"Chuck D." <pgsql-performance@nullmx.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