Re: why partition pruning doesn't work?

On Sun, Jul 15, 2018 at 1:02 PM, Tom Lane <tgl@sss.pgh.pa.us> wrote:
> What we'd be better off doing, if we go this route, is to install an
> assertion-build-only test that verifies during relation_open(NoLock)
> that some kind of lock is already held on the rel.  That would protect
> not only the executor, but a boatload of existing places that open
> rels with NoLock on the currently-unverified assumption that a lock is
> already held.

+1.  In fact, maybe we ought to go a little further and have a
relation_reopen(oid, mode) that verifies that a lock in the specified
mode is held.

And then maybe we ought to go even further and start trying to get rid
of all the places where we reopen already-opened relations.  A
distressing number of new patches add more places that do that, and
while I try to push back on those, I think they are proliferating, and
I think that they are not free.  Granted, a hash table lookup is
pretty cheap, but if you do a sufficient number of them in
commonlt-taken code paths, it's got to cost something.

-- 
Robert Haas
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