MySQL— 索引

--创建普通索引
CREATE INDEX index_name ON table_name(col_name);

--创建唯一索引
CREATE UNIQUE INDEX index_name ON table_name(col_name);

--创建普通组合索引
CREATE INDEX index_name ON table_name(col_name_1,col_name_2);

--创建唯一组合索引
CREATE UNIQUE INDEX index_name ON table_name(col_name_1,col_name_2);

ALTER TABLE table_name ADD INDEX index_name(col_name);

CREATE TABLE table_name (
    ID INT NOT NULL,
    col_name VARCHAR (16) NOT NULL,
    INDEX index_name (col_name)
);

--直接删除索引
DROP INDEX index_name ON table_name;

--修改表结构删除索引
ALTER TABLE table_name DROP INDEX index_name;

- 查看表结构
    desc table_name;
 
- 查看生成表的SQL
    show create table table_name;
 
- 查看索引
    show index from  table_name;
 
- 查看执行时间
    set profiling = 1;
    SQL...
    show profiles;

SELECT t.Name  FROM mytable_t LEFT JOIN mytable_m ON t.Name=m.username WHERE m.age=20 AND m.city=‘郑州‘ ;

- like ‘%xx‘
    select * from tb1 where name like ‘%cn‘;
- 使用函数
    select * from tb1 where reverse(name) = ‘wupeiqi‘;
- or
    select * from tb1 where nid = 1 or email = ‘seven@live.com‘;
    特别的：当or条件中有未建立索引的列才失效，以下会走索引
            select * from tb1 where nid = 1 or name = ‘seven‘;
            select * from tb1 where nid = 1 or email = ‘seven@live.com‘ and name = ‘alex‘
- 类型不一致
    如果列是字符串类型，传入条件是必须用引号引起来，不然...
    select * from tb1 where name = 999;
- !=
    select * from tb1 where name != ‘alex‘
    特别的：如果是主键，则还是会走索引
        select * from tb1 where nid != 123
- >
    select * from tb1 where name > ‘alex‘
    特别的：如果是主键或索引是整数类型，则还是会走索引
        select * from tb1 where nid > 123
        select * from tb1 where num > 123
- order by
    select email from tb1 order by name desc;
    当根据索引排序时候，选择的映射如果不是索引，则不走索引
    特别的：如果对主键排序，则还是走索引：
        select * from tb1 order by nid desc;
 
- 组合索引最左前缀
    如果组合索引为：(name,email)
    name and email       -- 使用索引
    name                 -- 使用索引
    email                -- 不使用索引

- 避免使用select *
- count(1)或count(列) 代替 count(*)
- 创建表时尽量时 char 代替 varchar
- 表的字段顺序固定长度的字段优先
- 组合索引代替多个单列索引（经常使用多个条件查询时）
- 尽量使用短索引
- 使用连接（JOIN）来代替子查询(Sub-Queries)
- 连表时注意条件类型需一致
- 索引散列值（重复多）不适合建索引，例：性别不适合

--第一页
SELECT * FROM table_name LIMIT 0,10;

--第二页
SELECT * FROM table_name LIMIT 10,10;

--第三页
SELECT * FROM table_name LIMIT 20,10;

--下一页
SELECT * FROM table_name WHERE id in (SELECT id FROM table_name WHERE id > max_id LIMIT 10);

--上一页
SELECT * FROM table_name WHERE id in (SELECT id FROM table_name WHERE id < min_id ORDER BY id DESC LIMIT 10);

--当前页之后的某一页
SELECT * FROM table_name WHERE id in (SELECT id FROM (SELECT id FROM (SELECT id FROM table_name WHERE id < min_id ORDER BY id desc LIMIT (页数差*10)) AS N ORDER BY N.id ASC LIMIT 10) AS P ORDER BY P.id ASC);

--当前页之前的某一页
SELECT * FROM table_name WHERE id in (SELECT id FROM (SELECT id FROM (SELECT id FROM table_name WHERE id > max_id LIMIT (页数差*10)) AS N ORDER BY N.id DESC LIMIT 10) AS P ORDER BY P.id ASC);

mysql> explain select * from tb2;
+----+-------------+-------+------+---------------+------+---------+------+------+-------+
| id | select_type | table | type | possible_keys | key  | key_len | ref  | rows | Extra |
+----+-------------+-------+------+---------------+------+---------+------+------+-------+
|  1 | SIMPLE      | tb2   | ALL  | NULL          | NULL | NULL    | NULL |    2 | NULL  |
+----+-------------+-------+------+---------------+------+---------+------+------+-------+
1 row in set (0.00 sec)

--查询配置命令
show variables like ‘%query%‘;

--当前配置参数
binlog_rows_query_log_events    OFF
ft_query_expansion_limit    20
have_query_cache    YES

--时间限制，超过此时间，则记录
long_query_time    10.000000

query_alloc_block_size    8192
query_cache_limit    1048576
query_cache_min_res_unit    4096
query_cache_size    1048576
query_cache_type    OFF
query_cache_wlock_invalidate    OFF
query_prealloc_size    8192

--是否开启慢日志记录
slow_query_log    OFF

--日志文件
slow_query_log_file    D:\Program Files (x86)\mysql-5.7.18-winx64\data\Jack-slow.log
--

set global 变量名 = 值;

--例如，修改时间限制为20s
long_query_time = 20;

mysqldumpslow -s at -a  /usr/local/var/mysql/MacBook-Pro-3-slow.log

"""
--verbose    版本
--debug      调试
--help       帮助
 
-v           版本
-d           调试模式
-s ORDER     排序方式
             what to sort by (al, at, ar, c, l, r, t), ‘at‘ is default
              al: average lock time
              ar: average rows sent
              at: average query time
               c: count
               l: lock time
               r: rows sent
               t: query time
-r           反转顺序，默认文件倒序拍。reverse the sort order (largest last instead of first)
-t NUM       显示前N条just show the top n queries
-a           不要将SQL中数字转换成N，字符串转换成S。don‘t abstract all numbers to N and strings to ‘S‘
-n NUM       abstract numbers with at least n digits within names
-g PATTERN   正则匹配；grep: only consider stmts that include this string
-h HOSTNAME  mysql机器名或者IP；hostname of db server for *-slow.log filename (can be wildcard),
             default is ‘*‘, i.e. match all
-i NAME      name of server instance (if using mysql.server startup script)
-l           总时间中不减去锁定时间；don‘t subtract lock time from total time
"""