Horton

Re: Declarative partitioning - another take

Amit Langote <langote_amit_f8@lab.ntt.co.jp>

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From: Amit Langote <Langote_Amit_f8@lab.ntt.co.jp>
To: Robert Haas <robertmhaas@gmail.com>
Cc: Ashutosh Bapat <ashutosh.bapat@enterprisedb.com>, Pg Hackers <pgsql-hackers@postgresql.org>
Date: 2016-08-31T07:07:50Z
Lists: pgsql-hackers 
On 2016/08/29 20:53, Robert Haas wrote:
> On Fri, Aug 26, 2016 at 1:33 PM, Amit Langote
> <Langote_Amit_f8@lab.ntt.co.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

Commits

  1. Fix typo.

  2. Document trigger-firing behavior for inheritance/partitioning.

  3. Fire per-statement triggers on partitioned tables.

  4. Set ecxt_scantuple correctly for tuple routing.

  5. Fix interaction of partitioned tables with BulkInsertState.

  6. Avoid core dump for empty prepared statement in an aborted transaction.

  7. Fix some problems in check_new_partition_bound().

  8. Remove unnecessary arguments from partitioning functions.

  9. Fix reporting of constraint violations for table partitioning.

  10. Fix tuple routing in cases where tuple descriptors don't match.

  11. Invalid parent's relcache after CREATE TABLE .. PARTITION OF.

  12. Doc: improve documentation about inheritance.