Re: Plan stability versus near-exact ties in cost estimates

Stephen Frost <sfrost@snowman.net> writes:
> * Tom Lane (tgl@sss.pgh.pa.us) wrote:
>> A variant idea would be to replace the exact cost comparison with a
>> second round of fuzzy cost comparison, but with a much tighter fuzz
>> factor, maybe 1e-6 instead of 0.01.

> Not impressed with this idea- the notion that our model is good enough
> to produce valid values out to that many digits is, well, unlikely.

While I remain convinced of the abstract truth of that position, I've
committed a patch that uses a second round of fuzzy comparison.  It
turned out that simply removing the exact comparison as per my first
proposal resulted in a surprisingly large number of changes in the
regression test results; apparently, a lot more cases than we realized
have multiple plans with indistinguishable costs.  I didn't feel like
going through and validating the test result changes, and besides that
this could have resulted in changes in behavior-in-the-field that people
would complain about.
        regards, tom lane