mysql的联合索引利用情况

目录

查询条件对应的列值的类型与列对应的类型不一致

只有一个联合索引且包含非主键外全部列

查询条件全部为等值查询

查询条件有范围查询

有联合索引未包含全部列

在使用 mysql 进行数据存储时，经常用到联合索引，但是使用联合索引有一些注意点，在此记录一下。

使用的 mysql 版本为 8.0.28

定义表结构如下

CREATE TABLE test
(
id INT,
a CHAR(1),
b CHAR(1),
c CHAR(1),
PRIMARY KEY(id),
INDEX idx_union(a,b,c)
);

INSERT INTO test(id,a,b,c) VALUES(1,1,1,1);
INSERT INTO test(id,a,b,c) VALUES(2,2,2,2);
INSERT INTO test(id,a,b,c) VALUES(3,3,3,3);
INSERT INTO test(id,a,b,c) VALUES(4,4,4,4);
INSERT INTO test(id,a,b,c) VALUES(5,5,5,5);
INSERT INTO test(id,a,b,c) VALUES(6,6,6,6);
INSERT INTO test(id,a,b,c) VALUES(7,7,7,7);

可以看到，这里的类型为数字，但是存储的类型为 char，在插入过程中会进行转化。

查询条件对应的列值的类型与列对应的类型不一致

这种情况会有索引无法充分利用的情况，在优化器执行时会有警告。

mysql> EXPLAIN SELECT * FROM test WHERE a = 1 and b = 1 and c = 1;
+----+-------------+-------+------------+-------+---------------+-----------+---------+------+------+----------+--------------------------+
| id | select_type | table | partitions | type  | possible_keys | key       | key_len | ref  | rows | filtered | Extra                    |
+----+-------------+-------+------------+-------+---------------+-----------+---------+------+------+----------+--------------------------+
|  1 | SIMPLE      | test  | NULL       | index | idx_union     | idx_union | 15      | NULL |    7 |    14.29 | Using where; Using index |
+----+-------------+-------+------------+-------+---------------+-----------+---------+------+------+----------+--------------------------+
1 row in set, 5 warnings (0.01 sec)mysql> show warnings;
+---------+------+----------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------+
| Level   | Code | Message                                                                                                                                                                                                                                |
+---------+------+----------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------+
| Warning | 1739 | Cannot use ref access on index 'idx_union' due to type or collation conversion on field 'a'                                                                                                                                            |
| Warning | 1739 | Cannot use range access on index 'idx_union' due to type or collation conversion on field 'a'                                                                                                                                          |
| Warning | 1739 | Cannot use range access on index 'idx_union' due to type or collation conversion on field 'b'                                                                                                                                          |
| Warning | 1739 | Cannot use range access on index 'idx_union' due to type or collation conversion on field 'c'                                                                                                                                          |
| Note    | 1003 | /* select#1 */ select `ican`.`test`.`id` AS `id`,`ican`.`test`.`a` AS `a`,`ican`.`test`.`b` AS `b`,`ican`.`test`.`c` AS `c` from `ican`.`test` where ((`ican`.`test`.`a` = 1) and (`ican`.`test`.`b` = 1) and (`ican`.`test`.`c` = 1)) |
+---------+------+----------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------+
5 rows in set (0.00 sec)mysql> EXPLAIN SELECT * FROM test WHERE a = 1 and b = 1;
+----+-------------+-------+------------+-------+---------------+-----------+---------+------+------+----------+--------------------------+
| id | select_type | table | partitions | type  | possible_keys | key       | key_len | ref  | rows | filtered | Extra                    |
+----+-------------+-------+------------+-------+---------------+-----------+---------+------+------+----------+--------------------------+
|  1 | SIMPLE      | test  | NULL       | index | idx_union     | idx_union | 15      | NULL |    7 |    14.29 | Using where; Using index |
+----+-------------+-------+------------+-------+---------------+-----------+---------+------+------+----------+--------------------------+
1 row in set, 4 warnings (0.00 sec)mysql> show warnings;
+---------+------+------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------+
| Level   | Code | Message                                                                                                                                                                                                    |
+---------+------+------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------+
| Warning | 1739 | Cannot use ref access on index 'idx_union' due to type or collation conversion on field 'a'                                                                                                                |
| Warning | 1739 | Cannot use range access on index 'idx_union' due to type or collation conversion on field 'a'                                                                                                              |
| Warning | 1739 | Cannot use range access on index 'idx_union' due to type or collation conversion on field 'b'                                                                                                              |
| Note    | 1003 | /* select#1 */ select `ican`.`test`.`id` AS `id`,`ican`.`test`.`a` AS `a`,`ican`.`test`.`b` AS `b`,`ican`.`test`.`c` AS `c` from `ican`.`test` where ((`ican`.`test`.`a` = 1) and (`ican`.`test`.`b` = 1)) |
+---------+------+------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------+
4 rows in set (0.00 sec)mysql> EXPLAIN SELECT * FROM test WHERE a = 1 and c = 1;
+----+-------------+-------+------------+-------+---------------+-----------+---------+------+------+----------+--------------------------+
| id | select_type | table | partitions | type  | possible_keys | key       | key_len | ref  | rows | filtered | Extra                    |
+----+-------------+-------+------------+-------+---------------+-----------+---------+------+------+----------+--------------------------+
|  1 | SIMPLE      | test  | NULL       | index | idx_union     | idx_union | 15      | NULL |    7 |    14.29 | Using where; Using index |
+----+-------------+-------+------------+-------+---------------+-----------+---------+------+------+----------+--------------------------+
1 row in set, 4 warnings (0.01 sec)mysql> show warnings;
+---------+------+------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------+
| Level   | Code | Message                                                                                                                                                                                                    |
+---------+------+------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------+
| Warning | 1739 | Cannot use ref access on index 'idx_union' due to type or collation conversion on field 'a'                                                                                                                |
| Warning | 1739 | Cannot use range access on index 'idx_union' due to type or collation conversion on field 'a'                                                                                                              |
| Warning | 1739 | Cannot use range access on index 'idx_union' due to type or collation conversion on field 'c'                                                                                                              |
| Note    | 1003 | /* select#1 */ select `ican`.`test`.`id` AS `id`,`ican`.`test`.`a` AS `a`,`ican`.`test`.`b` AS `b`,`ican`.`test`.`c` AS `c` from `ican`.`test` where ((`ican`.`test`.`a` = 1) and (`ican`.`test`.`c` = 1)) |
+---------+------+------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------+
4 rows in set (0.01 sec)mysql> EXPLAIN SELECT * FROM test WHERE b = 1 and c = 1;
+----+-------------+-------+------------+-------+---------------+-----------+---------+------+------+----------+--------------------------+
| id | select_type | table | partitions | type  | possible_keys | key       | key_len | ref  | rows | filtered | Extra                    |
+----+-------------+-------+------------+-------+---------------+-----------+---------+------+------+----------+--------------------------+
|  1 | SIMPLE      | test  | NULL       | index | NULL          | idx_union | 15      | NULL |    7 |    14.29 | Using where; Using index |
+----+-------------+-------+------------+-------+---------------+-----------+---------+------+------+----------+--------------------------+
1 row in set, 3 warnings (0.00 sec)mysql> show warnings;
+---------+------+------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------+
| Level   | Code | Message                                                                                                                                                                                                    |
+---------+------+------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------+
| Warning | 1739 | Cannot use range access on index 'idx_union' due to type or collation conversion on field 'b'                                                                                                              |
| Warning | 1739 | Cannot use range access on index 'idx_union' due to type or collation conversion on field 'c'                                                                                                              |
| Note    | 1003 | /* select#1 */ select `ican`.`test`.`id` AS `id`,`ican`.`test`.`a` AS `a`,`ican`.`test`.`b` AS `b`,`ican`.`test`.`c` AS `c` from `ican`.`test` where ((`ican`.`test`.`b` = 1) and (`ican`.`test`.`c` = 1)) |
+---------+------+------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------+
3 rows in set (0.00 sec)mysql> EXPLAIN SELECT * FROM test WHERE a = 1;
+----+-------------+-------+------------+-------+---------------+-----------+---------+------+------+----------+--------------------------+
| id | select_type | table | partitions | type  | possible_keys | key       | key_len | ref  | rows | filtered | Extra                    |
+----+-------------+-------+------------+-------+---------------+-----------+---------+------+------+----------+--------------------------+
|  1 | SIMPLE      | test  | NULL       | index | idx_union     | idx_union | 15      | NULL |    7 |    14.29 | Using where; Using index |
+----+-------------+-------+------------+-------+---------------+-----------+---------+------+------+----------+--------------------------+
1 row in set, 3 warnings (0.00 sec)mysql> show warnings;
+---------+------+------------------------------------------------------------------------------------------------------------------------------------------------------------------------------+
| Level   | Code | Message                                                                                                                                                                      |
+---------+------+------------------------------------------------------------------------------------------------------------------------------------------------------------------------------+
| Warning | 1739 | Cannot use ref access on index 'idx_union' due to type or collation conversion on field 'a'                                                                                  |
| Warning | 1739 | Cannot use range access on index 'idx_union' due to type or collation conversion on field 'a'                                                                                |
| Note    | 1003 | /* select#1 */ select `ican`.`test`.`id` AS `id`,`ican`.`test`.`a` AS `a`,`ican`.`test`.`b` AS `b`,`ican`.`test`.`c` AS `c` from `ican`.`test` where (`ican`.`test`.`a` = 1) |
+---------+------+------------------------------------------------------------------------------------------------------------------------------------------------------------------------------+
3 rows in set (0.00 sec)mysql> EXPLAIN SELECT * FROM test WHERE b = 1;
+----+-------------+-------+------------+-------+---------------+-----------+---------+------+------+----------+--------------------------+
| id | select_type | table | partitions | type  | possible_keys | key       | key_len | ref  | rows | filtered | Extra                    |
+----+-------------+-------+------------+-------+---------------+-----------+---------+------+------+----------+--------------------------+
|  1 | SIMPLE      | test  | NULL       | index | NULL          | idx_union | 15      | NULL |    7 |    14.29 | Using where; Using index |
+----+-------------+-------+------------+-------+---------------+-----------+---------+------+------+----------+--------------------------+
1 row in set, 2 warnings (0.01 sec)mysql> show warnings;
+---------+------+------------------------------------------------------------------------------------------------------------------------------------------------------------------------------+
| Level   | Code | Message                                                                                                                                                                      |
+---------+------+------------------------------------------------------------------------------------------------------------------------------------------------------------------------------+
| Warning | 1739 | Cannot use range access on index 'idx_union' due to type or collation conversion on field 'b'                                                                                |
| Note    | 1003 | /* select#1 */ select `ican`.`test`.`id` AS `id`,`ican`.`test`.`a` AS `a`,`ican`.`test`.`b` AS `b`,`ican`.`test`.`c` AS `c` from `ican`.`test` where (`ican`.`test`.`b` = 1) |
+---------+------+------------------------------------------------------------------------------------------------------------------------------------------------------------------------------+
2 rows in set (0.01 sec)mysql> EXPLAIN SELECT * FROM test WHERE c = 1;
+----+-------------+-------+------------+-------+---------------+-----------+---------+------+------+----------+--------------------------+
| id | select_type | table | partitions | type  | possible_keys | key       | key_len | ref  | rows | filtered | Extra                    |
+----+-------------+-------+------------+-------+---------------+-----------+---------+------+------+----------+--------------------------+
|  1 | SIMPLE      | test  | NULL       | index | NULL          | idx_union | 15      | NULL |    7 |    14.29 | Using where; Using index |
+----+-------------+-------+------------+-------+---------------+-----------+---------+------+------+----------+--------------------------+
1 row in set, 2 warnings (0.01 sec)mysql> show warnings;
+---------+------+------------------------------------------------------------------------------------------------------------------------------------------------------------------------------+
| Level   | Code | Message                                                                                                                                                                      |
+---------+------+------------------------------------------------------------------------------------------------------------------------------------------------------------------------------+
| Warning | 1739 | Cannot use range access on index 'idx_union' due to type or collation conversion on field 'c'                                                                                |
| Note    | 1003 | /* select#1 */ select `ican`.`test`.`id` AS `id`,`ican`.`test`.`a` AS `a`,`ican`.`test`.`b` AS `b`,`ican`.`test`.`c` AS `c` from `ican`.`test` where (`ican`.`test`.`c` = 1) |
+---------+------+------------------------------------------------------------------------------------------------------------------------------------------------------------------------------+
2 rows in set (0.00 sec)

修改类型为一致，则正常执行

mysql> EXPLAIN SELECT * FROM test WHERE a = '1' and b = '1' and c = '1';
+----+-------------+-------+------------+------+---------------+-----------+---------+-------------------+------+----------+--------------------------+
| id | select_type | table | partitions | type | possible_keys | key       | key_len | ref               | rows | filtered | Extra                    |
+----+-------------+-------+------------+------+---------------+-----------+---------+-------------------+------+----------+--------------------------+
|  1 | SIMPLE      | test  | NULL       | ref  | idx_union     | idx_union | 15      | const,const,const |    1 |   100.00 | Using where; Using index |
+----+-------------+-------+------------+------+---------------+-----------+---------+-------------------+------+----------+--------------------------+
1 row in set, 1 warning (0.01 sec)mysql> show warnings;
+-------+------+----------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------+
| Level | Code | Message                                                                                                                                                                                                                                      |
+-------+------+----------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------+
| Note  | 1003 | /* select#1 */ select `ican`.`test`.`id` AS `id`,`ican`.`test`.`a` AS `a`,`ican`.`test`.`b` AS `b`,`ican`.`test`.`c` AS `c` from `ican`.`test` where ((`ican`.`test`.`c` = '1') and (`ican`.`test`.`b` = '1') and (`ican`.`test`.`a` = '1')) |
+-------+------+----------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------+
1 row in set (0.01 sec)mysql> EXPLAIN SELECT * FROM test WHERE a = '1';
+----+-------------+-------+------------+------+---------------+-----------+---------+-------+------+----------+--------------------------+
| id | select_type | table | partitions | type | possible_keys | key       | key_len | ref   | rows | filtered | Extra                    |
+----+-------------+-------+------------+------+---------------+-----------+---------+-------+------+----------+--------------------------+
|  1 | SIMPLE      | test  | NULL       | ref  | idx_union     | idx_union | 5       | const |    1 |   100.00 | Using where; Using index |
+----+-------------+-------+------------+------+---------------+-----------+---------+-------+------+----------+--------------------------+
1 row in set, 1 warning (0.00 sec)mysql> show warnings;
+-------+------+--------------------------------------------------------------------------------------------------------------------------------------------------------------------------------+
| Level | Code | Message                                                                                                                                                                        |
+-------+------+--------------------------------------------------------------------------------------------------------------------------------------------------------------------------------+
| Note  | 1003 | /* select#1 */ select `ican`.`test`.`id` AS `id`,`ican`.`test`.`a` AS `a`,`ican`.`test`.`b` AS `b`,`ican`.`test`.`c` AS `c` from `ican`.`test` where (`ican`.`test`.`a` = '1') |
+-------+------+--------------------------------------------------------------------------------------------------------------------------------------------------------------------------------+
1 row in set (0.01 sec)mysql> EXPLAIN SELECT * FROM test WHERE b = '1';
+----+-------------+-------+------------+-------+---------------+-----------+---------+------+------+----------+--------------------------+
| id | select_type | table | partitions | type  | possible_keys | key       | key_len | ref  | rows | filtered | Extra                    |
+----+-------------+-------+------------+-------+---------------+-----------+---------+------+------+----------+--------------------------+
|  1 | SIMPLE      | test  | NULL       | index | idx_union     | idx_union | 15      | NULL |    7 |    14.29 | Using where; Using index |
+----+-------------+-------+------------+-------+---------------+-----------+---------+------+------+----------+--------------------------+
1 row in set, 1 warning (0.00 sec)mysql> show warnings;
+-------+------+--------------------------------------------------------------------------------------------------------------------------------------------------------------------------------+
| Level | Code | Message                                                                                                                                                                        |
+-------+------+--------------------------------------------------------------------------------------------------------------------------------------------------------------------------------+
| Note  | 1003 | /* select#1 */ select `ican`.`test`.`id` AS `id`,`ican`.`test`.`a` AS `a`,`ican`.`test`.`b` AS `b`,`ican`.`test`.`c` AS `c` from `ican`.`test` where (`ican`.`test`.`b` = '1') |
+-------+------+--------------------------------------------------------------------------------------------------------------------------------------------------------------------------------+
1 row in set (0.01 sec)mysql> EXPLAIN SELECT * FROM test WHERE c = '1';
+----+-------------+-------+------------+-------+---------------+-----------+---------+------+------+----------+--------------------------+
| id | select_type | table | partitions | type  | possible_keys | key       | key_len | ref  | rows | filtered | Extra                    |
+----+-------------+-------+------------+-------+---------------+-----------+---------+------+------+----------+--------------------------+
|  1 | SIMPLE      | test  | NULL       | index | idx_union     | idx_union | 15      | NULL |    7 |    14.29 | Using where; Using index |
+----+-------------+-------+------------+-------+---------------+-----------+---------+------+------+----------+--------------------------+
1 row in set, 1 warning (0.01 sec)mysql> show warnings;
+-------+------+--------------------------------------------------------------------------------------------------------------------------------------------------------------------------------+
| Level | Code | Message                                                                                                                                                                        |
+-------+------+--------------------------------------------------------------------------------------------------------------------------------------------------------------------------------+
| Note  | 1003 | /* select#1 */ select `ican`.`test`.`id` AS `id`,`ican`.`test`.`a` AS `a`,`ican`.`test`.`b` AS `b`,`ican`.`test`.`c` AS `c` from `ican`.`test` where (`ican`.`test`.`c` = '1') |
+-------+------+--------------------------------------------------------------------------------------------------------------------------------------------------------------------------------+
1 row in set (0.01 sec)

尤其是最左列的类型一致后，type 列从 index 变为 ref。

只有一个联合索引且包含非主键外全部列

查询条件全部为等值查询

sql查询条件typeExtrarefrowsfiltered示例sql说明
符合索引中全部列查询
或者按照索引定义的最左列顺序查询		refUsing where; Using index对应值为const

对应的数量为等值查询列数量		符合要求的行数100%EXPLAIN SELECT * FROM test WHERE a = '1' and b = '1' and c = '1';
EXPLAIN SELECT * FROM test WHERE a = '1' and b = '1';
EXPLAIN SELECT * FROM test WHERE a = '1';		充分利用了索引
符合索引中部分列查询
且包含索引定义的最左列		refUsing where; Using index对应值为const

对应的数量为等值查询列数量		符合要求的行数非100%EXPLAIN SELECT * FROM test WHERE a = '1' and c = '1';部分利用了索引
符合索引中部分列查询
且不包含索引定义的最左列		indexUsing where; Using indexNULL索引中数据总行数非100%EXPLAIN SELECT * FROM test WHERE b = '1' and c = '1';
EXPLAIN SELECT * FROM test WHERE b = '1';
EXPLAIN SELECT * FROM test WHERE c = '1';		部分利用了索引

可以看到，在使用联合索引的时候有一种情况特殊

非联合索引首列进行查询，最终 type 为 index。

查询条件有范围查询

sql查询条件typeExtrarefrowsfiltered示例sql说明
符合索引中全部列查询或者按照索引定义的最左列顺序查询
且只有一个范围查询		rangeUsing where; Using indexNULL符合要求的行数100%EXPLAIN SELECT * FROM test WHERE a = '1' and b = '1' and c > '1';
EXPLAIN SELECT * FROM test WHERE a = '1' and b > '1';
EXPLAIN SELECT * FROM test WHERE a > '1';		利用了索引，完全使用了最左匹配原则
符合索引中全部列查询或者按照索引定义的最左列顺序查询
且不止一个范围查询		rangeUsing where; Using indexNULL符合要求的行数非100%EXPLAIN SELECT * FROM test WHERE a = '1' and b > '1' and c > '1';
EXPLAIN SELECT * FROM test WHERE a > '1' and b > '1' and c > '1';
EXPLAIN SELECT * FROM test WHERE a > '1' and c > '1';		利用了索引，部分使用了最左匹配原则
符合索引中部分列查询
且包含索引定义的最左列
且最少一个范围查询		refUsing where; Using index对应值为const

对应的数量为等值查询列数量		符合要求的行数非100%EXPLAIN SELECT * FROM test WHERE a = '1' and c > '1';利用了索引，部分使用了最左匹配原则
符合索引中部分列查询
且不包含索引定义的最左列		indexUsing where; Using indexNULL索引中数据总行数非100%EXPLAIN SELECT * FROM test WHERE b = '1' and c > '1';
EXPLAIN SELECT * FROM test WHERE b > '1' and c > '1';
EXPLAIN SELECT * FROM test WHERE c > '1';		利用了索引，未使用最左匹配原则

可以看到，在使用联合索引的时候有两种情况特殊

  • 通过最左列进行等值查询，间隔的列执行范围查询，最终 type 为 ref。
  • 不包含最左列，使用了等值或者范围查询或者两者的组合。

有联合索引未包含全部列

ALTER TABLE test ADD COLUMN d CHAR(1);

sql查询条件typeExtrarefrowsfiltered示例sql说明
符合索引中全部列查询
或者按照索引定义的最左列顺序查询
且查询条件为全部列		refUsing index condition对应值为const

对应的数量为等值查询列数量		符合要求的行数100%EXPLAIN SELECT * FROM test WHERE a = '1' and b = '1' and c = '1';
EXPLAIN SELECT * FROM test WHERE a = '1' and b = '1';
EXPLAIN SELECT * FROM test WHERE a = '1';		部分利用了索引，部分数据需要通过回表查询缺失的数据
符合索引中全部列查询
或者按照索引定义的最左列顺序查询
且查询条件为联合索引全部列		refUsing where; Using index对应值为const

对应的数量为等值查询列数量		符合要求的行数100%EXPLAIN SELECT a,b,c FROM test WHERE a = '1' and b = '1' and c = '1';
EXPLAIN SELECT a,b,c FROM test WHERE a = '1' and b = '1';
EXPLAIN SELECT a,b,c FROM test WHERE a = '1';		充分利用了索引
符合索引中部分列查询
且包含索引定义的最左列
且查询条件为全部列		refUsing index condition对应值为const

对应的数量为等值查询列数量		符合要求的行数非100%EXPLAIN SELECT * FROM test WHERE a = '1' and c = '1';部分利用了索引，部分数据需要通过回表查询缺失的数据
符合索引中部分列查询
且包含索引定义的最左列
且查询条件为联合索引全部列		refUsing where; Using index对应值为const

对应的数量为等值查询列数量		符合要求的行数非100%EXPLAIN SELECT a,b,c FROM test WHERE a = '1' and c = '1';部分利用了索引
符合索引中部分列查询
且不包含索引定义的最左列
且查询条件为全部列		ALLUsing whereNULL索引中数据总行数非100%EXPLAIN SELECT * FROM test WHERE b = '1' and c = '1';
EXPLAIN SELECT * FROM test WHERE b = '1';
EXPLAIN SELECT * FROM test WHERE c = '1';		未利用索引
符合索引中部分列查询
且不包含索引定义的最左列
且查询条件为联合索引全部列		indexUsing where; Using indexNULL索引中数据总行数非100%EXPLAIN SELECT a,b,c FROM test WHERE b = '1' and c = '1';
EXPLAIN SELECT a,b,c FROM test WHERE b = '1';
EXPLAIN SELECT a,b,c FROM test WHERE c = '1';		部分利用了索引

有的 Extra 信息为 Using index condition，表明使用了回表查询。 

从 mysql 5.6 开始添加的功能，但是看官网链接里 5.6 相关的删除了，最老的是 5.7。

https://dev.mysql.com/doc/refman/5.6/en/index-condition-pushdown-optimization.html

https://dev.mysql.com/doc/refman/5.7/en/index-condition-pushdown-optimization.html

5.6 的链接重定向到 8.0 的链接。 

https://dev.mysql.com/doc/refman/8.0/en/index-condition-pushdown-optimization.html

查询时不包含首列，查询结果查询全部会直接全表扫描。

针对联合索引等值查询乱序的情况，优化器在执行 sql 之前会将查询条件进行调整来适应索引定义的列的顺序，通过 cbo 计算查询的代价，从而匹配适合的索引，如果一个都没匹配到，则执行全表扫描。

参考链接

https://www.cnblogs.com/ql211lin/p/11124574.html

https://www.cnblogs.com/zhp-king/p/7250810.html

https://www.cnblogs.com/wy123/p/7366486.html

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