Ed Tyrrill <tyrrill_ed@emc.com> writes:
> It seems to me that the first plan is the optimal one for this case, but
> when the planner has more information about the table it chooses not to
> use it. Do you think that if work_mem were higher it might choose the
> first plan again?
It's worth fooling around with work_mem just to see what happens. The
other thing that would be interesting is to force the other plan (set
enable_mergejoin = off) just to see what the planner is costing it at.
My suspicion is that the estimated costs are pretty close.
The ANALYZE stats affect this choice only in second-order ways AFAIR.
The planner penalizes hashes if it thinks there will be a lot of
duplicate values in the inner relation, but IIRC there is also a penalty
for inner duplicates in the mergejoin cost estimate. So I'm a bit
surprised that there'd be a change.
Can you show us the pg_stats rows for the join columns after analyzing
at target 10 and target 100?
regards, tom lane