote:
> I've been doing some benchmarking and profiling on the PostgreSQL
> query analyzer, and it seems that (at least for the sorts of queries
> that I typically run) the dominant cost is add_path(). I've been able
> to find two optimizations that seem to help significantly:
>
> 1. add_path() often calls compare_fuzzy_path_costs() twice on the same
> pair of paths, and when the paths compare equal on one criterion, some
> comparisons are duplicated. I've refactored this function to return
> the results of both calculations without repeating any floating-point
> arithmetic.
>
> 2. match_unsorted_outer() adds as many as 5 nested loop joins at a
> time with the same set of pathkeys. In my tests, it tended to be ~3 -
> cheapest inner, cheapest inner materialized, and cheapest inner index.
> Since these all have the same pathkeys, clearly only the one with the
> cheapest total cost is in the running for cheapest total cost for that
> set of pathkeys, and likewise for startup cost (and the two may be the
> same). Yet we compare all of them against the whole pathlist, one
> after the other, including (for the most part) the rather expensive
> pathkey comparison. I've added a function add_similar_paths() and
> refactored match_unsorted_outer() to use it.
>
> On a couple of complex (and proprietary) queries with 12+ joins each,
> I measure a planning time improvement of 8-12% with the attached patch
> applied. It would be interesting to try to replicate this on a
> publicly available data set, but I don't know of a good one to use.
> Suggestions welcome - results of performance testing on your own
> favorite big queries even more welcome. Simple test harness also
> attached. I took the approach of dropping caches, starting the
> server, and then running this 5 times each on several queries,
> dropping top and bottom results.
>
> ...Robert

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Bruce Momjian <bruce(at)momjian(dot)us> http://momjian.us
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