Difference between MySQL and MariaDB Query execution plan

Question

Is there difference between MySQL and MariaDB Query execution plan?

If yes, which one's is better?

CREATE TABLE `Table1` (
  `ID` int(11) NOT NULL,
  KEY `ID` (`ID`)
);

CREATE TABLE `Table2` (
  `ID` int(11) NOT NULL,
  KEY `ID` (`ID`)
);

CREATE TABLE `Table3` (
  `ID` int(11) NOT NULL,
  PRIMARY KEY (`ID`)
);

In Maria DB,

MariaDB [truepay_psr]> explain select T1.ID FROM Table1 T1 LEFT JOIN (SELECT T1.ID FROM Table3 T1 LEFT JOIN Table2 T2 ON T1.ID = T2.ID WHERE T2.ID IS NULL) T2 ON T1.ID=T2.ID WHERE T2.ID IS NULL;
+------+-------------+-------+--------+---------------+---------+---------+-------------------+------+--------------------------+
| id   | select_type | table | type   | possible_keys | key     | key_len | ref               | rows | Extra                    |
+------+-------------+-------+--------+---------------+---------+---------+-------------------+------+--------------------------+
|    1 | SIMPLE      | T1    | index  | NULL          | ID      | 4       | NULL              |    1 | Using index              |
|    1 | SIMPLE      | T1    | eq_ref | PRIMARY       | PRIMARY | 4       | truepay_psr.T1.ID |    1 | Using where; Using index |
|    1 | SIMPLE      | T2    | ref    | ID            | ID      | 4       | truepay_psr.T1.ID |    1 | Using where; Using index |
+------+-------------+-------+--------+---------------+---------+---------+-------------------+------+--------------------------+
3 rows in set (0.01 sec)

In MySQL,

mysql> explain select T1.ID FROM Table1 T1 LEFT JOIN (SELECT T1.ID FROM Table3 T1 LEFT JOIN Table2 T2 ON T1.ID = T2.ID WHERE T2.ID IS NULL) T2 ON T1.ID=T2.ID WHERE T2.ID IS NULL;
+----+-------------+------------+--------+---------------+---------+---------+------------+------+--------------------------------------+
| id | select_type | table      | type   | possible_keys | key     | key_len | ref        | rows | Extra                                |
+----+-------------+------------+--------+---------------+---------+---------+------------+------+--------------------------------------+
|  1 | PRIMARY     | <derived2> | system | NULL          | NULL    | NULL    | NULL       |    0 | const row not found                  | 
|  1 | PRIMARY     | T1         | index  | NULL          | ID      | 4       | NULL       |    1 | Using index                          | 
|  2 | DERIVED     | T1         | index  | NULL          | PRIMARY | 4       | NULL       |    1 | Using index                          | 
|  2 | DERIVED     | T2         | ref    | ID            | ID      | 4       | test.T1.ID |    1 | Using where; Using index; Not exists | 
+----+-------------+------------+--------+---------------+---------+---------+------------+------+--------------------------------------+
4 rows in set (0.00 sec)

Answer 1

You're seeing the effect of "Table elimination" optimization (MySQL plan has 4 rows while MariaDB one has only 3). The MariaDB plan should be the better one as there is "less work to do". Both should return the same results though.

The feature is explained in detail here:

http://s.petrunia.net/blog/?p=58

and here:

https://mariadb.com/kb/en/mariadb/documentation/optimization-and-tuning/query-optimizations/table-elimination/

If you want MariaDB to use the same plan as MySQL you can enforce this by disabling the table elimination optimization with:

SET optimizer_switch='table_elimination=off';