Re: Revisiting default_statistics_target

> Greg Smith <gsmith(at)gregsmith(dot)com> writes:
>> Yesterday Jignesh Shah presented his extensive benchmark results
>> comparing
>> 8.4-beta1 with 8.3.7 at PGCon:
>> http://blogs.sun.com/jkshah/entry/pgcon_2009_performance_comparison_of
>
>> 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 remainder
>> was from the constraint_exclusion change. That 80/20 proportion was
>> mentioned in the talk but not in the slides. Putting both those back to
>> the 8.3 defaults swapped things where 8.4b1 was ahead by 5% instead.
>
> Yeah, I saw that talk and I'm concerned too, but I think it's premature
> to conclude that the problem is precisely that stats_target is now too
> high. I'd like to see Jignesh check through the individual queries in
> the test and make sure that none of them had plans that changed for the
> worse. The stats change might have just coincidentally tickled some
> other planning issue.

Wouldn't he just need to rerun the tests with default_stats_target set to
the old value? I presume he has actually done this already in order to
come to the conclusion he did about the cause of the regression.

cheers

andrew