Horton

Re: executor relation handling

Andres Freund <andres@anarazel.de>

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From: Andres Freund <andres@anarazel.de>
To: Tom Lane <tgl@sss.pgh.pa.us>
Cc: Amit Langote <Langote_Amit_f8@lab.ntt.co.jp>, David Rowley <david.rowley@2ndquadrant.com>, Alvaro Herrera <alvherre@2ndquadrant.com>, Jesper Pedersen <jesper.pedersen@redhat.com>, Amit Langote <amitlangote09@gmail.com>, Pg Hackers <pgsql-hackers@postgresql.org>
Date: 2018-10-04T19:34:44Z
Lists: pgsql-hackers 
Hi,

On 2018-10-04 15:27:59 -0400, Tom Lane wrote:
> Andres Freund <andres@anarazel.de> writes:
> > I've not really followed this thread, and just caught up to here.  It
> > seems entirely unacceptable to not acquire locks on workers to me.
> > Maybe I'm missing something, but why do/did the patches in this thread
> > require that / introduce that? We didn't have that kind of concept
> > before, no?  The group locking stuff should rely / require that kind of
> > thing, no?
> 
> I'm possibly confused, but I thought that the design of parallel query
> involved an expectation that workers didn't need to get their own
> locks.

Not as far as I'm aware of - but I'm not exactly the expert
there. There's an exception that some lock classes don't conflict
between the leader and the workers - that's group locking
(a1c1af2a1f60). But the locks still have to be acquired, and I think
it's quite dangerous not to do so.  The group locking logic is required
because otherwise it'd be trivial to get into deadlocks, and some of the
restrictions around parallel query are required to make that safe.


> What we've determined so far in this thread is that workers *do* get
> their own locks (or did before yesterday), but I'd been supposing that
> that was accidental not intentional.

I don't think it was accidental.

Greetings,

Andres Freund

Commits

  1. Avoid O(N^2) cost in ExecFindRowMark().

  2. Remove some unnecessary fields from Plan trees.

  3. Restore sane locking behavior during parallel query.

  4. Remove more redundant relation locking during executor startup.

  5. In the executor, use an array of pointers to access the rangetable.

  6. Centralize executor's opening/closing of Relations for rangetable entries.

  7. Change executor to just Assert that table locks were already obtained.

  8. Change rewriter/planner/executor/plancache to depend on RTE rellockmode.

  9. Add assertions that we hold some relevant lock during relation open.

  10. Create an RTE field to record the query's lock mode for each relation.