Two MySQL table index optimization

Construction of the table statement

CREATE TABLE IF NOT EXISTS `class`(
`id` INT(10) UNSIGNED NOT NULL AUTO_INCREMENT,
`card` INT(10) UNSIGNED NOT NULL,
PRIMARY KEY(`id`)
);

CREATE TABLE IF NOT EXISTS `book`(
`bookid` INT(10) UNSIGNED NOT NULL AUTO_INCREMENT,
`card` INT(10) UNSIGNED NOT NULL,
PRIMARY KEY(`bookid`)
);

INSERT INTO class(card) VALUES(FLOOR(1 + (RAND() * 20)));
INSERT INTO class(card) VALUES(FLOOR(1 + (RAND() * 20)));
INSERT INTO class(card) VALUES(FLOOR(1 + (RAND() * 20)));
INSERT INTO class(card) VALUES(FLOOR(1 + (RAND() * 20)));
INSERT INTO class(card) VALUES(FLOOR(1 + (RAND() * 20)));
INSERT INTO class(card) VALUES(FLOOR(1 + (RAND() * 20)));
INSERT INTO class(card) VALUES(FLOOR(1 + (RAND() * 20)));
INSERT INTO class(card) VALUES(FLOOR(1 + (RAND() * 20)));
INSERT INTO class(card) VALUES(FLOOR(1 + (RAND() * 20)));
INSERT INTO class(card) VALUES(FLOOR(1 + (RAND() * 20)));
INSERT INTO class(card) VALUES(FLOOR(1 + (RAND() * 20)));
INSERT INTO class(card) VALUES(FLOOR(1 + (RAND() * 20)));
INSERT INTO class(card) VALUES(FLOOR(1 + (RAND() * 20)));
INSERT INTO class(card) VALUES(FLOOR(1 + (RAND() * 20)));
INSERT INTO class(card) VALUES(FLOOR(1 + (RAND() * 20)));
INSERT INTO class(card) VALUES(FLOOR(1 + (RAND() * 20)));
INSERT INTO class(card) VALUES(FLOOR(1 + (RAND() * 20)));
INSERT INTO class(card) VALUES(FLOOR(1 + (RAND() * 20)));
INSERT INTO class(card) VALUES(FLOOR(1 + (RAND() * 20)));
INSERT INTO class(card) VALUES(FLOOR(1 + (RAND() * 20)));
INSERT INTO class(card) VALUES(FLOOR(1 + (RAND() * 20)));
INSERT INTO class(card) VALUES(FLOOR(1 + (RAND() * 20)));
INSERT INTO class(card) VALUES(FLOOR(1 + (RAND() * 20)));
INSERT INTO class(card) VALUES(FLOOR(1 + (RAND() * 20)));

Explain the use of the sql check

mysql> explain SELECT * FROM class LEFT JOIN book ON class.card = book.card; 
+----+-------------+-------+------------+------+---------------+------+---------+------+------+----------+----------------------------------------------------+
| id | select_type | table | partitions | type | possible_keys | key  | key_len | ref  | rows | filtered | Extra                                              |
+----+-------------+-------+------------+------+---------------+------+---------+------+------+----------+----------------------------------------------------+
|  1 | SIMPLE      | class | NULL       | ALL  | NULL          | NULL | NULL    | NULL |   24 |   100.00 | NULL                                               |
|  1 | SIMPLE      | book  | NULL       | ALL  | NULL          | NULL | NULL    | NULL |    1 |   100.00 | Using where; Using join buffer (Block Nested Loop) |
+----+-------------+-------+------------+------+---------------+------+---------+------+------+----------+----------------------------------------------------+
2 rows in set, 1 warning (0.00 sec)

• We can see that type are all (perform a full table queries)
• Join buffer used (algorithm which is used in Block Nested-Loop (BNL))

Add index

• card table to add attributes to book index

alter table `book` add index Y(`card`);

• Check with sql prior explain again

  mysql> explain SELECT * FROM class LEFT JOIN book ON class.card = book.card;
  +----+-------------+-------+------------+------+---------------+------+---------+-----------------+------+----------+-------------+
  | id | select_type | table | partitions | type | possible_keys | key  | key_len | ref             | rows | filtered | Extra       |
  +----+-------------+-------+------------+------+---------------+------+---------+-----------------+------+----------+-------------+
  |  1 | SIMPLE      | class | NULL       | ALL  | NULL          | NULL | NULL    | NULL            |   24 |   100.00 | NULL        |
  |  1 | SIMPLE      | book  | NULL       | ref  | Y             | Y    | 4       | test.class.card |    1 |   100.00 | Using index |
  +----+-------------+-------+------------+------+---------------+------+---------+-----------------+------+----------+-------------+
  2 rows in set, 1 warning (0.00 sec)

  • You can see the type of order to optimize the book table type, and extra changes to the using index, a buffer is no longer used.

• card table to add attributes to class index

  alter table `class` add index Y(`card`);

• Check with sql prior explain again

  mysql> explain SELECT * FROM class LEFT JOIN book ON class.card = book.card;
  +----+-------------+-------+------------+-------+---------------+------+---------+-----------------+------+----------+-------------+
  | id | select_type | table | partitions | type  | possible_keys | key  | key_len | ref             | rows | filtered | Extra       |
  +----+-------------+-------+------------+-------+---------------+------+---------+-----------------+------+----------+-------------+
  |  1 | SIMPLE      | class | NULL       | index | NULL          | Y    | 4       | NULL            |   24 |   100.00 | Using index |
  |  1 | SIMPLE      | book  | NULL       | ref   | Y             | Y    | 4       | test.class.card |    1 |   100.00 | Using index |
  +----+-------------+-------+------------+-------+---------------+------+---------+-----------------+------+----------+-------------+
  2 rows in set, 1 warning (0.00 sec)
  

  • We can see the type becomes the index, ref and extra have become the Using index

It can be concluded

• When connected to the left, to the right table to optimize the construction of the index is more important (left table are retained, the inevitable full table scan)
• When connected to the right, left the table to optimize the construction of the index is more important (at right are reserved, the inevitable full table scan)