Bruce Momjian <bruce@momjian.us> writes:
> I never understood why we don't just keep the selectivity estimates of
> previous plans and just reuse the plan if the selectivity estimates are
> similar. Isn't parameter selectivity the only thing that distinguishes
> on plan with parameter from another?
> Checking selectivity estimates must be cheaper than replanning. This
> could be done at the second use of the prepared plan, and maybe for all
> plan reuses, rather than waiting for five and then perhaps getting this
> bad behavior.
You're imagining that a selectivity recheck could be separated out from
the rest of the planner. That's nowhere near feasible, IMO. Even if it
were, what would we do with it? There's no reliable way to determine
whether X% change in one or another selectivity number would change the
selected plan, other than by redoing practically all of the planning work.
regards, tom lane