MySQL/MariaDB cursors and temp tables

In MariaDB and MySQL, cursors create a temporary table.

Does this statement deserve a whole blog post? Apparently not. However, in some cases one does not expect a temporary table to be created:

  • SELECT ... FOR UPDATE: An exclusive lock is created, yes, but you still read data from a temporary table.
  • SELECT FROM a temporary table: you are reading from a temporary tables, yes, but an internal temporary table is created anyway.
  • Impossible WHERE and LIMIT 0.

A quick example:

CREATE TEMPORARY TABLE t
(
        id INT UNSIGNED AUTO_INCREMENT PRIMARY KEY
);

CREATE PROCEDURE p()
BEGIN
        DECLARE c CURSOR FOR
                SELECT id FROM t WHERE 0 LIMIT 0 FOR UPDATE;
        OPEN c;
        CLOSE c;
END;

MySQL [test]> SHOW STATUS LIKE 'Created_tmp_tables';
+--------------------+-------+
| Variable_name      | Value |
+--------------------+-------+
| Created_tmp_tables | 31    |
+--------------------+-------+
1 row in set (0.00 sec)

MySQL [test]> CALL p();
Query OK, 0 rows affected (0.00 sec)

MySQL [test]> SHOW STATUS LIKE 'Created_tmp_tables';
+--------------------+-------+
| Variable_name      | Value |
+--------------------+-------+
| Created_tmp_tables | 32    |
+--------------------+-------+
1 row in set (0.00 sec)

I am not complaining, and I don’t even know if this behavior can be changed. But one should certainly be aware of this behavior. For example, one could think that creating a temporary table one time and then loop on that table with cursors several times is an optimization – but that’s not the case.

Federico

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