Re: ATTACH/DETACH PARTITION CONCURRENTLY

On Fri, 21 Dec 2018 at 09:43, Robert Haas <robertmhaas@gmail.com> wrote:
> - I refactored expand_inherited_rtentry() to drive partition expansion
> entirely off of PartitionDescs. The reason why this is necessary is
> that it clearly will not work to have find_all_inheritors() use a
> current snapshot to decide what children we have and lock them, and
> then consult a different source of truth to decide which relations to
> open with NoLock.  There's nothing to keep the lists of partitions
> from being different in the two cases, and that demonstrably causes
> assertion failures if you SELECT with an ATTACH/DETACH loop running in
> the background. However, it also changes the order in which tables get
> locked.  Possibly that could be fixed by teaching
> expand_partitioned_rtentry() to qsort() the OIDs the way
> find_inheritance_children() does.  It also loses the infinite-loop
> protection which find_all_inheritors() has.  Not sure what to do about
> that.

I don't think you need to qsort() the Oids before locking. What the
qsort() does today is ensure we get a consistent locking order. Any
other order would surely do, providing we stick to it consistently. I
think PartitionDesc order is fine, as it's consistent.  Having it
locked in PartitionDesc order I think is what's needed for [1] anyway.
[2] proposes to relax the locking order taken during execution.

[1] https://commitfest.postgresql.org/21/1778/
[2] https://commitfest.postgresql.org/21/1887/

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