David Brown <time(at)bigpond(dot)net(dot)au> writes:
> Actually, I'm surprised the planner came up with such a low cost for the
> single column index, unless ... perhaps correlation statistics aren't
> used when determining costs for multi-column indexes?
The correlation calculation for multi-column indexes is pretty whacked
out pre-8.0. I don't think it's that great in 8.0 either --- we really
need to make ANALYZE calculate the correlation explicitly for each
index, probably, rather than trying to use per-column correlations.
regards, tom lane