Mysql索引
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参考：《MySQL高性能》 第五章
《MySQL技术内幕：innodb存储引擎》 第五章
B+tree树 索引
聚簇索引(一级索引)：主键(无主键时指定一个唯一非空索引代替，无唯一非空索引时 隐式的指定主键rowid代替)，叶子节点中存储行的所有数据
辅助索引(二级索引)：普通索引，叶子节点只存储索引列对应的数据和主键列(用于去聚簇索引查找数据)
一、索引是顺序组织存储的，可避免排序， 将随机I/O变为顺序I/O
二、组合索引的排序
三：测试如下：
--辅助索引 组合索引的使用规则，
--1、建立测试数据
mysql> create table test_idx_comb_table(id int ,name varchar(10),score int ,class int);
Query OK, 0 rows affected (0.86 sec)
mysql> alter table test_idx_comb_table add index idx_test_idx_comb_table(id,name,score);
Query OK, 0 rows affected (0.13 sec)
Records: 0 Duplicates: 0 Warnings: 0
mysql> insert into test_idx_comb_table values(1,'zhao',98,11),(2,'qian',99,11),(3,'sun',78,11),(4,'li',80,11);
Query OK, 4 rows affected (0.12 sec)
Records: 4 Duplicates: 0 Warnings: 0
mysql> commit;
Query OK, 0 rows affected (0.00 sec)
--2、查看测试数据
mysql> select * from test_idx_comb_table;
+------+------+-------+-------+
| id | name | score | class |
+------+------+-------+-------+
| 1 | zhao | 98 | 11 |
| 2 | qian | 99 | 11 |
| 3 | sun | 78 | 11 |
| 4 | li | 80 | 11 |
+------+------+-------+-------+
4 rows in set (0.00 sec)
--3、查看执行计划
--3.1、全值匹配
mysql> explain select id,name,score,class from test_idx_comb_table where id=3 and name='sun' and score='78';
+----+-------------+---------------------+------------+------+-------------------------+-------------------------+---------+-------------------+------+----------+-------+
| id | select_type | table | partitions | type | possible_keys | key | key_len | ref | rows | filtered | Extra |
+----+-------------+---------------------+------------+------+-------------------------+-------------------------+---------+-------------------+------+----------+-------+
| 1 | SIMPLE | test_idx_comb_table | NULL | ref | idx_test_idx_comb_table | idx_test_idx_comb_table | 43 | const,const,const | 1 | 100.00 | NULL |
+----+-------------+---------------------+------------+------+-------------------------+-------------------------+---------+-------------------+------+----------+-------+
1 row in set, 1 warning (0.00 sec)
--3.2、索引覆盖查询
--Using index 索引覆盖扫描，server层完成，无需回表
mysql> explain select id,name,score from test_idx_comb_table where id=3 and name='sun' and score='78';
+----+-------------+---------------------+------------+------+-------------------------+-------------------------+---------+-------------------+------+----------+-------------+
| id | select_type | table | partitions | type | possible_keys | key | key_len | ref | rows | filtered | Extra |
+----+-------------+---------------------+------------+------+-------------------------+-------------------------+---------+-------------------+------+----------+-------------+
| 1 | SIMPLE | test_idx_comb_table | NULL | ref | idx_test_idx_comb_table | idx_test_idx_comb_table | 43 | const,const,const | 1 | 100.00 | Using index |
+----+-------------+---------------------+------------+------+-------------------------+-------------------------+---------+-------------------+------+----------+-------------+
1 row in set, 1 warning (0.00 sec)
--3.3、匹配索引最左前缀
mysql> explain select id,name,score,class from test_idx_comb_table where id=3 ;
+----+-------------+---------------------+------------+------+-------------------------+-------------------------+---------+-------+------+----------+-------+
| id | select_type | table | partitions | type | possible_keys | key | key_len | ref | rows | filtered | Extra |
+----+-------------+---------------------+------------+------+-------------------------+-------------------------+---------+-------+------+----------+-------+
| 1 | SIMPLE | test_idx_comb_table | NULL | ref | idx_test_idx_comb_table | idx_test_idx_comb_table | 5 | const | 1 | 100.00 | NULL |
+----+-------------+---------------------+------------+------+-------------------------+-------------------------+---------+-------+------+----------+-------+
1 row in set, 1 warning (0.00 sec)
--3.4、匹配索引范围值
mysql> explain select id,name,score,class from test_idx_comb_table where id between 2 and 3 ;
+----+-------------+---------------------+------------+-------+-------------------------+-------------------------+---------+------+------+----------+-----------------------+
| id | select_type | table | partitions | type | possible_keys | key | key_len | ref | rows | filtered | Extra |
+----+-------------+---------------------+------------+-------+-------------------------+-------------------------+---------+------+------+----------+-----------------------+
| 1 | SIMPLE | test_idx_comb_table | NULL | range | idx_test_idx_comb_table | idx_test_idx_comb_table | 5 | NULL | 2 | 100.00 | Using index condition |
+----+-------------+---------------------+------------+-------+-------------------------+-------------------------+---------+------+------+----------+-----------------------+
1 row in set, 1 warning (0.00 sec)
--3.5、匹配索引一列并范围匹配另一列
--Using index condition (Index Condition Pushdown，ICP) 按照索引条件过滤数据行的同时 where条件中的其他条件去过滤这些数据行
--开启时(5.6特性) 在查询id=3的数据行的同时 会根据 name like 's%'条件过滤，然后检索的结果指向聚集索引， 最后返回给用户， 减少了访问表的次数
--未开启时(5.5及之前版本) 先查询id=3的记录 ，然后检索的结果指向聚集索引，再根据name like 's%'条件过滤， 最后返回给用户
mysql> explain select id,name,score,class from test_idx_comb_table where id=3 and name like 's%';
+----+-------------+---------------------+------------+-------+-------------------------+-------------------------+---------+------+------+----------+-----------------------+
| id | select_type | table | partitions | type | possible_keys | key | key_len | ref | rows | filtered | Extra |
+----+-------------+---------------------+------------+-------+-------------------------+-------------------------+---------+------+------+----------+-----------------------+
| 1 | SIMPLE | test_idx_comb_table | NULL | range | idx_test_idx_comb_table | idx_test_idx_comb_table | 38 | NULL | 1 | 100.00 | Using index condition |
+----+-------------+---------------------+------------+-------+-------------------------+-------------------------+---------+------+------+----------+-----------------------+
1 row in set, 1 warning (0.00 sec)
--3.6、MRR特性，开启MRR优化特性(mrr_cost_based默认为on 是成本算法自行判断是否采用MRR优化， 这里设置为off 后 强制采用MRR优化)
--read_rnd_buffer
mysql> set optimizer_switch='mrr=on,mrr_cost_based=off';
Query OK, 0 rows affected (0.00 sec)
--MRR特性：通过辅助索引找到的主键 缓存起来 然后根据 主键排序后 找到记录， 这样就可以把 辅助索引找主键的随机I/O 变为根据主键排序的 顺序I/O了
--在read_rnd_buffer 中缓存起来
mysql> explain select id,name,score,class from test_idx_comb_table where id=3 and name like 's%';
+----+-------------+---------------------+-------+-------------------------+-------------------------+---------+------+------+----------------------------------+
| id | select_type | table | type | possible_keys | key | key_len | ref | rows | Extra |
+----+-------------+---------------------+-------+-------------------------+-------------------------+---------+------+------+----------------------------------+
| 1 | SIMPLE | test_idx_comb_table | range | idx_test_idx_comb_table | idx_test_idx_comb_table | 38 | NULL | 1 | Using index condition; Using MRR |
+----+-------------+---------------------+-------+-------------------------+-------------------------+---------+------+------+----------------------------------+
1 row in set (0.00 sec)
--3.7、无法使用索引查询不是最左前缀的列，如name,score字段匹配
--Using where 从数据表返回数据，再过滤掉不需要的数据
mysql> explain select id,name,score,class from test_idx_comb_table where name='sun' ;
+----+-------------+---------------------+------------+------+---------------+------+---------+------+------+----------+-------------+
| id | select_type | table | partitions | type | possible_keys | key | key_len | ref | rows | filtered | Extra |
+----+-------------+---------------------+------------+------+---------------+------+---------+------+------+----------+-------------+
| 1 | SIMPLE | test_idx_comb_table | NULL | ALL | NULL | NULL | NULL | NULL | 4 | 25.00 | Using where |
+----+-------------+---------------------+------------+------+---------------+------+---------+------+------+----------+-------------+
1 row in set, 1 warning (0.00 sec)
mysql> explain select id,name,score,class from test_idx_comb_table where score='78';
+----+-------------+---------------------+------------+------+---------------+------+---------+------+------+----------+-------------+
| id | select_type | table | partitions | type | possible_keys | key | key_len | ref | rows | filtered | Extra |
+----+-------------+---------------------+------------+------+---------------+------+---------+------+------+----------+-------------+
| 1 | SIMPLE | test_idx_comb_table | NULL | ALL | NULL | NULL | NULL | NULL | 4 | 25.00 | Using where |
+----+-------------+---------------------+------------+------+---------------+------+---------+------+------+----------+-------------+
1 row in set, 1 warning (0.00 sec)
--4、查看索引状态：
测试已test_stat表为例，测试如下：
--1、查看表数据
mysql> select * from test_stat;
+---+---+------+------+------+------+
| a | b | c | d | e | f |
+---+---+------+------+------+------+
| 1 | 1 | 10 | 11 | 100 | 101 |
| 1 | 2 | 10 | 11 | 200 | 102 |
| 1 | 3 | 10 | 11 | 100 | 103 |
| 1 | 4 | 10 | 12 | 200 | 104 |
| 1 | 5 | 10 | 12 | 100 | 105 |
+---+---+------+------+------+------+
5 rows in set (0.07 sec)
--2、查看表信息
mysql> show create table test_stat\\G
*************************** 1. row ***************************
Table: test_stat
Create Table: CREATE TABLE `test_stat` (
`a` int(11) NOT NULL,
`b` int(11) NOT NULL,
`c` int(11) DEFAULT NULL,
`d` int(11) DEFAULT NULL,
`e` int(11) DEFAULT NULL,
`f` int(11) DEFAULT NULL,
PRIMARY KEY (`a`,`b`),
UNIQUE KEY `i2uniq` (`e`,`f`),
KEY `i1` (`c`,`d`)
) ENGINE=InnoDB DEFAULT CHARSET=latin1
1 row in set (0.00 sec)
--3、查看索引信息
mysql> show index from test_stat\\G
*************************** 1. row ***************************
Table: test_stat --表名
Non_unique: 0 --非唯一索引是1，唯一索引是0
Key_name: PRIMARY --索引名称
Seq_in_index: 1 --索引中的位置
Column_name: a --索引列的名称
Collation: A --存储方式，B+ 树是A
Cardinality: 1 --唯一值的估算，越接近1 重复值越多，如果不准 ANALYZE TABLE 可以收集表统计信息 但最好业务低估收集
Sub_part: NULL --是否列的部分为索引，整列为索引时 显示null， 部分为索引时显示 具体数值 比如 key idx_a (a(2)) 就是只对前两个字段进行索引操作
Packed: NULL --如何被压缩，没有压缩为null
Null: --索引列是否允许为null，可以为空表示为YES
Index_type: BTREE --索引类型，B+ 树是BTREE
Comment: --注释
Index_comment:
*************************** 2. row ***************************
Table: test_stat
Non_unique: 0
Key_name: PRIMARY
Seq_in_index: 2
Column_name: b
Collation: A
Cardinality: 5
Sub_part: NULL
Packed: NULL
Null:
Index_type: BTREE
Comment:
Index_comment:
*************************** 3. row ***************************
Table: test_stat
Non_unique: 0
Key_name: i2uniq
Seq_in_index: 1
Column_name: e
Collation: A
Cardinality: 2
Sub_part: NULL
Packed: NULL
Null: YES
Index_type: BTREE
Comment:
Index_comment:
*************************** 4. row ***************************
Table: test_stat
Non_unique: 0
Key_name: i2uniq
Seq_in_index: 2
Column_name: f
Collation: A
Cardinality: 5
Sub_part: NULL
Packed: NULL
Null: YES
Index_type: BTREE
Comment:
Index_comment:
*************************** 5. row ***************************
Table: test_stat
Non_unique: 1
Key_name: i1
Seq_in_index: 1
Column_name: c
Collation: A
Cardinality: 1
Sub_part: NULL
Packed: NULL
Null: YES
Index_type: BTREE
Comment:
Index_comment:
*************************** 6. row ***************************
Table: test_stat
Non_unique: 1
Key_name: i1
Seq_in_index: 2
Column_name: d
Collation: A
Cardinality: 2
Sub_part: NULL
Packed: NULL
Null: YES
Index_type: BTREE
Comment:
Index_comment:
6 rows in set (0.00 sec)
--五、在线ddl
--在线ddl时，会将insert、update、delete等dml操作放到缓存中，待ddl完成后 再重做应用到表上 到达数据一致性
--缓存大小为innodb_online_alter_log_max_size，默认128M， 但缓存的dml操作空间不够时会报错 建议1G
mysql> show variables like 'innodb_online_alter_log_max_size';
+----------------------------------+-----------+
| Variable_name | Value |
+----------------------------------+-----------+
| innodb_online_alter_log_max_size | 134217728 |
+----------------------------------+-----------+
1 row in set (0.00 sec)
mysql> select 134217728/1024/1024 M;
+--------------+
| M |
+--------------+
| 128.00000000 |
+--------------+
1 row in set (0.00 sec)
--六、参数优化
--optimizer_search_depth //搜索深度
--默认 62， 越小越好
--如果连接7个表 可将值设置为0 、或者让优化器 自动选择表的数量的值7
--七、强制和忽略索引
--1、测试数据
mysql> CREATE TABLE test_hint_index_table (
-> a int(11) ,
-> b int(11) ,
-> KEY (a),
-> KEY (b)
-> );
mysql> insert into test_hint_index_table values(1,1),(1,2),(2,3),(2,4),(1,2);
Query OK, 5 rows affected (0.04 sec)
Records: 5 Duplicates: 0 Warnings: 0
mysql> commit;
Query OK, 0 rows affected (0.00 sec)
--2、初始执行计划，走的b，a的索引
mysql> explain select * from test_hint_index_table where a=1 and b=2\\G
*************************** 1. row ***************************
id: 1
select_type: SIMPLE
table: test_hint_index_table
partitions: NULL
type: index_merge
possible_keys: a,b
key: b,a
key_len: 5,5
ref: NULL
rows: 1
filtered: 100.00
Extra: Using intersect(b,a); Using where; Using index
1 row in set, 1 warning (0.00 sec)
--3、使用use index(a)让其走a的索引，但实际没有走key为null
mysql> explain select * from test_hint_index_table use index(a) where a=1 and b=2\\G
*************************** 1. row ***************************
id: 1
select_type: SIMPLE
table: test_hint_index_table
partitions: NULL
type: ALL
possible_keys: a
key: NULL
key_len: NULL
ref: NULL
rows: 5
filtered: 20.00
Extra: Using where
1 row in set, 1 warning (0.02 sec)
--4、使用force参数强制走a索引，可以走到a的索引上
mysql> explain select * from test_hint_index_table force index(a) where a=1 and b=2\\G
*************************** 1. row ***************************
id: 1
select_type: SIMPLE
table: test_hint_index_table
partitions: NULL
type: ref
possible_keys: a
key: a
key_len: 5
ref: const
rows: 3
filtered: 20.00
Extra: Using where
1 row in set, 1 warning (0.00 sec)
--5、可以使用ignore参数，来忽略索引的使用
mysql> explain select * from test_hint_index_table ignore index(a,b) where a=1 and b=2\\G
*************************** 1. row ***************************
id: 1
select_type: SIMPLE
table: test_hint_index_table
partitions: NULL
type: ALL
possible_keys: NULL
key: NULL
key_len: NULL
ref: NULL
rows: 5
filtered: 20.00
Extra: Using where
1 row in set, 1 warning (0.00 sec)
--八、不走索引的情况
--1、统计信息不准确，需要表分析
--设置持久化保存，统计信息的表， 查看last_update 最后更新的情况
mysql.innodb_table_stats 和 mysql.innodb_index_stats
--表分析
--对整张表
analyze table test_create_table;
--对分区表的分区
alter table t_exchange analyze partition p2;
--2、隐式转换
--建立测试表，b字段为char类型、b字段建立索引
mysql> create table test_conver_table (a int ,b varchar(10));
mysql> create index idx_test_conver_table on test_conver_table(b);
--插入数据
DROP PROCEDURE IF EXISTS p_test_conver_table;
DELIMITER $
CREATE PROCEDURE p_test_conver_table()
BEGIN
DECLARE i INT DEFAULT 1;
WHILE i<=10000 DO
INSERT INTO test_conver_table(a,b) VALUES(i,i);
SET i = i+1;
END WHILE;
COMMIT;
END $innodb——
CALL p_test_conver_table();
--执行语句，b带入的是整型、不是字符型，发生隐式转换，不走索引，走的全表
mysql> explain extended select * from test_conver_table where b=10;
+----+-------------+-------------------+------------+------+-----------------------+------+---------+------+---------+----------+-------------+
| id | select_type | table | partitions | type | possible_keys | key | key_len | ref | rows | filtered | Extra |
+----+-------------+-------------------+------------+------+-----------------------+------+---------+------+---------+----------+-------------+
| 1 | SIMPLE | test_conver_table | NULL | ALL | idx_test_conver_table | NULL | NULL | NULL | 9760364 | 10.00 | Using where |
+----+-------------+-------------------+------------+------+-----------------------+------+---------+------+---------+----------+-------------+
1 row in set, 4 warnings (0.04 sec)
--查看警告，无法匹配
mysql> show warnings;
+---------+------+-----------------------------------------------------------------------------------------------------------------------------------------------------------------------------------+
| Level | Code | Message |
+---------+------+-----------------------------------------------------------------------------------------------------------------------------------------------------------------------------------+
| Warning | 1681 | 'EXTENDED' is deprecated and will be removed in a future release. |
| Warning | 1739 | Cannot use ref access on index 'idx_test_conver_table' due to type or collation conversion on field 'b' |
| Warning | 1739 | Cannot use range access on index 'idx_test_conver_table' due to type or collation conversion on field 'b' |
| Note | 1003 | /* select#1 */ select `flydb`.`test_conver_table`.`a` AS `a`,`flydb`.`test_conver_table`.`b` AS `b` from `flydb`.`test_conver_table` where (`flydb`.`test_conver_table`.`b` = 10) |
+---------+------+-----------------------------------------------------------------------------------------------------------------------------------------------------------------------------------+
4 rows in set (0.00 sec)
--带入b='10' 字符串类型， 或者修改表b字段为整型即可
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