On 2016/11/22 4:49, Tom Lane wrote:
Etsuro Fujita <fujita.ets...@lab.ntt.co.jp> writes:
On 2016/11/10 5:17, Tom Lane wrote:
I think there's a very good argument that we should just treat any inval
in pg_foreign_data_wrapper as a reason to blow away the whole plan cache.
People aren't going to alter FDW-level options often enough to make it
worth tracking things more finely.  Personally I'd put pg_foreign_server
changes in the same boat, though maybe those are changed slightly more
often.  If it's worth doing anything more than that, it would be to note
which plans mention foreign tables at all, and not invalidate those that
don't.  We could do that with, say, a hasForeignTables bool in

I agree on that point.

OK, please update the patch to handle those catalogs that way.

Will do.

That leaves the question of whether it's worth detecting table-level
option changes this way, or whether we should just handle those by forcing
a relcache inval in ATExecGenericOptions, as in Amit's original patch in
<5702298d.4090...@lab.ntt.co.jp>.  I kind of like that approach; that
patch was short and sweet, and it put the cost on the unusual path (ALTER
TABLE) not the common path (every time we create a query plan).

I think it'd be better to detect table-level option changes because that
could allow us to do more; we could avoid invalidating cached plans that
need not to be invalidated, by tracking the plan dependencies more
exactly, ie, avoid collecting the dependencies of foreign tables in a
plan that are in dead subqueries or excluded by constraint exclusion.
The proposed patch doesn't do that, though.  I think updates on
pg_foreign_table would be more frequent, so it might be worth
complicating the code.  But the relcache-inval approach couldn't do
that, IIUC.

Why not?  But the bigger picture here is that relcache inval is the
established practice for forcing replanning due to table-level changes,
and I have very little sympathy for inventing a new mechanism for that
just for foreign tables.  If it were worth bypassing replanning for
changes in tables in dead subqueries, for instance, it would surely be
worth making that happen for all table types not just foreign tables.

My point here is that FDW-option changes wouldn't affect replanning by core; even if forcing a replan, we would have the same foreign tables excluded by constraint exclusion, for example. So, considering updates on pg_foreign_table to be rather frequent, it might be better to avoid replanning for the pg_foreign_table changes to foreign tables excluded by constraint exclusion, for example. My concern about the relcache-inval approach is: that approach wouldn't allow us to do that, IIUC.

Best regards,
Etsuro Fujita

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