Horton

Re: ALTER TABLE .. DETACH PARTITION CONCURRENTLY

Alvaro Herrera <alvherre@alvh.no-ip.org>

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From: Álvaro Herrera <alvherre@alvh.no-ip.org>
To: "Amit Langote" <amitlangote09@gmail.com>
Cc: "Justin Pryzby" <pryzby@telsasoft.com>, "Tom Lane" <tgl@sss.pgh.pa.us>, "pgsql-hackers@lists.postgresql.org" <pgsql-hackers@lists.postgresql.org>
Date: 2021-04-28T16:11:12Z
Lists: pgsql-hackers

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Thanks for re-reviewing! This one I hope is the last version.

On Wed, Apr 28, 2021, at 10:21 AM, Amit Langote wrote:
> I noticed that rd_partdesc_nodetached_xmin can sometimes end up with
> value 0. While you seem to be already aware of that, because otherwise
> you wouldn't have added TransactionIdIsValid(...) in condition in
> RelationGetPartitionDesc(), the comments nearby don't mention why such
> a thing might happen.  Also, I guess it can't be helped that the
> partdesc_nodetached will have to be leaked when the xmin is 0, but
> that shouldn't be as problematic as the case we discussed earlier.

The only case I am aware where that can happen is if the pg_inherits tuple is frozen. (That's exactly what the affected test case was testing, note the "VACUUM FREEZE pg_inherits" there).  So that test case blew up immediately; but I think the real-world chances that people are going to be doing that are pretty low, so I'm not really concerned about the leak.

> Would it be a bit more readable to just duplicate this stanza in the
> blocks that assign to rd_partdesc_nodetached and rd_partdesc,
> respectively?  That's not much code to duplicate and it'd be easier to
> see which context is for which partdesc.

Sure .. that's how I first wrote this code.  We don't use that style much, so I'm OK with backing out of it.

> +   TransactionId rd_partdesc_nodetached_xmin;  /* xmin for the above */
> 
> Could you please expand this description a bit?

Done.

Commits

  1. Make detach-partition-concurrently-4 less timing sensitive

  2. Improve documentation on DETACH PARTITION lock levels

  3. Track detached partitions more accurately in partdescs

  4. Allow a partdesc-omitting-partitions to be cached

  5. Fix relcache inconsistency hazard in partition detach

  6. Don't add a redundant constraint when detaching a partition

  7. ALTER TABLE ... DETACH PARTITION ... CONCURRENTLY

  8. Let ALTER TABLE Phase 2 routines manage the relation pointer

  9. Check default partitions constraints while descending