[HACKERS] constraint exclusion analysis caching

Yesterday a client and I were sad to discover that the overhead of
constraint exclusion is apparently O(n) in the number of partitions, and
that where we had ~180 partitions each with a simple constraint (check
(field = nnn)) the overhead appeared to amount to about 0.25s on some
quite performant hardware, which is way too high for our application.
Actual execution of the query in question was talking one tenth of that
time.

For now we're going to work around this by directing the queries
directly to the child tables, although this does involve fairly large
application changes.

However, I wondered if we couldn't mitigate this by caching the results
of constraint exclusion analysis for a particular table + condition. I
have no idea how hard this would be, but in principle it seems silly to
keep paying the same penalty over and over again.

Thoughts?

cheers

andrew

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