Re: Plan uses wrong index, preferring to scan pkey index instead

Yuri Kunde Schlesner <yuriks+lists@yuriks.net> writes:
> Does anyone know if there's any tweaking I can do in Postgres so that it
> uses the appropriate plan?

I suspect that the reason the planner likes the backlog_pkey is that it's
almost perfectly correlated with table order, which greatly reduces the
number of table fetches that need to happen over the course of a indexscan
compared to using the less-well-correlated bufferid+messageid index.
So that way is estimated to be cheaper than using the less-correlated
index ... and that may even be true except for outlier bufferid values
with no recent messages.

You could try fooling around with the planner cost parameters
(particularly random_page_cost) to see if that changes the decision;
but it's usually a bad idea to alter cost parameters on the basis of
tweaking a single query, and even more so for tweaking an outlier
case of a single query.

What I think might be a workable solution, assuming you can stand a little
downtime to do it, is to CLUSTER the table on the bufferid+messageid
index.  This would reverse the correlation advantage and thereby solve
your problem.  Now, ordinarily CLUSTER is only a temporary solution
because the cluster-induced ordering degrades over time.  But I think it
would likely be a very long time until you accumulate so many new messages
that the table as a whole looks well-correlated on messageid alone.

            regards, tom lane