Re: Revisiting default_statistics_target - Mailing list pgsql-hackers
From | Greg Sabino Mullane |
---|---|
Subject | Re: Revisiting default_statistics_target |
Date | |
Msg-id | 2f42d7264d2d0fe46810615466a71cb4@biglumber.com Whole thread Raw |
In response to | Revisiting default_statistics_target (Greg Smith <gsmith@gregsmith.com>) |
Responses |
Re: Revisiting default_statistics_target
("Joshua D. Drake" <jd@commandprompt.com>)
Re: Revisiting default_statistics_target (Stephen Frost <sfrost@snowman.net>) Re: Revisiting default_statistics_target ("Jignesh K. Shah" <J.K.Shah@Sun.COM>) Re: Revisiting default_statistics_target (Tom Lane <tgl@sss.pgh.pa.us>) Re: Revisiting default_statistics_target (Greg Smith <gsmith@gregsmith.com>) |
List | pgsql-hackers |
-----BEGIN PGP SIGNED MESSAGE----- Hash: RIPEMD160 > While most cases were dead even or a modest improvement, his dbt-2 results > suggest a 15-20% regression in 8.4. Changing the default_statistics_taget > to 100 was responsible for about 80% of that regression. ... > The situation where the stats target being so low hurts things the most > are the data warehouse use cases. Er...so why should we change our defaults to support data warehousing users? Certainly the rest of the postgresql.conf settings don't attempt to do that. > The bump from 10 to 100 was supported by microbenchmarks that suggested it > would be tolerable. No, the 10 to 100 was supported by years of people working in the field who routinely did that adjustment (and >100) and saw great gains. Also, as the one who originally started the push to 100, my original goal was to get it over the "magic 99" bump, at which the planner started acting very differently. This caused a huge performance regression in one of the Postgres releases (don't remember which one exactly), which severely impacted one of our large clients. > That doesn't seem to be reality here though, and it's questionable whether > this change really helps the people who need to fool with the value the most. The goal of defaults is not to help people who fool with the value - it's to get a good default out of the box for people who *don't* fool with all the values. :) > But unless someone has some compelling evidence to the contrary, it looks like > the stats target needs to go back to a lower value. Please don't. This is a very good change, and I don't see why changing it back because it might hurt people doing DW is a good thing, when most of users are not doing DW. > As for the change to constraint_exclusion, the regression impact there is > much less severe and the downside of getting it wrong is pretty bad. Similarly, the people who are affected by something like presumably are not running a default postgresql.conf anyway, so they can toggle it back to squeeze a little more performance out of their system. - -- Greg Sabino Mullane greg@turnstep.com End Point Corporation PGP Key: 0x14964AC8 200905221239 http://biglumber.com/x/web?pk=2529DF6AB8F79407E94445B4BC9B906714964AC8 -----BEGIN PGP SIGNATURE----- iEYEAREDAAYFAkoW1iEACgkQvJuQZxSWSsh1gACgqHBcwEd0zLsfbZJvCnXywlGp jZ8AoNn79heFG+iLE2uh6eZ0lxRmwuHR =/A/F -----END PGP SIGNATURE-----
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