MySQL索引

Posted 2021-02-07

mysql基础

单进程多线程：
    用户连接：连接线程   

    MySQL数据文件类型：
        数据文件、索引文件
        重做日志、撤消日志、二进制日志、错误日志、查询日志、慢查询日志、(中继日志)

DDL & DML:
    索引管理：
            按特定数据结构存储的数据；

        索引类型：
            聚集索引、非聚集索引：数据是否与索引存储在一起；
            主键索引、辅助索引
            稠密索引、稀疏索引：是否索引了每一个数据项；
            B+ TREE、HASH、R TREE
            简单索引、组合索引

            左前缀索引
            覆盖索引

        管理索引的途径：
            创建索引：创建表时指定；CREATE INDEX
            创建或删除索引：修改表的命令
            删除索引：DROP INDEX

            查看表上的索引：
                SHOW {INDEX | INDEXES | KEYS}
                    {FROM | IN} tbl_name
                    [{FROM | IN} db_name]
                    [WHERE expr]

        索引的创建可以使用ALTER、CREATE命令创建：
        mysql> CREATE INDEX name ON student (name);
        Query OK, 0 rows affected (0.12 sec)
        Records: 0  Duplicates: 0  Warnings: 0

        mysql> SHOW INDEXES FROM student;
        +---------+------------+----------+--------------+-------------+-----------+-------------+----------+--------+------+------------+---------+---------------+
        | Table   | Non_unique | Key_name | Seq_in_index | Column_name | Collation | Cardinality | Sub_part | Packed | Null | Index_type | Comment | Index_comment |
        +---------+------------+----------+--------------+-------------+-----------+-------------+----------+--------+------+------------+---------+---------------+
        | student |          0 | PRIMARY  |            1 | id          | A         |          10 |     NULL | NULL   |      | BTREE      |         |               |
        | student |          1 | name     |            1 | name        | A         |          10 |     NULL | NULL   |      | BTREE      |         |               |
        +---------+------------+----------+--------------+-------------+-----------+-------------+----------+--------+------+------------+---------+---------------+
        2 rows in set (0.00 sec)

        mysql>

            EXPLAIN   #确认是否有使用到索引进行查询。
            mysql> EXPLAIN SELECT * FROM students WHERE id=3;
            mysql> select * from student;
            +----+----------+---------------+------+
            | id | name     | register_date | sex  |
            +----+----------+---------------+------+
            |  1 | ZhanYang | 2018-06-20    | NULL |
            |  4 | LiuJia   | 2018-05-30    | NULL |
            |  8 | JiaLiu   | 2018-06-20    | NULL |
            |  9 | JiaLiu   | 2018-04-20    | NULL |
            | 10 | gaoyf    | 2018-04-20    | NULL |
            | 11 | zhujh    | 2018-04-20    | NULL |
            | 12 | zhouha   | 2018-04-20    | NULL |
            | 13 | hanzb    | 2018-04-21    | M    |
            | 16 | ZhanXing | 2018-05-21    | M    |
            | 17 | XingYan  | 2018-05-21    | M    |
            +----+----------+---------------+------+
            10 rows in set (0.01 sec)

            mysql> EXPLAIN SELECT * FROM student  WHERE id=4;
            +----+-------------+---------+-------+---------------+---------+---------+-------+------+-------+
            | id | select_type | table   | type  | possible_keys | key     | key_len | ref   | rows | Extra |
            +----+-------------+---------+-------+---------------+---------+---------+-------+------+-------+
            |  1 | SIMPLE      | student | const | PRIMARY       | PRIMARY | 4       | const |    1 | NULL  |
            +----+-------------+---------+-------+---------------+---------+---------+-------+------+-------+
            1 row in set (0.01 sec)

            mysql> EXPLAIN SELECT * FROM student  WHERE id=4G
            *************************** 1. row ***************************
                       id: 1
              select_type: SIMPLE
                    table: student
                     type: const
            possible_keys: PRIMARY
                      key: PRIMARY
                  key_len: 4
                      ref: const
                     rows: 1
                    Extra: NULL
            1 row in set (0.00 sec)

        mysql> EXPLAIN SELECT * FROM student WHERE name LIKE ‘%X%‘;
        +----+-------------+---------+------+---------------+------+---------+------+------+-------------+
        | id | select_type | table   | type | possible_keys | key  | key_len | ref  | rows | Extra       |
        +----+-------------+---------+------+---------------+------+---------+------+------+-------------+
        |  1 | SIMPLE      | student | ALL  | NULL          | NULL | NULL    | NULL |   10 | Using where |
        +----+-------------+---------+------+---------------+------+---------+------+------+-------------+
        1 row in set (0.00 sec)

        mysql> EXPLAIN SELECT * FROM student WHERE name LIKE ‘%X%‘G
        *************************** 1. row ***************************
                   id: 1
          select_type: SIMPLE
                table: student
                 type: ALL
        possible_keys: NULL
                  key: NULL
              key_len: NULL
                  ref: NULL
                 rows: 10
                Extra: Using where
        1 row in set (0.01 sec)

    DESC命令用于获取数据表结构