Date: Tue, 29 Nov 2022 20:59:49 -0500
From: Bruce Momjian <bruce@momjian.us>
To: Tom Lane <tgl@sss.pgh.pa.us>
Cc: Greg Stark <stark@mit.edu>, David Zhang <david.zhang@highgo.ca>,
	Simon Riggs <simon.riggs@enterprisedb.com>,
	Cary Huang <cary.huang@highgo.ca>,
	Pgsql Hackers <pgsql-hackers@lists.postgresql.org>
Subject: Re: Patch: Global Unique Index
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On Tue, Nov 29, 2022 at 06:13:56PM -0500, Tom Lane wrote:
> Greg Stark <stark@mit.edu> writes:
> > If I understand correctly you're going to insert into the local index
> > for the partition using the normal btree uniqueness implementation.
> > Then while holding an exclusive lock on the index do lookups on every
> > partition for the new key. Effectively serializing inserts to the
> > table?
> 
> ... not to mention creating a high probability of deadlocks between
> concurrent insertions to different partitions.  If they each
> ex-lock their own partition's index before starting to look into
> other partitions' indexes, it seems like a certainty that such
> cases would fail.  The rule of thumb about locking multiple objects
> is that all comers had better do it in the same order, and this
> isn't doing that.

I am not sure why they would need to exclusive lock anything more than
the unique index entry they are adding, just like UPDATE does.

> I still think this is a dead end that will never get committed.
> If folks want to put time into perhaps finding an ingenious
> way around these problems, okay; but they'd better realize that
> there's a high probability of failure, or at least coming out
> with something nobody will want to use.

Agreed, my earlier point was that this would need a lot of thought to
get right since we don't do this often.  The exclusion constraint is a
close example, though that is in a single index.

-- 
  Bruce Momjian  <bruce@momjian.us>        https://momjian.us
  EDB                                      https://enterprisedb.com

Embrace your flaws.  They make you human, rather than perfect,
which you will never be.