Re: Rewriting DISTINCT and losing performance

[Tom Lane <tgl(at)sss(dot)pgh(dot)pa(dot)us>]

"Chuck D." <pgsql-performance(at)nullmx(dot)com> writes:
> Doesn't that seem a bit strange?  Does it have to do with the smaller size of
> the new table maybe?

No, it seems to be a planner bug:
http://archives.postgresql.org/pgsql-hackers/2007-05/msg00920.php

I imagine that your table statistics are close to the critical point
where a bitmap scan looks cheaper or more expensive than a plain index
scan, and so the chosen plan varies depending on more-or-less chance
factors.  Certainly getting rid of NULLs shouldn't have had any direct
impact on this choice.

			regards, tom lane