Re: Delay locking partitions during INSERT and UPDATE

On Wed, 20 Feb 2019 at 06:36, Robert Haas <robertmhaas(at)gmail(dot)com> wrote:
>
> On Mon, Feb 18, 2019 at 6:15 PM Tom Lane <tgl(at)sss(dot)pgh(dot)pa(dot)us> wrote:
> > I'm inclined to think that if we already have lock on the parent
> > partitioned table (thereby, IIUC, guaranteeing that its partitioning
> > info can't change) that the order in which we acquire the same lock
> > level on its partition(s) isn't very important.
>
> Well, as it turns out, there's also a pending patch (series) to remove
> that guarantee which I would like to get committed. The thread for
> that is "ATTACH/DETACH PARTITION CONCURRENTLY". I believe I've more
> or less got the issues with that patch sorted out, but I'm concerned
> about how it interacts with all of the other things that are currently
> in flight. Delaying or skipping lock acquisition in some cases and
> weakeni ng the locks we take in others is not a process that can
> continue indefinitely without at some point hitting a wall.

I'd say that here we should only discuss what this patch is doing, not
anything else that's in flight that you're concerned will conflict
with the ATTACH/DETACH PARTITION CONCURRENTLY patch.

During INSERT and UPDATE, not all partitions will always be locked
before executor startup. This patch removing the find_all_inheritors()
call from ExecSetupPartitionTupleRouting() is not going to break
ATTACH/DETACH PARTITION CONCURRENTLY if it wasn't already broken in
the first place. With this patch, we're still obtaining locks after
execution has begun, it just may delay the locking until a bit later.
It was previously already happening after executor startup had begun,
so the window for the problems that you describe were already
non-zero.

--
David Rowley http://www.2ndQuadrant.com/
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