6. Analyze the query statement: EXPLAIN
6.1 Basic syntax
The syntax of the EXPLAIN or DESCRIBE statement is as follows:
EXPLAIN SELECT select_options
或者
DESCRIBE SELECT select_options
If we want to see the execution plan of a certain query, we can add an EXPLAIN in front of the specific query statement, like this:
mysql> EXPLAIN SELECT 1;
The functions of each column output by the EXPLAIN statement are as follows:
6.2 Data preparation
CREATE TABLE s1 (
id INT AUTO_INCREMENT,
key1 VARCHAR ( 100 ),
key2 INT,
key3 VARCHAR ( 100 ),
key_part1 VARCHAR ( 100 ),
key_part2 VARCHAR ( 100 ),
key_part3 VARCHAR ( 100 ),
common_field VARCHAR ( 100 ),
PRIMARY KEY ( id ),
INDEX idx_key1 ( key1 ),
UNIQUE INDEX idx_key2 ( key2 ),
INDEX idx_key3 ( key3 ),
INDEX idx_key_part ( key_part1, key_part2, key_part3 )
) ENGINE = INNODB CHARSET = utf8;
CREATE TABLE s2 (
id INT AUTO_INCREMENT,
key1 VARCHAR ( 100 ),
key2 INT,
key3 VARCHAR ( 100 ),
key_part1 VARCHAR ( 100 ),
key_part2 VARCHAR ( 100 ),
key_part3 VARCHAR ( 100 ),
common_field VARCHAR ( 100 ),
PRIMARY KEY ( id ),
INDEX idx_key1 ( key1 ),
UNIQUE INDEX idx_key2 ( key2 ),
INDEX idx_key3 ( key3 ),
INDEX idx_key_part ( key_part1, key_part2, key_part3 )
) ENGINE = INNODB CHARSET = utf8;
Set parameter log_bin_trust_function_creators
Create a function. If an error is reported, you need to enable the following command: Allow creation of function settings:
set global log_bin_trust_function_creators=1; # 不加global只是当前窗口有效。
Create function
DELIMITER //
CREATE FUNCTION rand_string1 ( n INT ) RETURNS VARCHAR ( 255 ) #该函数会返回一个字符串
BEGIN
DECLARE
chars_str VARCHAR ( 100 ) DEFAULT 'abcdefghijklmnopqrstuvwxyzABCDEFJHIJKLMNOPQRSTUVWXYZ';
DECLARE
return_str VARCHAR ( 255 ) DEFAULT '';
DECLARE
i INT DEFAULT 0;
WHILE
i < n DO
SET return_str = CONCAT(
return_str,
SUBSTRING( chars_str, FLOOR( 1+RAND ()* 52 ), 1 ));
SET i = i + 1;
END WHILE;
RETURN return_str;
END //
DELIMITER;
Create stored procedure
Create a stored procedure to insert data into the s1 table:
DELIMITER //
CREATE PROCEDURE insert_s1 (
IN min_num INT ( 10 ),
IN max_num INT ( 10 )) BEGIN
DECLARE
i INT DEFAULT 0;
SET autocommit = 0;
REPEAT
SET i = i + 1;
INSERT INTO s1
VALUES
(
( min_num + i ),
rand_string1 ( 6 ),
( min_num + 30 * i + 5 ),
rand_string1 ( 6 ),
rand_string1 ( 10 ),
rand_string1 ( 5 ),
rand_string1 ( 10 ),
rand_string1 ( 10 ));
UNTIL i = max_num
END REPEAT;
COMMIT;
END //
DELIMITER;
Create a stored procedure to insert data into the s2 table:
DELIMITER //
CREATE PROCEDURE insert_s2 (
IN min_num INT ( 10 ),
IN max_num INT ( 10 )) BEGIN
DECLARE
i INT DEFAULT 0;
SET autocommit = 0;
REPEAT
SET i = i + 1;
INSERT INTO s2
VALUES
(
( min_num + i ),
rand_string1 ( 6 ),
( min_num + 30 * i + 5 ),
rand_string1 ( 6 ),
rand_string1 ( 10 ),
rand_string1 ( 5 ),
rand_string1 ( 10 ),
rand_string1 ( 10 ));
UNTIL i = max_num
END REPEAT;
COMMIT;
END //
DELIMITER;
Call stored procedure
Adding s1 table data: adding 10,000 records:
CALL insert_s1(10001,10000);
Adding s2 table data: adding 10,000 records:
CALL insert_s2(10001,10000);
6.4 The functions of each EXPLAIN column
1. table
No matter how complex our query statement is or how many tables it contains, in the end each table will need to be accessed by a single table. Therefore, MySQL stipulates that each record output by the EXPLAIN statement corresponds to a single table access. Method , the table column of the record represents the table name of the table (sometimes not the real table name, it may be the abbreviation).
2. id
The query statements we write generally start with the SELECT keyword. A relatively simple query statement has only one SELECT keyword, such as the following query statement:
mysql> EXPLAIN SELECT * FROM s1 WHERE key1 = 'a';
+----+-------------+-------+------------+------+---------------+----------+---------+-------+------+----------+-------+
| id | select_type | table | partitions | type | possible_keys | key | key_len | ref | rows | filtered | Extra |
+----+-------------+-------+------------+------+---------------+----------+---------+-------+------+----------+-------+
| 1 | SIMPLE | s1 | NULL | ref | idx_key1 | idx_key1 | 303 | const | 1 | 100.00 | NULL |
+----+-------------+-------+------------+------+---------------+----------+---------+-------+------+----------+-------+
1 row in set, 1 warning (0.00 sec)
mysql> EXPLAIN SELECT * FROM s1 INNER JOIN s2;
+----+-------------+-------+------------+------+---------------+------+---------+------+------+----------+---------------------------------------+
| id | select_type | table | partitions | type | possible_keys | key | key_len | ref | rows | filtered | Extra |
+----+-------------+-------+------------+------+---------------+------+---------+------+------+----------+---------------------------------------+
| 1 | SIMPLE | s1 | NULL | ALL | NULL | NULL | NULL | NULL | 9895 | 100.00 | NULL |
| 1 | SIMPLE | s2 | NULL | ALL | NULL | NULL | NULL | NULL | 9895 | 100.00 | Using join buffer (Block Nested Loop) |
+----+-------------+-------+------------+------+---------------+------+---------+------+------+----------+---------------------------------------+
2 rows in set, 1 warning (0.00 sec)
mysql> EXPLAIN SELECT * FROM s1 WHERE key1 IN (SELECT key1 FROM s2) OR key3 = 'a';
+----+-------------+-------+------------+-------+---------------+----------+---------+------+------+----------+-------------+
| id | select_type | table | partitions | type | possible_keys | key | key_len | ref | rows | filtered | Extra |
+----+-------------+-------+------------+-------+---------------+----------+---------+------+------+----------+-------------+
| 1 | PRIMARY | s1 | NULL | ALL | idx_key3 | NULL | NULL | NULL | 9895 | 100.00 | Using where |
| 2 | SUBQUERY | s2 | NULL | index | idx_key1 | idx_key1 | 303 | NULL | 9895 | 100.00 | Using index |
+----+-------------+-------+------------+-------+---------------+----------+---------+------+------+----------+-------------+
2 rows in set, 1 warning (0.00 sec)
mysql> EXPLAIN SELECT * FROM s1 WHERE key1 IN (SELECT key2 FROM s2 WHERE common_field
-> = 'a');
+----+-------------+-------+------------+------+---------------+----------+---------+--------------------+------+----------+------------------------------------+
| id | select_type | table | partitions | type | possible_keys | key | key_len | ref | rows | filtered | Extra |
+----+-------------+-------+------------+------+---------------+----------+---------+--------------------+------+----------+------------------------------------+
| 1 | SIMPLE | s1 | NULL | ALL | idx_key1 | NULL | NULL | NULL | 9895 | 100.00 | Using where |
| 1 | SIMPLE | s2 | NULL | ref | idx_key2 | idx_key2 | 5 | atguigudb1.s1.key1 | 1 | 10.00 | Using index condition; Using where |
+----+-------------+-------+------------+------+---------------+----------+---------+--------------------+------+----------+------------------------------------+
2 rows in set, 2 warnings (0.00 sec)
mysql> EXPLAIN SELECT * FROM s1 UNION SELECT * FROM s2;
+----+--------------+------------+------------+------+---------------+------+---------+------+------+----------+-----------------+
| id | select_type | table | partitions | type | possible_keys | key | key_len | ref | rows | filtered | Extra |
+----+--------------+------------+------------+------+---------------+------+---------+------+------+----------+-----------------+
| 1 | PRIMARY | s1 | NULL | ALL | NULL | NULL | NULL | NULL | 9895 | 100.00 | NULL |
| 2 | UNION | s2 | NULL | ALL | NULL | NULL | NULL | NULL | 9895 | 100.00 | NULL |
| NULL | UNION RESULT | <union1,2> | NULL | ALL | NULL | NULL | NULL | NULL | NULL | NULL | Using temporary |
+----+--------------+------------+------------+------+---------------+------+---------+------+------+----------+-----------------+
3 rows in set, 1 warning (0.00 sec)
mysql> EXPLAIN SELECT * FROM s1 UNION ALL SELECT * FROM s2;
+----+-------------+-------+------------+------+---------------+------+---------+------+------+----------+-------+
| id | select_type | table | partitions | type | possible_keys | key | key_len | ref | rows | filtered | Extra |
+----+-------------+-------+------------+------+---------------+------+---------+------+------+----------+-------+
| 1 | PRIMARY | s1 | NULL | ALL | NULL | NULL | NULL | NULL | 9895 | 100.00 | NULL |
| 2 | UNION | s2 | NULL | ALL | NULL | NULL | NULL | NULL | 9895 | 100.00 | NULL |
+----+-------------+-------+------------+------+---------------+------+---------+------+------+----------+-------+
2 rows in set, 1 warning (0.00 sec)
- If the IDs are the same, they can be considered as a group and executed sequentially from top to bottom.
- In all groups, the larger the id value is, the higher the priority is and the execution is performed first.
- Point of concern: Each ID number represents an independent query. The fewer the number of queries in a SQL statement, the better.
3. select_type
mysql> EXPLAIN SELECT * FROM s1 INNER JOIN s2;
+----+-------------+-------+------------+------+---------------+------+---------+------+------+----------+---------------------------------------+
| id | select_type | table | partitions | type | possible_keys | key | key_len | ref | rows | filtered | Extra |
+----+-------------+-------+------------+------+---------------+------+---------+------+------+----------+---------------------------------------+
| 1 | SIMPLE | s1 | NULL | ALL | NULL | NULL | NULL | NULL | 9895 | 100.00 | NULL |
| 1 | SIMPLE | s2 | NULL | ALL | NULL | NULL | NULL | NULL | 9895 | 100.00 | Using join buffer (Block Nested Loop) |
+----+-------------+-------+------------+------+---------------+------+---------+------+------+----------+---------------------------------------+
2 rows in set, 1 warning (0.00 sec)
mysql> EXPLAIN SELECT * FROM s1 UNION SELECT * FROM s2;
+----+--------------+------------+------------+------+---------------+------+---------+------+------+----------+-----------------+
| id | select_type | table | partitions | type | possible_keys | key | key_len | ref | rows | filtered | Extra |
+----+--------------+------------+------------+------+---------------+------+---------+------+------+----------+-----------------+
| 1 | PRIMARY | s1 | NULL | ALL | NULL | NULL | NULL | NULL | 9895 | 100.00 | NULL |
| 2 | UNION | s2 | NULL | ALL | NULL | NULL | NULL | NULL | 9895 | 100.00 | NULL |
| NULL | UNION RESULT | <union1,2> | NULL | ALL | NULL | NULL | NULL | NULL | NULL | NULL | Using temporary |
+----+--------------+------------+------------+------+---------------+------+---------+------+------+----------+-----------------+
3 rows in set, 1 warning (0.00 sec)
mysql> EXPLAIN SELECT * FROM s1 WHERE key1 IN (SELECT key1 FROM s2) OR key3 = 'a';
+----+-------------+-------+------------+-------+---------------+----------+---------+------+------+----------+-------------+
| id | select_type | table | partitions | type | possible_keys | key | key_len | ref | rows | filtered | Extra |
+----+-------------+-------+------------+-------+---------------+----------+---------+------+------+----------+-------------+
| 1 | PRIMARY | s1 | NULL | ALL | idx_key3 | NULL | NULL | NULL | 9895 | 100.00 | Using where |
| 2 | SUBQUERY | s2 | NULL | index | idx_key1 | idx_key1 | 303 | NULL | 9895 | 100.00 | Using index |
+----+-------------+-------+------------+-------+---------------+----------+---------+------+------+----------+-------------+
2 rows in set, 1 warning (0.00 sec)
mysql> EXPLAIN SELECT * FROM s1 WHERE key1 IN (SELECT key1 FROM s2 WHERE s1.key2 =
-> s2.key2) OR key3 = 'a';
+----+--------------------+-------+------------+------+-------------------+----------+---------+--------------------+------+----------+-------------+
| id | select_type | table | partitions | type | possible_keys | key | key_len | ref | rows | filtered | Extra |
+----+--------------------+-------+------------+------+-------------------+----------+---------+--------------------+------+----------+-------------+
| 1 | PRIMARY | s1 | NULL | ALL | idx_key3 | NULL | NULL | NULL | 9895 | 100.00 | Using where |
| 2 | DEPENDENT SUBQUERY | s2 | NULL | ref | idx_key2,idx_key1 | idx_key2 | 5 | atguigudb1.s1.key2 | 1 | 10.00 | Using where |
+----+--------------------+-------+------------+------+-------------------+----------+---------+--------------------+------+----------+-------------+
2 rows in set, 2 warnings (0.00 sec)
mysql> EXPLAIN SELECT * FROM s1 WHERE key1 IN (SELECT key1 FROM s2 WHERE key1 =
-> 'a' UNION SELECT key1 FROM s1 WHERE key1 = 'b');
+----+--------------------+------------+------------+------+---------------+----------+---------+-------+------+----------+--------------------------+
| id | select_type | table | partitions | type | possible_keys | key | key_len | ref | rows | filtered | Extra |
+----+--------------------+------------+------------+------+---------------+----------+---------+-------+------+----------+--------------------------+
| 1 | PRIMARY | s1 | NULL | ALL | NULL | NULL | NULL | NULL | 9895 | 100.00 | Using where |
| 2 | DEPENDENT SUBQUERY | s2 | NULL | ref | idx_key1 | idx_key1 | 303 | const | 1 | 100.00 | Using where; Using index |
| 3 | DEPENDENT UNION | s1 | NULL | ref | idx_key1 | idx_key1 | 303 | const | 1 | 100.00 | Using where; Using index |
| NULL | UNION RESULT | <union2,3> | NULL | ALL | NULL | NULL | NULL | NULL | NULL | NULL | Using temporary |
+----+--------------------+------------+------------+------+---------------+----------+---------+-------+------+----------+--------------------------+
4 rows in set, 1 warning (0.00 sec)
mysql> EXPLAIN SELECT * FROM (SELECT key1, count(*) as c FROM s1 GROUP BY key1) AS
-> derived_s1 where c > 1;
+----+-------------+------------+------------+-------+---------------+----------+---------+------+------+----------+-------------+
| id | select_type | table | partitions | type | possible_keys | key | key_len | ref | rows | filtered | Extra |
+----+-------------+------------+------------+-------+---------------+----------+---------+------+------+----------+-------------+
| 1 | PRIMARY | <derived2> | NULL | ALL | NULL | NULL | NULL | NULL | 9895 | 33.33 | Using where |
| 2 | DERIVED | s1 | NULL | index | idx_key1 | idx_key1 | 303 | NULL | 9895 | 100.00 | Using index |
+----+-------------+------------+------------+-------+---------------+----------+---------+------+------+----------+-------------+
2 rows in set, 1 warning (0.00 sec)
mysql> EXPLAIN SELECT * FROM s1 WHERE key1 IN (SELECT key1 FROM s2);
+----+--------------+-------------+------------+--------+---------------+------------+---------+--------------------+------+----------+-------------+
| id | select_type | table | partitions | type | possible_keys | key | key_len | ref | rows | filtered | Extra |
+----+--------------+-------------+------------+--------+---------------+------------+---------+--------------------+------+----------+-------------+
| 1 | SIMPLE | s1 | NULL | ALL | idx_key1 | NULL | NULL | NULL | 9895 | 100.00 | Using where |
| 1 | SIMPLE | <subquery2> | NULL | eq_ref | <auto_key> | <auto_key> | 303 | atguigudb1.s1.key1 | 1 | 100.00 | NULL |
| 2 | MATERIALIZED | s2 | NULL | index | idx_key1 | idx_key1 | 303 | NULL | 9895 | 100.00 | Using index |
+----+--------------+-------------+------------+--------+---------------+------------+---------+--------------------+------+----------+-------------+
3 rows in set, 1 warning (0.00 sec)
4.type ☆
The complete access methods are as follows: system, const, eq_ref, ref, fulltext, ref_or_null, index_merge, unique_subquery, index_subquery, range, index, ALL.
system
mysql> CREATE TABLE t(i int) Engine=MyISAM;
Query OK, 0 rows affected (0.05 sec)
mysql> INSERT INTO t VALUES(1);
Query OK, 1 row affected (0.01 sec)
Then we look at the execution plan for querying this table:
mysql> EXPLAIN SELECT * FROM t;
+----+-------------+-------+------------+--------+---------------+------+---------+------+------+----------+-------+
| id | select_type | table | partitions | type | possible_keys | key | key_len | ref | rows | filtered | Extra |
+----+-------------+-------+------------+--------+---------------+------+---------+------+------+----------+-------+
| 1 | SIMPLE | t | NULL | system | NULL | NULL | NULL | NULL | 1 | 100.00 | NULL |
+----+-------------+-------+------------+--------+---------------+------+---------+------+------+----------+-------+
1 row in set, 1 warning (0.00 sec)
const
mysql> EXPLAIN SELECT * FROM s1 WHERE id = 10005;
+----+-------------+-------+------------+-------+---------------+---------+---------+-------+------+----------+-------+
| id | select_type | table | partitions | type | possible_keys | key | key_len | ref | rows | filtered | Extra |
+----+-------------+-------+------------+-------+---------------+---------+---------+-------+------+----------+-------+
| 1 | SIMPLE | s1 | NULL | const | PRIMARY | PRIMARY | 4 | const | 1 | 100.00 | NULL |
+----+-------------+-------+------------+-------+---------------+---------+---------+-------+------+----------+-------+
1 row in set, 1 warning (0.00 sec)
eq_ref
mysql> EXPLAIN SELECT * FROM s1 INNER JOIN s2 ON s1.id = s2.id;
+----+-------------+-------+------------+--------+---------------+---------+---------+------------------+------+----------+-------+
| id | select_type | table | partitions | type | possible_keys | key | key_len | ref | rows | filtered | Extra |
+----+-------------+-------+------------+--------+---------------+---------+---------+------------------+------+----------+-------+
| 1 | SIMPLE | s1 | NULL | ALL | PRIMARY | NULL | NULL | NULL | 9895 | 100.00 | NULL |
| 1 | SIMPLE | s2 | NULL | eq_ref | PRIMARY | PRIMARY | 4 | atguigudb1.s1.id | 1 | 100.00 | NULL |
+----+-------------+-------+------------+--------+---------------+---------+---------+------------------+------+----------+-------+
2 rows in set, 1 warning (0.00 sec)
It can be seen from the results of the execution plan that MySQL intends to use s2 as the driving table and s1 as the driven table. The access method focusing on s1 is eq_ref, which indicates that when accessing the s1 table, it can be accessed through the equivalent matching of the primary key. .
ref
mysql> EXPLAIN SELECT * FROM s1 WHERE key1 = 'a';
+----+-------------+-------+------------+------+---------------+----------+---------+-------+------+----------+-------+
| id | select_type | table | partitions | type | possible_keys | key | key_len | ref | rows | filtered | Extra |
+----+-------------+-------+------------+------+---------------+----------+---------+-------+------+----------+-------+
| 1 | SIMPLE | s1 | NULL | ref | idx_key1 | idx_key1 | 303 | const | 1 | 100.00 | NULL |
+----+-------------+-------+------------+------+---------------+----------+---------+-------+------+----------+-------+
1 row in set, 1 warning (0.00 sec)
ref_or_null
mysql> EXPLAIN SELECT * FROM s1 WHERE key1 = 'a' OR key1 IS NULL;
+----+-------------+-------+------------+-------------+---------------+----------+---------+-------+------+----------+-----------------------+
| id | select_type | table | partitions | type | possible_keys | key | key_len | ref | rows | filtered | Extra |
+----+-------------+-------+------------+-------------+---------------+----------+---------+-------+------+----------+-----------------------+
| 1 | SIMPLE | s1 | NULL | ref_or_null | idx_key1 | idx_key1 | 303 | const | 2 | 100.00 | Using index condition |
+----+-------------+-------+------------+-------------+---------------+----------+---------+-------+------+----------+-----------------------+
1 row in set, 1 warning (0.00 sec)
index_merge
mysql> EXPLAIN SELECT * FROM s1 WHERE key1 = 'a' OR key3 = 'a';
+----+-------------+-------+------------+-------------+-------------------+-------------------+---------+------+------+----------+---------------------------------------------+
| id | select_type | table | partitions | type | possible_keys | key | key_len | ref | rows | filtered | Extra |
+----+-------------+-------+------------+-------------+-------------------+-------------------+---------+------+------+----------+---------------------------------------------+
| 1 | SIMPLE | s1 | NULL | index_merge | idx_key1,idx_key3 | idx_key1,idx_key3 | 303,303 | NULL | 2 | 100.00 | Using union(idx_key1,idx_key3); Using where |
+----+-------------+-------+------------+-------------+-------------------+-------------------+---------+------+------+----------+---------------------------------------------+
1 row in set, 1 warning (0.00 sec)
It can be seen from the value of the type column of the execution plan that it is index_merge. MySQL intends to use index merging to execute queries on the s1 table.
unique_subquery
mysql> EXPLAIN SELECT * FROM s1 WHERE key2 IN (SELECT id FROM s2 where s1.key1 =
-> s2.key1) OR key3 = 'a';
+----+--------------------+-------+------------+-----------------+------------------+---------+---------+------+------+----------+-------------+
| id | select_type | table | partitions | type | possible_keys | key | key_len | ref | rows | filtered | Extra |
+----+--------------------+-------+------------+-----------------+------------------+---------+---------+------+------+----------+-------------+
| 1 | PRIMARY | s1 | NULL | ALL | idx_key3 | NULL | NULL | NULL | 9895 | 100.00 | Using where |
| 2 | DEPENDENT SUBQUERY | s2 | NULL | unique_subquery | PRIMARY,idx_key1 | PRIMARY | 4 | func | 1 | 10.00 | Using where |
+----+--------------------+-------+------------+-----------------+------------------+---------+---------+------+------+----------+-------------+
2 rows in set, 2 warnings (0.00 sec)
index_subquery
mysql> EXPLAIN SELECT * FROM s1 WHERE common_field IN (SELECT key3 FROM s2 where
-> s1.key1 = s2.key1) OR key3 = 'a';
+----+--------------------+-------+------------+------+-------------------+----------+---------+--------------------+------+----------+-------------+
| id | select_type | table | partitions | type | possible_keys | key | key_len | ref | rows | filtered | Extra |
+----+--------------------+-------+------------+------+-------------------+----------+---------+--------------------+------+----------+-------------+
| 1 | PRIMARY | s1 | NULL | ALL | idx_key3 | NULL | NULL | NULL | 9895 | 100.00 | Using where |
| 2 | DEPENDENT SUBQUERY | s2 | NULL | ref | idx_key1,idx_key3 | idx_key1 | 303 | atguigudb1.s1.key1 | 1 | 10.00 | Using where |
+----+--------------------+-------+------------+------+-------------------+----------+---------+--------------------+------+----------+-------------+
2 rows in set, 2 warnings (0.00 sec)
range
mysql> EXPLAIN SELECT * FROM s1 WHERE key1 IN ('a', 'b', 'c');
+----+-------------+-------+------------+-------+---------------+----------+---------+------+------+----------+-----------------------+
| id | select_type | table | partitions | type | possible_keys | key | key_len | ref | rows | filtered | Extra |
+----+-------------+-------+------------+-------+---------------+----------+---------+------+------+----------+-----------------------+
| 1 | SIMPLE | s1 | NULL | range | idx_key1 | idx_key1 | 303 | NULL | 3 | 100.00 | Using index condition |
+----+-------------+-------+------------+-------+---------------+----------+---------+------+------+----------+-----------------------+
1 row in set, 1 warning (0.00 sec)
range
mysql> EXPLAIN SELECT * FROM s1 WHERE key1 > 'a' AND key1 < 'b';
+----+-------------+-------+------------+-------+---------------+----------+---------+------+------+----------+-----------------------+
| id | select_type | table | partitions | type | possible_keys | key | key_len | ref | rows | filtered | Extra |
+----+-------------+-------+------------+-------+---------------+----------+---------+------+------+----------+-----------------------+
| 1 | SIMPLE | s1 | NULL | range | idx_key1 | idx_key1 | 303 | NULL | 369 | 100.00 | Using index condition |
+----+-------------+-------+------------+-------+---------------+----------+---------+------+------+----------+-----------------------+
1 row in set, 1 warning (0.00 sec)
index
mysql> EXPLAIN SELECT key_part2 FROM s1 WHERE key_part3 = 'a';
+----+-------------+-------+------------+-------+---------------+--------------+---------+------+------+----------+--------------------------+
| id | select_type | table | partitions | type | possible_keys | key | key_len | ref | rows | filtered | Extra |
+----+-------------+-------+------------+-------+---------------+--------------+---------+------+------+----------+--------------------------+
| 1 | SIMPLE | s1 | NULL | index | NULL | idx_key_part | 909 | NULL | 9895 | 10.00 | Using where; Using index |
+----+-------------+-------+------------+-------+---------------+--------------+---------+------+------+----------+--------------------------+
1 row in set, 1 warning (0.00 sec)
ALL
mysql> EXPLAIN SELECT * FROM s1;
+----+-------------+-------+------------+------+---------------+------+---------+------+------+----------+-------+
| id | select_type | table | partitions | type | possible_keys | key | key_len | ref | rows | filtered | Extra |
+----+-------------+-------+------------+------+---------------+------+---------+------+------+----------+-------+
| 1 | SIMPLE | s1 | NULL | ALL | NULL | NULL | NULL | NULL | 9895 | 100.00 | NULL |
+----+-------------+-------+------------+------+---------------+------+---------+------+------+----------+-------+
1 row in set, 1 warning (0.00 sec)
The resulting values from best to worst are: system > const > eq_ref > ref > fulltext > ref_or_null > index_merge >unique_subquery > index_subquery > range > index > ALL The more important ones are extracted (see the blue in the picture above) ). The goal of SQL performance optimization: at least reach the range level, the requirement is the ref level, preferably the consts level. (Required by Alibaba Development Manual)
5.possible_keys和key
mysql> EXPLAIN SELECT * FROM s1 WHERE key1 > 'z' AND key3 = 'a';
+----+-------------+-------+------------+------+-------------------+----------+---------+-------+------+----------+-------------+
| id | select_type | table | partitions | type | possible_keys | key | key_len | ref | rows | filtered | Extra |
+----+-------------+-------+------------+------+-------------------+----------+---------+-------+------+----------+-------------+
| 1 | SIMPLE | s1 | NULL | ref | idx_key1,idx_key3 | idx_key3 | 303 | const | 1 | 5.00 | Using where |
+----+-------------+-------+------------+------+-------------------+----------+---------+-------+------+----------+-------------+
1 row in set, 1 warning (0.00 sec)
6. key_len ☆
mysql> EXPLAIN SELECT * FROM s1 WHERE key2 = 10126;
+----+-------------+-------+------------+-------+---------------+----------+---------+-------+------+----------+-------+
| id | select_type | table | partitions | type | possible_keys | key | key_len | ref | rows | filtered | Extra |
+----+-------------+-------+------------+-------+---------------+----------+---------+-------+------+----------+-------+
| 1 | SIMPLE | s1 | NULL | const | idx_key2 | idx_key2 | 5 | const | 1 | 100.00 | NULL |
+----+-------------+-------+------------+-------+---------------+----------+---------+-------+------+----------+-------+
1 row in set, 1 warning (0.00 sec)
The length calculation formula of key_len:
varchar(10)变长字段且允许NULL = 10 * ( character set：
utf8=3,gbk=2,latin1=1)+1(NULL)+2(变长字段)
varchar(10)变长字段且不允许NULL = 10 * ( character set：utf8=3,gbk=2,latin1=1)+2(变长字段)
char(10)固定字段且允许NULL = 10 * ( character set：utf8=3,gbk=2,latin1=1)+1(NULL)
char(10)固定字段且不允许NULL = 10 * ( character set：utf8=3,gbk=2,latin1=1)
7. ref
mysql> EXPLAIN SELECT * FROM s1 INNER JOIN s2 ON s1.id = s2.id;
+----+-------------+-------+------------+--------+---------------+---------+---------+------------------+------+----------+-------+
| id | select_type | table | partitions | type | possible_keys | key | key_len | ref | rows | filtered | Extra |
+----+-------------+-------+------------+--------+---------------+---------+---------+------------------+------+----------+-------+
| 1 | SIMPLE | s1 | NULL | ALL | PRIMARY | NULL | NULL | NULL | 9895 | 100.00 | NULL |
| 1 | SIMPLE | s2 | NULL | eq_ref | PRIMARY | PRIMARY | 4 | atguigudb1.s1.id | 1 | 100.00 | NULL |
+----+-------------+-------+------------+--------+---------------+---------+---------+------------------+------+----------+-------+
2 rows in set, 1 warning (0.00 sec)
mysql> EXPLAIN SELECT * FROM s1 INNER JOIN s2 ON s2.key1 = UPPER(s1.key1);
+----+-------------+-------+------------+------+---------------+----------+---------+------+------+----------+-----------------------+
| id | select_type | table | partitions | type | possible_keys | key | key_len | ref | rows | filtered | Extra |
+----+-------------+-------+------------+------+---------------+----------+---------+------+------+----------+-----------------------+
| 1 | SIMPLE | s1 | NULL | ALL | NULL | NULL | NULL | NULL | 9895 | 100.00 | NULL |
| 1 | SIMPLE | s2 | NULL | ref | idx_key1 | idx_key1 | 303 | func | 1 | 100.00 | Using index condition |
+----+-------------+-------+------------+------+---------------+----------+---------+------+------+----------+-----------------------+
2 rows in set, 1 warning (0.00 sec)
8. rows ☆
mysql> EXPLAIN SELECT * FROM s1 WHERE key1 > 'z';
+----+-------------+-------+------------+-------+---------------+----------+---------+------+------+----------+-----------------------+
| id | select_type | table | partitions | type | possible_keys | key | key_len | ref | rows | filtered | Extra |
+----+-------------+-------+------------+-------+---------------+----------+---------+------+------+----------+-----------------------+
| 1 | SIMPLE | s1 | NULL | range | idx_key1 | idx_key1 | 303 | NULL | 364 | 100.00 | Using index condition |
+----+-------------+-------+------------+-------+---------------+----------+---------+------+------+----------+-----------------------+
1 row in set, 1 warning (0.00 sec)
9. filtered
mysql> EXPLAIN SELECT * FROM s1 WHERE key1 > 'z' AND common_field = 'a';
+----+-------------+-------+------------+-------+---------------+----------+---------+------+------+----------+------------------------------------+
| id | select_type | table | partitions | type | possible_keys | key | key_len | ref | rows | filtered | Extra |
+----+-------------+-------+------------+-------+---------------+----------+---------+------+------+----------+------------------------------------+
| 1 | SIMPLE | s1 | NULL | range | idx_key1 | idx_key1 | 303 | NULL | 364 | 10.00 | Using index condition; Using where |
+----+-------------+-------+------------+-------+---------------+----------+---------+------+------+----------+------------------------------------+
1 row in set, 1 warning (0.00 sec)
10.Extra ☆
mysql> EXPLAIN SELECT * FROM s1 WHERE 1 != 1;
+----+-------------+-------+------------+------+---------------+------+---------+------+------+----------+------------------+
| id | select_type | table | partitions | type | possible_keys | key | key_len | ref | rows | filtered | Extra |
+----+-------------+-------+------------+------+---------------+------+---------+------+------+----------+------------------+
| 1 | SIMPLE | NULL | NULL | NULL | NULL | NULL | NULL | NULL | NULL | NULL | Impossible WHERE |
+----+-------------+-------+------------+------+---------------+------+---------+------+------+----------+------------------+
1 row in set, 1 warning (0.00 sec)
mysql> EXPLAIN SELECT * FROM s1 WHERE common_field = 'a';
+----+-------------+-------+------------+------+---------------+------+---------+------+------+----------+-------------+
| id | select_type | table | partitions | type | possible_keys | key | key_len | ref | rows | filtered | Extra |
+----+-------------+-------+------------+------+---------------+------+---------+------+------+----------+-------------+
| 1 | SIMPLE | s1 | NULL | ALL | NULL | NULL | NULL | NULL | 9895 | 10.00 | Using where |
+----+-------------+-------+------------+------+---------------+------+---------+------+------+----------+-------------+
1 row in set, 1 warning (0.00 sec)
mysql> EXPLAIN SELECT MIN(key1) FROM s1 WHERE key1 = 'abcdefg';
+----+-------------+-------+------------+------+---------------+------+---------+------+------+----------+-------------------------+
| id | select_type | table | partitions | type | possible_keys | key | key_len | ref | rows | filtered | Extra |
+----+-------------+-------+------------+------+---------------+------+---------+------+------+----------+-------------------------+
| 1 | SIMPLE | NULL | NULL | NULL | NULL | NULL | NULL | NULL | NULL | NULL | No matching min/max row |
+----+-------------+-------+------------+------+---------------+------+---------+------+------+----------+-------------------------+
1 row in set, 1 warning (0.00 sec)
mysql> EXPLAIN SELECT key1 FROM s1 WHERE key1 = 'a';
+----+-------------+-------+------------+------+---------------+----------+---------+-------+------+----------+-------------+
| id | select_type | table | partitions | type | possible_keys | key | key_len | ref | rows | filtered | Extra |
+----+-------------+-------+------------+------+---------------+----------+---------+-------+------+----------+-------------+
| 1 | SIMPLE | s1 | NULL | ref | idx_key1 | idx_key1 | 303 | const | 1 | 100.00 | Using index |
+----+-------------+-------+------------+------+---------------+----------+---------+-------+------+----------+-------------+
1 row in set, 1 warning (0.00 sec)
mysql> EXPLAIN SELECT * FROM s1 ORDER BY key1 LIMIT 10;
+----+-------------+-------+------------+-------+---------------+----------+---------+------+------+----------+-------+
| id | select_type | table | partitions | type | possible_keys | key | key_len | ref | rows | filtered | Extra |
+----+-------------+-------+------------+-------+---------------+----------+---------+------+------+----------+-------+
| 1 | SIMPLE | s1 | NULL | index | NULL | idx_key1 | 303 | NULL | 10 | 100.00 | NULL |
+----+-------------+-------+------------+-------+---------------+----------+---------+------+------+----------+-------+
1 row in set, 1 warning (0.00 sec)
mysql> EXPLAIN SELECT key1, COUNT(*) AS amount FROM s1 GROUP BY key1;
+----+-------------+-------+------------+-------+---------------+----------+---------+------+------+----------+-------------+
| id | select_type | table | partitions | type | possible_keys | key | key_len | ref | rows | filtered | Extra |
+----+-------------+-------+------------+-------+---------------+----------+---------+------+------+----------+-------------+
| 1 | SIMPLE | s1 | NULL | index | idx_key1 | idx_key1 | 303 | NULL | 9895 | 100.00 | Using index |
+----+-------------+-------+------------+-------+---------------+----------+---------+------+------+----------+-------------+
1 row in set, 1 warning (0.00 sec)
- EXPLAIN does not consider various caches
- EXPLAIN cannot display the optimization work done by MySQL when executing the query
- EXPLAIN doesn't tell you about triggers, stored procedures, or how user-defined functions affect your query.
- Some statistics are estimates, not exact values
7. Further use of EXPLAIN
JSON format: Add FORMAT=JSON between the EXPLAIN word and the actual query statement.
EXPLAIN FORMAT=JSON SELECT ....
mysql> EXPLAIN FORMAT=JSON SELECT s1.key1, s2.key1 FROM s1 LEFT JOIN s2 ON s1.key1 = s2.key1 WHERE s2.common_field is not null;
| {
"query_block": {
"select_id": 1,
"cost_info": {
"query_cost": "12762.60"
},
"nested_loop": [
{
"table": {
"table_name": "s2",
"access_type": "ALL",
"possible_keys": [
"idx_key1"
],
"rows_examined_per_scan": 9895,
"rows_produced_per_join": 8905,
"filtered": "90.00",
"cost_info": {
"read_cost": "294.90",
"eval_cost": "1781.10",
"prefix_cost": "2076.00",
"data_read_per_join": "15M"
},
"used_columns": [
"key1",
"common_field"
],
"attached_condition": "((`atguigudb1`.`s2`.`common_field` is not null) and (`atguigudb1`.`s2`.`key1` is not null))"
}
},
{
"table": {
"table_name": "s1",
"access_type": "ref",
"possible_keys": [
"idx_key1"
],
"key": "idx_key1",
"used_key_parts": [
"key1"
],
"key_length": "303",
"ref": [
"atguigudb1.s2.key1"
],
"rows_examined_per_scan": 1,
"rows_produced_per_join": 8905,
"filtered": "100.00",
"using_index": true,
"cost_info": {
"read_cost": "8905.50",
"eval_cost": "1781.10",
"prefix_cost": "12762.60",
"data_read_per_join": "15M"
},
"used_columns": [
"key1"
]
}
}
]
}
} |
1 row in set, 1 warning (0.00 sec)
"cost_info": {
"read_cost": "294.90",
"eval_cost": "1781.10",
"prefix_cost": "2076.00",
"data_read_per_join": "15M"
},
read_cost is composed of the following two parts:
- IO cost
- CPU cost of detecting rows × (1 - filter) records
Tips: rows and filter are the output columns of the execution plan we introduced earlier. In the execution plan in JSON format, rows is equivalent to rows_examined_per_scan, and the filtered name remains unchanged.
- eval_cost is calculated like this:
Cost of detecting rows × filter records.
-
prefix_cost is the cost of querying the s1 table alone, that is: read_cost + eval_cost
-
data_read_per_join indicates the amount of data that needs to be read in this query.
The "cost_info" part of the s2 table looks like this:
"cost_info": {
"read_cost": "8905.50",
"eval_cost": "1781.10",
"prefix_cost": "12762.60",
"data_read_per_join": "15M"
},
Since the s2 table is a driven table, it may be read multiple times. The sum here read_costis eval_cost the accumulated value after accessing the s2 table multiple times. Everyone is mainly concerned prefix_cost that the value inside represents the estimated cost of the entire connection query, and It is the sum of the costs after a single query of the s1 table and multiple queries of the s2 table, that is:
8905.50+1781.1+2076=12762.6
8. MySQL monitoring analysis view-sys schema
- Host-related: Starting with host_summary, it mainly summarizes IO delay information.
- Innodb related: Starting with innodb, it summarizes the innodb buffer information and information about transactions waiting for innodb locks.
- I/O related: Starting with io, it summarizes waiting for I/O and I/O usage.
- Memory usage: starting with memory, showing memory usage from the perspective of host, thread, event, etc.
- Connection and session information: processlist and session-related views summarize session-related information.
- Table related: Views starting with schema_table show statistical information of the table.
- Index information: Statistics of index usage, including redundant indexes and unused indexes.
- Statement-related: Starting with statement, it contains statement information for performing full table scan, using temporary tables, sorting, etc.
- User related: The view starting with user counts the file I/O and execution statement statistics used by the user.
- Information related to waiting events: starting with wait, showing the delay of waiting events.
8.1Sys schema view usage scenarios
Index status
#1. 查询冗余索引
select * from sys.schema_redundant_indexes;
#2. 查询未使用过的索引
select * from sys.schema_unused_indexes;
#3. 查询索引的使用情况
select index_name,rows_selected,rows_inserted,rows_updated,rows_deleted
from sys.schema_index_statistics where table_schema='dbname' ;
table related
# 1. 查询表的访问量
select table_schema,table_name,sum(io_read_requests+io_write_requests) as io from
sys.schema_table_statistics group by table_schema,table_name order by io desc;
# 2. 查询占用bufferpool较多的表
select object_schema,object_name,allocated,data
from sys.innodb_buffer_stats_by_table order by allocated limit 10;
# 3. 查看表的全表扫描情况
select * from sys.statements_with_full_table_scans where db='dbname';
Statement related
#1. 监控SQL执行的频率
select db,exec_count,query from sys.statement_analysis
order by exec_count desc;
#2. 监控使用了排序的SQL
select db,exec_count,first_seen,last_seen,query
from sys.statements_with_sorting limit 1;
#3. 监控使用了临时表或者磁盘临时表的SQL
select db,exec_count,tmp_tables,tmp_disk_tables,query
from sys.statement_analysis where tmp_tables>0 or tmp_disk_tables >0
order by (tmp_tables+tmp_disk_tables) desc;
IO related
#1. 查看消耗磁盘IO的文件
select file,avg_read,avg_write,avg_read+avg_write as avg_io
from sys.io_global_by_file_by_bytes order by avg_read limit 10;
Innodb related
#1. 行锁阻塞情况
select * from sys.innodb_lock_waits;