Details
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Type:
Bug
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Status: Closed
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Priority:
Minor
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Resolution: Fixed
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Affects Version/s: N/A
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Fix Version/s: 10.0.14
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Component/s: None
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Labels:
Description
In the following test case, EXPLAIN seems to acquire MDL.
Console1:
MariaDB [test]> SHOW CREATE TABLE myisam1; +---------+----------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------+ | Table | Create Table | +---------+----------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------+ | myisam1 | CREATE TABLE `myisam1` ( `a` int(11) NOT NULL, `b2` int(11) DEFAULT NULL, PRIMARY KEY (`a`) ) ENGINE=MyISAM DEFAULT CHARSET=latin1 /*!50100 PARTITION BY KEY (a) PARTITIONS 3 */ | +---------+----------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------+ 1 row in set (0.00 sec) MariaDB [test]> START TRANSACTION; Query OK, 0 rows affected (0.01 sec) MariaDB [test]> EXPLAIN SELECT * FROM myisam1 WHERE 0; +------+-------------+-------+------+---------------+------+---------+------+------+------------------+ | id | select_type | table | type | possible_keys | key | key_len | ref | rows | Extra | +------+-------------+-------+------+---------------+------+---------+------+------+------------------+ | 1 | SIMPLE | NULL | NULL | NULL | NULL | NULL | NULL | NULL | Impossible WHERE | +------+-------------+-------+------+---------------+------+---------+------+------+------------------+ 1 row in set (0.00 sec)
Console2:
MariaDB [test]> ALTER TABLE myisam1 CHANGE b2 b INT; Stage: 2 of 2 'enabling keys' 0% of stage done
(it's frozen)
Console3:
MariaDB [(none)]> SELECT * FROM information_schema.METADATA_LOCK_INFO; +-----------+-------------------------+-----------------+----------------------+--------------+------------+ | THREAD_ID | LOCK_MODE | LOCK_DURATION | LOCK_TYPE | TABLE_SCHEMA | TABLE_NAME | +-----------+-------------------------+-----------------+----------------------+--------------+------------+ | 5 | MDL_INTENTION_EXCLUSIVE | MDL_STATEMENT | Global read lock | | | | 4 | MDL_SHARED_READ | MDL_TRANSACTION | Table metadata lock | test | myisam1 | | 5 | MDL_SHARED_NO_WRITE | MDL_TRANSACTION | Table metadata lock | test | myisam1 | | 5 | MDL_INTENTION_EXCLUSIVE | MDL_TRANSACTION | Schema metadata lock | test | | +-----------+-------------------------+-----------------+----------------------+--------------+------------+
Not sure if this is a bug. If it isn't, I suggest to document non-obvious cases when a MDL is acquired. (When doing something similar to this, I locked metadata for some minutes, in production.)
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I have added a note about EXPLAIN acquiring MDL in the KB.
I think it's not very surprising — EXPLAIN SELECT acquires metadata locks, just like SELECT does. Because EXPLAIN needs to know table metadata and, sometimes, data too.