From: | Tom Lane <tgl(at)sss(dot)pgh(dot)pa(dot)us> |
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To: | Peter Eisentraut <peter_e(at)gmx(dot)net> |
Cc: | pgsql-hackers(at)postgresql(dot)org, Robert Haas <robertmhaas(at)gmail(dot)com>, Andrew Dunstan <andrew(at)dunslane(dot)net>, Josh Berkus <josh(at)agliodbs(dot)com> |
Subject: | Re: machine-readable explain output |
Date: | 2009-06-14 15:21:18 |
Message-ID: | 11242.1244992878@sss.pgh.pa.us |
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Peter Eisentraut <peter_e(at)gmx(dot)net> writes:
> A regression test for EXPLAIN, however, should primarily check whether the
> output format is stable. We are planning to offer this as a public interface,
> after all. You could use faked up statistics and all but one or two plan
> types turned off, and then the results should be pretty stable.
You'd be surprised :-(. We've found in the past that queries in the
regression tests get different plans across different platforms just
because of alignment-rule differences (leading to different numbers
of rows per page, etc etc). I think that test cases could be chosen
to be relatively stable points in the plan space, but it's hopeless
to imagine that the low-order digits of cost estimates will be the
same across all platforms.
regards, tom lane
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