Subject: Re: BUG #17141: SELECT LIMIT WITH TIES FOR UPDATE SKIP LOCKED returns
 wrong number of rows
To: pgsql-bugs@lists.postgresql.org
References: <17141-913d78b9675aac8e@postgresql.org>
From: Emil Iggland <emil@iggland.com>
Message-ID: <88d7b60d-045e-27c5-a2f4-c669371128c5@iggland.com>
Date: Wed, 11 Aug 2021 19:29:02 +0200
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Precedence: bulk

As does

SELECT * FROM queue
ORDER BY task DESC
FETCH NEXT 1 ROWS WITH TIES
FOR UPDATE SKIP LOCKED;

Note the use of NEXT instead of FIRST


On 2021-08-11 18:38, PG Bug reporting form wrote:
> The following bug has been logged on the website:
> 
> Bug reference:      17141
> Logged by:          Emil Iggland
> Email address:      emil@iggland.com
> PostgreSQL version: 13.3
> Operating system:   Windows
> Description:        
> 
> I am trying to create a queue which should assign multiple tasks to a
> worker. I use row locking with FOR UPDATE SKIP LOCKED, but the number of
> rows returned are inconsistent with what I expect. 
> 
> Minimum example:
>  
> CREATE TABLE queue (task INTEGER); 
> INSERT INTO queue (task)
> VALUES (180),(280),(380),(480),(580),(180),(280),(380),(480),(580);
> 
> BEGIN; 
> SELECT * FROM queue
> ORDER BY task DESC
> FETCH FIRST 1 ROWS WITH TIES
> FOR UPDATE SKIP LOCKED;
> /* Some work to be done here */
> COMMIT;
> 
> select version();
> PostgreSQL 13.3, compiled by Visual C++ build 1914, 64-bit
> 
> Expected result Worker 1: (580), (580), Actual result Worker 1: (580),
> (580)
> Expected result Worker 2: (480), (480), Actual result Worker 2: (480)
>