Date: Tue, 29 Nov 2022 21:42:14 -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
Message-ID: <Y4bDBqClgJfmMVjH@momjian.us>
References: <184879c5306.12490ea581628934.7312528450011769010@highgo.ca>
 <CANbhV-Eo4D2Njpa=YiBr7BxOMDi5B54wenjgoamRKjzj-9gBGQ@mail.gmail.com>
 <Y4D/3EbEibKkJW2C@momjian.us>
 <46a0f6fb-1c0b-bbd8-ef3e-366216a89501@highgo.ca>
 <CAM-w4HNFyX2upz6pyeWXew-p2sPXiSy2q3tn_Zfa4Xvtu+SLhw@mail.gmail.com>
 <1239139.1669763636@sss.pgh.pa.us>
 <Y4a5FRbhRpbJIeo5@momjian.us>
 <1257837.1669774583@sss.pgh.pa.us>
MIME-Version: 1.0
Content-Type: text/plain; charset=us-ascii
Content-Disposition: inline
In-Reply-To: <1257837.1669774583@sss.pgh.pa.us>
Archived-At: 
 <https://www.postgresql.org/message-id/Y4bDBqClgJfmMVjH%40momjian.us>
Precedence: bulk

On Tue, Nov 29, 2022 at 09:16:23PM -0500, Tom Lane wrote:
> Assuming that you are inserting into index X, and you've checked
> index Y to find that it has no conflicts, what prevents another
> backend from inserting a conflict into index Y just after you look?
> AIUI the idea is to prevent that by continuing to hold an exclusive
> lock on the whole index Y until you've completed the insertion.
> Perhaps there's a better way to do that, but it's not what was
> described.

As I understood it, you insert into index X and then scan all other
indexes to look for a conflict --- if you find one, you abort with a
unique index conflict.  Other index changes do the same.

So, for example, one session inserts into index X and then scans all
other indexes.  During the index scan, another session inserts into
index Y, but its scan sees the index X addition and gets a uniqueness
conflict error.

> I actually think that that problem should be soluble with a
> slightly different approach.  The thing that feels insoluble
> is that you can't do this without acquiring sufficient locks
> to prevent addition of new partitions while the insertion is
> in progress.  That will be expensive in itself, and it will
> turn ATTACH PARTITION into a performance disaster.

Yes, that would require index locks.

-- 
  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.