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POMySQL deadlocks with stored procedure generating UID
 

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    <p>I have a stored procedure generating UID's from a "ticket" table, but under load I'm getting lots of deadlocks. I'm calling this procedure many times from multiple concurrent connections whenever my task needs a new UID.</p> <pre><code>BEGIN DECLARE a_uid BIGINT(20) UNSIGNED; START TRANSACTION; SELECT uid INTO a_uid FROM uid_data FOR UPDATE; # Lock INSERT INTO uid_data (stub) VALUES ('a') ON DUPLICATE KEY UPDATE uid=uid+1; SELECT a_uid+1 AS `uid`; COMMIT; END </code></pre> <p>I did consider using:</p> <pre><code>BEGIN REPLACE INTO uid_data (stub) VALUES ('a'); SELECT LAST_INSERT_ID(); END </code></pre> <p>However I wasn't sure if that would be safe with concurrent connections as there's no locking, unlike the first procedure with the <code>SELECT FOR UPDATE</code>.</p> <p>Here's the table:</p> <pre><code>mysql&gt; DESCRIBE uid_data; +-------+---------------------+------+-----+---------+----------------+ | Field | Type | Null | Key | Default | Extra | +-------+---------------------+------+-----+---------+----------------+ | uid | bigint(20) unsigned | NO | PRI | NULL | auto_increment | | stub | char(1) | NO | UNI | NULL | | +-------+---------------------+------+-----+---------+----------------+ </code></pre> <p>I've setup for read-committed transaction isolation:</p> <pre><code>mysql&gt; SHOW VARIABLES LIKE 'tx_isolation'; +---------------+-----------------+ | Variable_name | Value | +---------------+-----------------+ | tx_isolation | READ-COMMITTED | +---------------+-----------------+ </code></pre> <p>Here's what I'm getting back from <code>SHOW ENGINE INNODB STATUS;</code></p> <pre><code>... ... dozens and dozens of the following record locks... Record lock, heap no 1046 PHYSICAL RECORD: n_fields 2; compact format; info bits 32 0: len 1; hex 61; asc a;; 1: len 8; hex 00000000000335f2; asc 5 ;; Record lock, heap no 1047 PHYSICAL RECORD: n_fields 2; compact format; info bits 32 0: len 1; hex 61; asc a;; 1: len 8; hex 00000000000335f1; asc 5 ;; *** (2) WAITING FOR THIS LOCK TO BE GRANTED: RECORD LOCKS space id 13 page no 4 n bits 1120 index `stub` of table `my_db`.`uid_data` trx id 13AA89 lock_mode X waiting Record lock, heap no 583 PHYSICAL RECORD: n_fields 2; compact format; info bits 32 0: len 1; hex 61; asc a;; 1: len 8; hex 00000000000334a8; asc 4 ;; *** WE ROLL BACK TRANSACTION (1) </code></pre> <p>I'd be grateful if someone could explain what's happening and how they can be avoided.</p>
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    1. COFor information: the deadlock occurs even when using this simple sequence: `START TRANSACTION; SELECT uid FROM uid_data FOR UPDATE; UPDATE uid_data SET uid = uid +1 [[possible deadlock here]] ; COMMIT;` (therefore it has nothing to do with the `ON DUPLICATE` clause). However, no deadlock occurs with an isolation level of `REPEATABLE READ;`. I still don't know what to conclude from this point.
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