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From: Bruce Momjian <[email protected]>
To: Tom Lane <[email protected]>
Cc: Greg Stark <[email protected]>
Cc: David Zhang <[email protected]>
Cc: Simon Riggs <[email protected]>
Cc: Cary Huang <[email protected]>
Cc: Pgsql Hackers <[email protected]>
Subject: Re: Patch: Global Unique Index
Date: Tue, 29 Nov 2022 21:42:14 -0500
Message-ID: <[email protected]> (raw)
In-Reply-To: <[email protected]>
References: <[email protected]>
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	<[email protected]>
	<[email protected]>
	<[email protected]>

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  <[email protected]>        https://momjian.us
  EDB                                      https://enterprisedb.com

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

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