MySQL 索引
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概述：
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★定义：
索引：提取索引的创建在的表上字段中的数据，构建出一个独特的数据结构；
★作用：加速查询操作；
表中数据子集：把表中某个或某些字段的数据提取出来另存为一个特定数据结构组织的数据；
某个字段或某些字段：WHERE子句中用到的字段；
1.B+ TREE 索引
★B+ TREE：
顺序存储，每一个叶子结点到根结点的距离相同；
左前缀索引，适合于范围类型的数据查询；
★适用于B+ TREE索引的查询类型：全键值、键值范围或键前缀；
全值匹配：精确匹配某个值；
·eg：WHERE CLOUMN = ‘value‘;
匹配最左前缀：只精确匹配起头的部分；
·eg：WEHRE COLUMN LIKE ‘PREFIX%‘;
匹配范围值：（顺序排列）
·eg：WEHRE Age > 30;
精确匹配某一列，范围匹配另一列；
只用访问索引的查询：覆盖索引；（仅通过索引就能得到最终数据）
·eg: index(Name)
SELECT Name FROM students WHERE Name LIKE ‘L%‘;
★不适用B+ TREE索引的查询类型：
如查询条件不是从最左侧列开始，索引无效；
·eg:index(age,name), WHERE name=‘Jerry‘(无效);WHERE age>30(有效);
不能跳过索引中的某列；
·eg:index(name,age,gender)
WHERE name LIKE ‘j%‘ and gender=‘M‘ (无效，不能跳过阿age字段)
如果查询中的某个列是为范围查询，那么其右侧的列都无法再使用索引优化查询；
·eg:index(name,age,gender)
WHERE second_name=‘black‘ and age > 30;（有效，如果把范围放前面就无效了）
2.Hash索引：
★基于哈希表实现，特别适用于值的精确匹配查询；
★适用场景：
只支持等值比较查询，例如=, IN(), <=>
★不用场景：
所有非精确值查询；
MySQL仅对memory存储引擎支持显式的hash索引；也就是说在mysql中只能使用B+ TREE 索引
3.索引优点及高性能索引策略：
★索引优点：
降低需要扫描的数据量，减少了I/O次数；
可以帮助避免排序操作，避免使用临时表；
帮助将随机I/O转为顺序I/O；
★高性能索引策略：
☉在WHERE中独立使用列，尽量避免其参与运算；
☉左前缀索引：索引构建于字段的最左侧的多少个字符，要通过索引选择性来评估;
索引选择性：不重复的索引值和数据表的记录总数的比值；
尽量使用主键来构建索引，因为出现的重复率较小，效率最高
☉多列索引：
AND连接的多个查询条件更适合使用多列索引，而非多个单键索引；
☉选择合适的索引列次序：选择性最高的放左侧；
3.索引的创建、查看、修改和删除：
★创建
☉语法：
CREATE [UNIQUE|FULLTEXT|SPATIAL] INDEX index_name ON tbl_name (index_col_name,...) 索引字段名称
index_col_name:
col_name [(length)] [ASC | DESC]
{INDEX|KEY} 普通索引创建
{FULLTEXT|SPATIAL} 全文索引，空间索引
★删除：
DROP INDEX index_name ON tbl_name
★查看：
SHOW {INDEX | INDEXES | KEYS} {FROM | IN} tbl_name [{FROM | IN} db_name] [WHERE expr]
★使用ALTER 命令添加和删除索引
ALTER TABLE tbl_name ADD PRIMARY KEY (column_list):
该语句添加一个主键，这意味着索引值必须是唯一的，且不能为NULL。
ALTER TABLE tbl_name ADD UNIQUE index_name (column_list):
这条语句创建索引的值必须是唯一的（除了NULL外，NULL可能会出现多次）。
ALTER TABLE tbl_name ADD INDEX index_name (column_list):
添加普通索引，索引值可出现多次。
ALTER TABLE tbl_name ADD FULLTEXT index_name (column_list):
该语句指定了索引为 FULLTEXT ，用于全文索引。
4.EXPLAIN来分析索引有效性：
★语法：
EXPLAIN [explain_type] SELECT select_options
☉explain_type:
EXTENDED
PARTITIONS
★输出结果：
☉id：当前查询语句中，第几个SELECT语句的编号；
※复杂的查询的类型主要三种：
简单子查询；
用于FROM中的子查询；
联合查询
注意：联合查询的分析结果会出现一个额外的匿名临时表；
☉select_type：查询类型：
※简单查询：SIMPLE
※复杂查询：
简单子查询：SUBQUERY
用于FROM中的子查询：DERIVED
联合查询中的第一个查询：PRIMARY
联合查询中的第一个查询之后的其它查询：UNION
联合查询生成的临时表：UNION RESULT
☉table：查询针对的表；
☉type：关联类型，或称为访问类型，即MySQL如何去查询表中的行
ALL：全表扫描；
index：根据索引的顺序进行的全表扫描；但同时如果Extra列出现了"Using index”表示使用了覆盖索引；
range：有范围限制地根据索引实现范围扫描；扫描位置始于索引中的某一项，结束于另一项；
ref：根据索引返回的表中匹配到某单个值的所有行（匹配给定值的行不止一个）；
eq_ref：根据索引返回的表中匹配到某单个值的单一行，仅返回一个行，但需要与某个额外的参考值比较，而不是常数；
const，system：与某个常数比较，且只返回一行；
☉possiable_keys：查询中可能会用到的索引；
☉key：查询中使用的索引；
☉key_len：查询中用到的索引长度；
☉ref：在利用key字段所显示的索引完成查询操作时所引用的列或常量值；
☉rows：MySQL估计出的为找到所有的目标项而需要读取的行数；
☉Extra：额外信息
Using index：使用了覆盖索引进行的查询；
Using where：拿到数据后还要再次进行过滤；
Using temporary：使用了临时表以完成查询；
Using filesort：对结果使用了一个外部索引排序；
演示：
1.在hellodb库上创建索引，并使用EXLLAIN测试是否用到索引；
# 查看当前索引，可以发现只有一个主键索引 MariaDB [hellodb]> SHOW INDEX FROM students; +----------+------------+----------+--------------+-------------+-----------+-------------+----------+--------+------+------------+---------+---------------+ | Table | Non_unique | Key_name | Seq_in_index | Column_name | Collation | Cardinality | Sub_part | Packed | Null | Index_type | Comment | Index_comment | +----------+------------+----------+--------------+-------------+-----------+-------------+----------+--------+------+------------+---------+---------------+ | students | 0 | PRIMARY | 1 | StuID | A | 25 | NULL | NULL | | BTREE | | | +----------+------------+----------+--------------+-------------+-----------+-------------+----------+--------+------+------------+---------+---------------+ 1 row in set (0.04 sec) # 在Name字段上创建name索引； MariaDB [hellodb]> CREATE INDEX name ON students(Name); Query OK, 0 rows affected (0.05 sec) Records: 0 Duplicates: 0 Warnings: 0 # 查看索引 MariaDB [hellodb]> SHOW INDEX FROM students; +----------+------------+----------+--------------+-------------+-----------+-------------+----------+--------+------+------------+---------+---------------+ | Table | Non_unique | Key_name | Seq_in_index | Column_name | Collation | Cardinality | Sub_part | Packed | Null | Index_type | Comment | Index_comment | +----------+------------+----------+--------------+-------------+-----------+-------------+----------+--------+------+------------+---------+---------------+ | students | 0 | PRIMARY | 1 | StuID | A | 25 | NULL | NULL | | BTREE | | | | students | 1 | name | 1 | Name | A | 25 | NULL | NULL | | BTREE | | | +----------+------------+----------+--------------+-------------+-----------+-------------+----------+--------+------+------------+---------+---------------+ 2 rows in set (0.00 sec) # 查询数据 MariaDB [hellodb]> SELECT * FROM students WHERE Name = ‘Xi Ren‘; +-------+--------+-----+--------+---------+-----------+ | StuID | Name | Age | Gender | ClassID | TeacherID | +-------+--------+-----+--------+---------+-----------+ | 7 | Xi Ren | 19 | F | 3 | NULL | +-------+--------+-----+--------+---------+-----------+ 1 row in set (0.01 sec) # 使用EXPLAIN测试可以发现可能用到的字段为name，用到的key为name，可见索引是有效的，只查找了一行就可以找到数据 MariaDB [hellodb]> EXPLAIN SELECT * FROM students WHERE Name = ‘Xi Ren‘; +------+-------------+----------+------+---------------+------+---------+-------+------+-----------------------+ | id | select_type | table | type | possible_keys | key | key_len | ref | rows | Extra | +------+-------------+----------+------+---------------+------+---------+-------+------+-----------------------+ | 1 | SIMPLE | students | ref | name | name | 152 | const | 1 | Using index condition | +------+-------------+----------+------+---------------+------+---------+-------+------+-----------------------+ 1 row in set (0.00 sec) # 为了测试不加索引的效果，我把刚创建的Name字段的索引删除掉 MariaDB [hellodb]> DROP INDEX name ON students; Query OK, 0 rows affected (0.02 sec) Records: 0 Duplicates: 0 Warnings: 0 MariaDB [hellodb]> SHOW INDEX FROM students; +----------+------------+----------+--------------+-------------+-----------+-------------+----------+--------+------+------------+---------+---------------+ | Table | Non_unique | Key_name | Seq_in_index | Column_name | Collation | Cardinality | Sub_part | Packed | Null | Index_type | Comment | Index_comment | +----------+------------+----------+--------------+-------------+-----------+-------------+----------+--------+------+------------+---------+---------------+ | students | 0 | PRIMARY | 1 | StuID | A | 25 | NULL | NULL | | BTREE | | | +----------+------------+----------+--------------+-------------+-----------+-------------+----------+--------+------+------------+---------+---------------+ 1 row in set (0.02 sec) # 再次使用EXPLAIN可以发现，这次可用的键为空，查找了25行才能找到在数据，可见添加索引的功效 MariaDB [hellodb]> EXPLAIN SELECT * FROM students WHERE Name = ‘Xi Ren‘; +------+-------------+----------+------+---------------+------+---------+------+------+-------------+ | id | select_type | table | type | possible_keys | key | key_len | ref | rows | Extra | +------+-------------+----------+------+---------------+------+---------+------+------+-------------+ | 1 | SIMPLE | students | ALL | NULL | NULL | NULL | NULL | 25 | Using where | +------+-------------+----------+------+---------------+------+---------+------+------+-------------+ 1 row in set (0.00 sec)
MariaDB [hellodb]> CREATE INDEX name ON students(Name); Query OK, 0 rows affected (0.07 sec) Records: 0 Duplicates: 0 Warnings: 0 MariaDB [hellodb]> CREATE INDEX age ON students(Age); Query OK, 0 rows affected (0.03 sec) Records: 0 Duplicates: 0 Warnings: 0 # 在Name，Age字段创建索引，查看如下： MariaDB [hellodb]> SHOW INDEX FROM students; +----------+------------+----------+--------------+-------------+-----------+-------------+----------+--------+------+------------+---------+---------------+ | Table | Non_unique | Key_name | Seq_in_index | Column_name | Collation | Cardinality | Sub_part | Packed | Null | Index_type | Comment | Index_comment | +----------+------------+----------+--------------+-------------+-----------+-------------+----------+--------+------+------------+---------+---------------+ | students | 0 | PRIMARY | 1 | StuID | A | 25 | NULL | NULL | | BTREE | | | | students | 1 | name | 1 | Name | A | 25 | NULL | NULL | | BTREE | | | | students | 1 | age | 1 | Age | A | 25 | NULL | NULL | | BTREE | | | +----------+------------+----------+--------------+-------------+-----------+-------------+----------+--------+------+------------+---------+---------------+ 3 rows in set (0.01 sec) # 查询Age>30的可以发现，用到了Age字段的age索引，仅查询了5行就把结果输出来了 MariaDB [hellodb]> EXPLAIN SELECT * FROM students WHERE Age > 30; +------+-------------+----------+-------+---------------+------+---------+------+------+-----------------------+ | id | select_type | table | type | possible_keys | key | key_len | ref | rows | Extra | +------+-------------+----------+-------+---------------+------+---------+------+------+-----------------------+ | 1 | SIMPLE | students | range | age | age | 1 | NULL | 5 | Using index condition | +------+-------------+----------+-------+---------------+------+---------+------+------+-----------------------+ 1 row in set (0.00 sec) # 可以发现是结果是按顺序排列的，不用做全表扫描，直接截取符合条件的行； MariaDB [hellodb]> SELECT * FROM students WHERE Age > 30; +-------+--------------+-----+--------+---------+-----------+ | StuID | Name | Age | Gender | ClassID | TeacherID | +-------+--------------+-----+--------+---------+-----------+ | 4 | Ding Dian | 32 | M | 4 | 4 | | 13 | Tian Boguang | 33 | M | 2 | NULL | | 6 | Shi Qing | 46 | M | 5 | NULL | | 3 | Xie Yanke | 53 | M | 2 | 16 | | 25 | Sun Dasheng | 100 | M | NULL | NULL | +-------+--------------+-----+--------+---------+-----------+ 5 rows in set (0.00 sec)
2.创建多键索引，测试，得出结论：要想使索引生效，查询条件一定要从最左侧开始，不能跳过字段；
# 首先删除原来创建的索引； MariaDB [hellodb]> DROP INDEX name ON students; Query OK, 0 rows affected (0.04 sec) Records: 0 Duplicates: 0 Warnings: 0 MariaDB [hellodb]> DROP INDEX age ON students; Query OK, 0 rows affected (0.04 sec) Records: 0 Duplicates: 0 Warnings: 0 # 创建一个多级索引 MariaDB [hellodb]> CREATE INDEX name_age ON students(name,age); Query OK, 0 rows affected (0.06 sec) Records: 0 Duplicates: 0 Warnings: 0 # 查看可以发现，两个字段共同使用一个索引 name_age MariaDB [hellodb]> SHOW INDEX FROM students; +----------+------------+----------+--------------+-------------+-----------+-------------+----------+--------+------+------------+---------+---------------+ | Table | Non_unique | Key_name | Seq_in_index | Column_name | Collation | Cardinality | Sub_part | Packed | Null | Index_type | Comment | Index_comment | +----------+------------+----------+--------------+-------------+-----------+-------------+----------+--------+------+------------+---------+---------------+ | students | 0 | PRIMARY | 1 | StuID | A | 25 | NULL | NULL | | BTREE | | | | students | 1 | name_age | 1 | Name | A | 25 | NULL | NULL | | BTREE | | | | students | 1 | name_age | 2 | Age | A | 25 | NULL | NULL | | BTREE | | | +----------+------------+----------+--------------+-------------+-----------+-------------+----------+--------+------+------------+---------+---------------+ 3 rows in set (0.02 sec) # 现在，基于Name为过滤条件，发现多键索引依然有效，可见索引为左前缀精确匹配； MariaDB [hellodb]> EXPLAIN SELECT * FROM students WHERE Name = ‘Xi Ren‘; +------+-------------+----------+------+---------------+----------+---------+-------+------+-----------------------+ | id | select_type | table | type | possible_keys | key | key_len | ref | rows | Extra | +------+-------------+----------+------+---------------+----------+---------+-------+------+-----------------------+ | 1 | SIMPLE | students | ref | name_age | name_age | 152 | const | 1 | Using index condition | +------+-------------+----------+------+---------------+----------+---------+-------+------+-----------------------+ 1 row in set (0.00 sec) # 但是，如果过滤条件为Age的Where子句，发现索引无效，可见索引的次序非常重要，必须要从最左侧开始 MariaDB [hellodb]> EXPLAIN SELECT * FROM students WHERE Age > 30; +------+-------------+----------+------+---------------+------+---------+------+------+-------------+ | id | select_type | table | type | possible_keys | key | key_len | ref | rows | Extra | +------+-------------+----------+------+---------------+------+---------+------+------+-------------+ | 1 | SIMPLE | students | ALL | NULL | NULL | NULL | NULL | 25 | Using where | +------+-------------+----------+------+---------------+------+---------+------+------+-------------+ 1 row in set (0.00 sec) # 创建age字段位于最左侧的索引age_name，测试 MariaDB [hellodb]> CREATE INDEX age_name ON students(age,name); Query OK, 0 rows affected (0.05 sec) Records: 0 Duplicates: 0 Warnings: 0 # 可以发现成功使用索引，可见索引的次序非常重要，查询条件必须要从最左侧开始！！！ MariaDB [hellodb]> EXPLAIN SELECT * FROM students WHERE Age > 30; +------+-------------+----------+-------+---------------+----------+---------+------+------+-----------------------+ | id | select_type | table | type | possible_keys | key | key_len | ref | rows | Extra | +------+-------------+----------+-------+---------------+----------+---------+------+------+-----------------------+ | 1 | SIMPLE | students | range | age_name | age_name | 1 | NULL | 5 | Using index condition | +------+-------------+----------+-------+---------------+----------+---------+------+------+-----------------------+ 1 row in set (0.00 sec) MariaDB [hellodb]> SHOW INDEX FROM students; +----------+------------+----------+--------------+-------------+-----------+-------------+----------+--------+------+------------+---------+---------------+ | Table | Non_unique | Key_name | Seq_in_index | Column_name | Collation | Cardinality | Sub_part | Packed | Null | Index_type | Comment | Index_comment | +----------+------------+----------+--------------+-------------+-----------+-------------+----------+--------+------+------------+---------+---------------+ | students | 0 | PRIMARY | 1 | StuID | A | 25 | NULL | NULL | | BTREE | | | | students | 1 | name_age | 1 | Name | A | 25 | NULL | NULL | | BTREE | | | | students | 1 | name_age | 2 | Age | A | 25 | NULL | NULL | | BTREE | | | | students | 1 | age_name | 1 | Age | A | 25 | NULL | NULL | | BTREE | | | | students | 1 | age_name | 2 | Name | A | 25 | NULL | NULL | | BTREE | | | +----------+------------+----------+--------------+-------------+-----------+-------------+----------+--------+------+------------+---------+---------------+ 5 rows in set (0.00 sec)
3.要想使索引生效，左前缀一定要做精确匹配，如果为范围匹配，索引是无效的
# 查找条件左侧为精确匹配，然后再是范围匹配，可以看到用到的索引为name_age，过滤出来后再做范围匹配 MariaDB [hellodb]> EXPLAIN SELECT * FROM students WHERE Name = ‘Xi Ren‘ AND Age > 30; +------+-------------+----------+-------+-------------------+----------+---------+------+------+-----------------------+ | id | select_type | table | type | possible_keys | key | key_len | ref | rows | Extra | +------+-------------+----------+-------+-------------------+----------+---------+------+------+-----------------------+ | 1 | SIMPLE | students | range | name_age,age_name | name_age | 153 | NULL | 1 | Using index condition | +------+-------------+----------+-------+-------------------+----------+---------+------+------+-----------------------+ 1 row in set (0.00 sec) # 查找结果为空，没有符合条件这个数据 MariaDB [hellodb]> SELECT * FROM students WHERE Name = ‘Xi Ren‘ AND Age > 30; Empty set (0.00 sec) # 左前缀为范围匹配，发现左侧的索引无效，有效的仅为右侧的查找条件的索引 MariaDB [hellodb]> EXPLAIN SELECT * FROM students WHERE Name LIKE ‘X%‘ AND Age > 30; +------+-------------+----------+-------+-------------------+----------+---------+------+------+-----------------------+ | id | select_type | table | type | possible_keys | key | key_len | ref | rows | Extra | +------+-------------+----------+-------+-------------------+----------+---------+------+------+-----------------------+ | 1 | SIMPLE | students | range | name_age,age_name | age_name | 1 | NULL | 5 | Using index condition | +------+-------------+----------+-------+-------------------+----------+---------+------+------+-----------------------+ 1 row in set (0.00 sec) # 删除右侧查找条件的索引 MariaDB [hellodb]> DROP INDEX age_name ON students; Query OK, 0 rows affected (0.04 sec) Records: 0 Duplicates: 0 Warnings: 0 # 再次执行，可以得出结论：要想使索引有效，左前缀一定要做精确匹配，如果为范围匹配，索引是无效的 MariaDB [hellodb]> EXPLAIN SELECT * FROM students WHERE Name LIKE ‘X%‘ AND Age > 30; +------+-------------+----------+------+---------------+------+---------+------+------+-------------+ | id | select_type | table | type | possible_keys | key | key_len | ref | rows | Extra | +------+-------------+----------+------+---------------+------+---------+------+------+-------------+ | 1 | SIMPLE | students | ALL | name_age | NULL | NULL | NULL | 25 | Using where | +------+-------------+----------+------+---------------+------+---------+------+------+-------------+ 1 row in set (0.00 sec)
4.多表连接构建索引
MariaDB [hellodb]> SHOW INDEX FROM students; +----------+------------+----------+--------------+-------------+-----------+-------------+----------+--------+------+------------+---------+---------------+ | Table | Non_unique | Key_name | Seq_in_index | Column_name | Collation | Cardinality | Sub_part | Packed | Null | Index_type | Comment | Index_comment | +----------+------------+----------+--------------+-------------+-----------+-------------+----------+--------+------+------------+---------+---------------+ | students | 0 | PRIMARY | 1 | StuID | A | 25 | NULL | NULL | | BTREE | | | | students | 1 | age_name | 1 | Age | A | 25 | NULL | NULL | | BTREE | | | | students | 1 | age_name | 2 | Name | A | 25 | NULL | NULL | | BTREE | | | | students | 1 | name_age | 1 | Name | A | 25 | NULL | NULL | | BTREE | | | | students | 1 | name_age | 2 | Age | A | 25 | NULL | NULL | | BTREE | | | +----------+------------+----------+--------------+-------------+-----------+-------------+----------+--------+------+------------+---------+---------------+ 5 rows in set (0.01 sec) # 查询学生对应的老师信息，就要使用多表连接 MariaDB [hellodb]> SELECT s.name,t.name FROM students AS s,teachers AS t WHERE s.TeacherID=t.TID; +-------------+---------------+ | name | name | +-------------+---------------+ | Yu Yutong | Song Jiang | | Shi Zhongyu | Miejue Shitai | | Ding Dian | Lin Chaoying | +-------------+---------------+ 3 rows in set (0.00 sec) # 分析可见，老师的那张表有一个主键索引，学生的TezcherID字段没有索引，所以，一共要查找100行数据 MariaDB [hellodb]> EXPLAIN SELECT s.name,t.name FROM students AS s,teachers AS t WHERE s.TeacherID=t.TID; +------+-------------+-------+------+---------------+------+---------+------+------+-------------------------------------------------+ | id | select_type | table | type | possible_keys | key | key_len | ref | rows | Extra | +------+-------------+-------+------+---------------+------+---------+------+------+-------------------------------------------------+ | 1 | SIMPLE | s | ALL | NULL | NULL | NULL | NULL | 25 | | | 1 | SIMPLE | t | ALL | PRIMARY | NULL | NULL | NULL | 4 | Using where; Using join buffer (flat, BNL join) | +------+-------------+-------+------+---------------+------+---------+------+------+-------------------------------------------------+ 2 rows in set (0.00 sec) # 创建学生TeacherID的索引 MariaDB [hellodb]> CREATE INDEX tid ON students(TeacherID); Query OK, 0 rows affected (0.05 sec) Records: 0 Duplicates: 0 Warnings: 0 # 再次使用EXPLAIN分析，可以发现这次紧紧查找了4行数据，即teacher表的4行都直接对应stuents的一行数据 MariaDB [hellodb]> EXPLAIN SELECT s.name,t.name FROM students AS s,teachers AS t WHERE s.TeacherID=t.TID; +------+-------------+-------+------+---------------+------+---------+---------------+------+-----------------------+ | id | select_type | table | type | possible_keys | key | key_len | ref | rows | Extra | +------+-------------+-------+------+---------------+------+---------+---------------+------+-----------------------+ | 1 | SIMPLE | t | ALL | PRIMARY | NULL | NULL | NULL | 4 | | | 1 | SIMPLE | s | ref | tid | tid | 5 | hellodb.t.TID | 1 | Using index condition | +------+-------------+-------+------+---------------+------+---------+---------------+------+-----------------------+ 2 rows in set (0.00 sec)
5.覆盖索引和多表联合查找构建索引
MariaDB [hellodb]> SHOW INDEX FROM students; +----------+------------+----------+--------------+-------------+-----------+-------------+----------+--------+------+------------+---------+---------------+ | Table | Non_unique | Key_name | Seq_in_index | Column_name | Collation | Cardinality | Sub_part | Packed | Null | Index_type | Comment | Index_comment | +----------+------------+----------+--------------+-------------+-----------+-------------+----------+--------+------+------------+---------+---------------+ | students | 0 | PRIMARY | 1 | StuID | A | 25 | NULL | NULL | | BTREE | | | | students | 1 | age_name | 1 | Age | A | 25 | NULL | NULL | | BTREE | | | | students | 1 | age_name | 2 | Name | A | 25 | NULL | NULL | | BTREE | | | | students | 1 | name_age | 1 | Name | A | 25 | NULL | NULL | | BTREE | | | | students | 1 | name_age | 2 | Age | A | 25 | NULL | NULL | | BTREE | | | | students | 1 | tid | 1 | TeacherID | A | 25 | NULL | NULL | YES | BTREE | | | +----------+------------+----------+--------------+-------------+-----------+-------------+----------+--------+------+------------+---------+---------------+ 6 rows in set (0.01 sec) # 最后显示Using index的为覆盖索引，查询数据直接从索引中返回 MariaDB [hellodb]> EXPLAIN SELECT Name FROM students; +------+-------------+----------+-------+---------------+----------+---------+------+------+-------------+ | id | select_type | table | type | possible_keys | key | key_len | ref | rows | Extra | +------+-------------+----------+-------+---------------+----------+---------+------+------+-------------+ | 1 | SIMPLE | students | index | NULL | age_name | 153 | NULL | 25 | Using index | +------+-------------+----------+-------+---------------+----------+---------+------+------+-------------+ 1 row in set (0.00 sec) # 用到的是主键索引 MariaDB [hellodb]> EXPLAIN SELECT StuID FROM students; +------+-------------+----------+-------+---------------+------+---------+------+------+-------------+ | id | select_type | table | type | possible_keys | key | key_len | ref | rows | Extra | +------+-------------+----------+-------+---------------+------+---------+------+------+-------------+ | 1 | SIMPLE | students | index | NULL | tid | 5 | NULL | 25 | Using index | +------+-------------+----------+-------+---------------+------+---------+------+------+-------------+ 1 row in set (0.00 sec) MariaDB [hellodb]> EXPLAIN SELECT Age FROM students; +------+-------------+----------+-------+---------------+----------+---------+------+------+-------------+ | id | select_type | table | type | possible_keys | key | key_len | ref | rows | Extra | +------+-------------+----------+-------+---------------+----------+---------+------+------+-------------+ | 1 | SIMPLE | students | index | NULL | age_name | 153 | NULL | 25 | Using index | +------+-------------+----------+-------+---------------+----------+---------+------+------+-------------+ 1 row in set (0.00 sec) # ClassID字段上没有索引，所以扫描全表 MariaDB [hellodb]> EXPLAIN SELECT ClassID FROM students; +------+-------------+----------+------+---------------+------+---------+------+------+-------+ | id | select_type | table | type | possible_keys | key | key_len | ref | rows | Extra | +------+-------------+----------+------+---------------+------+---------+------+------+-------+ | 1 | SIMPLE | students | ALL | NULL | NULL | NULL | NULL | 25 | | +------+-------------+----------+------+---------------+------+---------+------+------+-------+ 1 row in set (0.00 sec) # ======================================================================================= # 联合查找，可以看到生成了三张表，第三张表为两张表联合生成后的表；因为没有加where子句，所以， # 扫描的是整个表，但是students因为有age_name索引并且为覆盖索引所以直接从索引中返回数据， # teacher因为没有相应的索引，所以全表扫描返回数据 MariaDB [hellodb]> EXPLAIN SELECT Name FROM teachers UNION SELECT Name FROM students; +------+--------------+------------+-------+---------------+----------+---------+------+------+-------------+ | id | select_type | table | type | possible_keys | key | key_len | ref | rows | Extra | +------+--------------+------------+-------+---------------+----------+---------+------+------+-------------+ | 1 | PRIMARY | teachers | ALL | NULL | NULL | NULL | NULL | 4 | | | 2 | UNION | students | index | NULL | age_name | 153 | NULL | 25 | Using index | | NULL | UNION RESULT | <union1,2> | ALL | NULL | NULL | NULL | NULL | NULL | | +------+--------------+------------+-------+---------------+----------+---------+------+------+-------------+ 3 rows in set (0.00 sec) # 可以发现，teachers表为主键索引，students为age_name的索引，并且为覆盖索引 MariaDB [hellodb]> EXPLAIN SELECT Name FROM teachers WHERE TID < 3 UNION SELECT Name FROM students; +------+--------------+------------+-------+---------------+----------+---------+------+------+-------------+ | id | select_type | table | type | possible_keys | key | key_len | ref | rows | Extra | +------+--------------+------------+-------+---------------+----------+---------+------+------+-------------+ | 1 | PRIMARY | teachers | range | PRIMARY | PRIMARY | 2 | NULL | 2 | Using where | | 2 | UNION | students | index | NULL | age_name | 153 | NULL | 25 | Using index | | NULL | UNION RESULT | <union1,2> | ALL | NULL | NULL | NULL | NULL | NULL | | +------+--------------+------------+-------+---------------+----------+---------+------+------+-------------+ 3 rows in set (0.00 sec) MariaDB [hellodb]> SELECT Name FROM teachers WHERE TID UNION SELECT Name FROM students; +---------------+ | Name | +---------------+ | Song Jiang | | Zhang Sanfeng | | Miejue Shitai | | Lin Chaoying | | Lin Daiyu | | Lu Wushuang | | Xue Baochai | | Diao Chan | | Wen Qingqing | | Xi Ren | | Yue Lingshan | | Ren Yingying | | Xiao Qiao | | Duan Yu | | Xu Zhu | | Huang Yueying | | Shi Potian | | Shi Zhongyu | | Hua Rong | | Ma Chao | | Yuan Chengzhi | | Lin Chong | | Yu Yutong | | Xu Xian | | Ding Dian | | Tian Boguang | | Shi Qing | | Xie Yanke | | Sun Dasheng | +---------------+ 29 rows in set (0.01 sec)
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