"Markus Wollny" <Markus(dot)Wollny(at)computec(dot)de> writes:
>> Could we see the pg_stats row for answer.session_id in both
>> 8.0 and 8.1?
> Here you are:
> 8.1:
> Correlation -0.0736492
> 8.0.3:
> Correlation -0.237136
Interesting --- if the 8.1 database is a dump and restore of the 8.0,
you'd expect the physical ordering to be similar. Why is 8.1 showing
a significantly lower correlation? That has considerable impact on the
estimated cost of an indexscan (plain not bitmap), and so it might
explain why 8.1 is mistakenly avoiding the indexscan ...
regards, tom lane