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MySQL locking in Duplicate Key Error
Posted by: Priyansh Goel
Date: June 22, 2016 10:58AM

From Docs :

Suppose that an InnoDB table t1 has the following structure:

CREATE TABLE t1 (i INT, PRIMARY KEY (i)) ENGINE = InnoDB;
Now suppose that three sessions perform the following operations in order:

Session 1:

START TRANSACTION;
INSERT INTO t1 VALUES(1);
Session 2:

START TRANSACTION;
INSERT INTO t1 VALUES(1);
Session 3:

START TRANSACTION;
INSERT INTO t1 VALUES(1);
Session 1:

ROLLBACK;


The first operation by session 1 acquires an exclusive lock for the row. The operations by sessions 2 and 3 both result in a duplicate-key error and they both request a shared lock for the row. When session 1 commits, it releases its exclusive lock on the row and the queued shared lock requests for sessions 2 and 3 are granted. At this point, sessions 2 and 3 deadlock: Neither can acquire an exclusive lock for the row because of the shared lock held by the other.

*******************************************************************************

I have some questions :

1) The insert query takes an exclusive lock on the row it is inserting. So, suppose T1 is inserting on row 1, it will lock row 1. Now when T2 comes to write, will INNODB evaluate the query before executing it and find out that it is going to write the same PK (row with i = 1) and make T2 wait? Or will it start execution of T2 and find that it gives duplicate key error or PK violation.

2) Why are T2 and T3 taking shared locks? How do shared locks come into picture during insert?

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