| From: | Tom Lane <tgl(at)sss(dot)pgh(dot)pa(dot)us> |
|---|---|
| To: | Tomas Vondra <tomas(dot)vondra(at)2ndquadrant(dot)com> |
| Cc: | Josh Berkus <josh(at)agliodbs(dot)com>, pgsql-hackers(at)postgresql(dot)org, Mark Wong <markwkm(at)gmail(dot)com> |
| Subject: | Re: nested loop semijoin estimates |
| Date: | 2015-06-02 21:27:32 |
| Message-ID: | 5926.1433280452@sss.pgh.pa.us |
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| Lists: | pgsql-hackers |
I wrote:
> Hm. In principle, add_path_precheck shouldn't be doing anything except
> rejecting paths that would get rejected anyway later on. However it
> strikes me that there's a logic error in it; specifically, this
> simplifying assumption is wrong:
> * For speed, we make exact rather than fuzzy cost comparisons. If an
> * old path dominates the new path exactly on both costs, it will
> * surely do so fuzzily.
After staring at this awhile longer, I still don't see exactly why it's
changing the results. Do we have instructions around here anyplace on
how to set up/use TPC-DS? I couldn't find anything about it on the
wiki ...
regards, tom lane
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