Re: Declarative partitioning - another take

On 2016/08/29 20:53, Robert Haas wrote:
> On Fri, Aug 26, 2016 at 1:33 PM, Amit Langote
> <Langote_Amit_f8(at)lab(dot)ntt(dot)co(dot)jp> wrote:
>> We do take a lock on the parent because we would be changing its partition
>> descriptor (relcache). I changed MergeAttributes() such that an
>> AccessExclusiveLock instead of ShareUpdateExclusiveLock is taken if the
>> parent is a partitioned table.
>
> Hmm, that seems both good and bad. On the good side, as mentioned,
> being able to rely on the partition descriptor not changing under us
> makes this sort of thing much easier to reason about. On the bad
> side, it isn't good for this feature to have worse concurrency than
> regular inheritance. Not sure what to do about this.
>
>> If we need an AccessExclusiveLock on parent to add/remove a partition
>> (IOW, changing that child table's partitioning information), then do we
>> need to lock the individual partitions when reading partition's
>> information? I mean to ask why the simple syscache look-ups to get each
>> partition's bound wouldn't do.
>
> Well, if X can't be changed without having an AccessExclusiveLock on
> the parent, then an AccessShareLock on the parent is sufficient to
> read X, right? Because those lock modes conflict.

Yes. And hence we can proceed with performing partition elimination
before locking any of children. Lock on parent (AccessShareLock) will
prevent any of existing partitions to be removed and any new partitions to
be added because those operations require AccessExclusiveLock on the
parent. What I was trying to understand is why this would not be possible
with a design where partition bound is stored in the catalog as a property
of individual partitions instead of a design where we store collection of
partition bounds as a property of the parent.

Thanks,
Amit