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From: Alpha Shuro <[email protected]>
To: Laurenz Albe <[email protected]>
Cc: [email protected]
Subject: Re: (docs): add missing info about ShareLocks
Date: Thu, 27 Nov 2025 12:33:42 +0100
Message-ID: <[email protected]> (raw)
In-Reply-To: <[email protected]>
References: <[email protected]>
	<[email protected]>

Ah thanks for pointing this out, I've moved it to the xact-locking page instead




> Den 22. nov. 2025 kl. 13.07 skrev Laurenz Albe <[email protected]>:
> 
> On Sat, 2025-11-22 at 09:07 +0100, Alpha Shuro wrote:
>> I had a production incident a few weeks ago while using deferred indexes, where
>> the Postgres docs lead me down the wrong path of investigation due to missing details.
>> Specifically, the docs implied that a `ShareLock` was only acquired when creating
>> indexes, but only after looking at the code did I learn that this lock is also
>> acquired when transactions are waiting for other transactions to complete.
>> I think this would be helpful to someone who might find themselves on the same path
>> in the future, and as I understand it this mailing list is the way to submit patches
>> to the docs?
> 
> No, that is wrong.  This section is about table locks, and a lock on a transaction ID
> should, if anywhere, be documented elsewhere.  Actually, there is already something
> about transaction ID locks in https://www.postgresql.org/docs/current/xact-locking.html
> 
> Perhaps you could improve that short documentation?
> 
> Yours,
> Laurenz Albe
> 
> PS: There are also SHARE locks on rows.



Attachments:

  [application/octet-stream] document-ShareLock-when-using-deferred-unique.patch (661B, 2-document-ShareLock-when-using-deferred-unique.patch)
  download | inline diff:
diff --git a/doc/src/sgml/xact.sgml b/doc/src/sgml/xact.sgml
index 3aa7ee1383e..d753869cc88 100644
--- a/doc/src/sgml/xact.sgml
+++ b/doc/src/sgml/xact.sgml
@@ -102,6 +102,13 @@
    of multixact IDs (<literal>mxid</literal>;  see <xref
    linkend="vacuum-for-multixact-wraparound"/>).
   </para>
+
+  <para>
+   If there is a DEFERRED UNIQUE constraint on a row inserted in a transaction,
+   and there are multiple concurrent transactions inserting the same unique key,
+   the earlier transactions will wait for later transactions by
+   acquiring a <structfield>ShareLock</structfield> on the transaction ID.
+  </para>
  </sect1>
 
  <sect1 id="subxacts">

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