Horton

Re: why partition pruning doesn't work?

Robert Haas <robertmhaas@gmail.com>

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From: Robert Haas <robertmhaas@gmail.com>
To: Tom Lane <tgl@sss.pgh.pa.us>
Cc: Amit Langote <Langote_Amit_f8@lab.ntt.co.jp>, David Rowley <david.rowley@2ndquadrant.com>, Dmitry Dolgov <9erthalion6@gmail.com>, Jeff Janes <jeff.janes@gmail.com>, Ashutosh Bapat <ashutosh.bapat@enterprisedb.com>, Pavel Stehule <pavel.stehule@gmail.com>, PostgreSQL Hackers <pgsql-hackers@lists.postgresql.org>
Date: 2018-07-16T00:12:37Z
Lists: pgsql-hackers 
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
EnterpriseDB: http://www.enterprisedb.com
The Enterprise PostgreSQL Company

Commits

  1. Fix up run-time partition pruning's use of relcache's partition data.

  2. Fix access to just-closed relcache entry.

  3. Improve ExecFindInitialMatchingSubPlans's subplan renumbering logic.

  4. Improve commentary about run-time partition pruning data structures.

  5. Fix run-time partition pruning code to handle NULL values properly.

  6. Assorted cosmetic cleanup of run-time-partition-pruning code.

  7. Relocate partition pruning structs to a saner place.

  8. Improve run-time partition pruning to handle any stable expression.

  9. Support partition pruning at execution time