MySQL中怎样快速找出超长索引

墨蓝 2022-10-27 12:11 66阅读 0赞

大家好，我是知数堂SQL 优化班老师 网名：骑龟的兔子

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需求：

想要查找哪些索引太长了，这个SQL在5.7下跑的特别慢，8.0则挺快的，帮看下有啥优化方案没

具体SQL 和执行计划如下 ：

SELECT c.TABLE_SCHEMA AS DB, 
     c .TABLE_NAME AS TBL,
     c.COLUMN_NAME AS COL,
     c.CHARACTER_OCTET_LENGTH AS COL_LEN_BYTES, 
     s.INDEX_NAME,
      s.SUB_PART * CHARACTER_OCTET_LENGTH/CHARACTER_MAXIMUM_LENGTH AS SUB_PART_LEN
    FROM information_schema.COLUMNS c 
    INNER JOIN information_schema.STATISTICS s USING(TABLE_SCHEMA, TABLE_NAME, COLUMN_NAME)
    INNER JOIN information_schema.TABLES t USING(TABLE_SCHEMA, TABLE_NAME)  
    WHERE c.TABLE_SCHEMA not in ('information_schema','performance_schema','mysql','sys', 'test')
    AND c.DATA_TYPE IN ("varchar", "char", "text", "blob") 
    AND ((CHARACTER_OCTET_LENGTH > 50 and SUB_PART is null) or
    SUB_PART * CHARACTER_OCTET_LENGTH/CHARACTER_MAXIMUM_LENGTH > 50)
    AND t.TABLE_ROWS > 10000
    ORDER BY COL_LEN_BYTES DESC;
    
    
    执行计划
    
    
    *************************** 1. row ***************************
               id: 1
      select_type: SIMPLE
            table: c
       partitions: NULL
             type: ALL
    possible_keys: NULL
              key: NULL
          key_len: NULL
              ref: NULL
             rows: NULL
         filtered: NULL
            Extra: Using where; Open_frm_only; Scanned all databases; Using temporary; Using filesort
    *************************** 2. row ***************************
               id: 1
      select_type: SIMPLE
            table: s
       partitions: NULL
             type: ALL
    possible_keys: NULL
              key: NULL
          key_len: NULL
              ref: NULL
             rows: NULL
         filtered: NULL
            Extra: Using where; Open_frm_only; Scanned all databases; Using join buffer (Block Nested Loop)
    *************************** 3. row ***************************
               id: 1
      select_type: SIMPLE
            table: t
       partitions: NULL
             type: ALL
    possible_keys: NULL
              key: NULL
          key_len: NULL
              ref: NULL
             rows: NULL
         filtered: NULL
            Extra: Using where; Open_full_table; Scanned all databases; Using join buffer (Block Nested Loop)
    3 rows in set, 1 warning (0.01 sec)
    
    
    
    
    select count(*) from information_schema.tables;
    +----------+
    | count(*) |
    +----------+
    |    33600 |
    +----------+
    
    
    select count(*) from information_schema.COLUMNS;
    +----------+
    | count(*) |
    +----------+
    |   342967 |
    +----------+
    select count(*) from information_schema.STATISTICS;
    +----------+
    | count(*) |
    +----------+
    |   135167 |
    +----------+

上面的SQL 运行450+ s 也运行不出来，最后kill掉了。

我们初步分析一下，从执行计划中 可以看出三个表都是ALL 所以很慢

那添加索引不就行了吗，因为是系统表，所以不能随便添加！

那该怎么办？想到了AUTOKEY 就是临时索引，那思路就是改写SQL

达到生成临时索引，最终达到优化效果

改写的SQL 如下

SELECT c.TABLE_SCHEMA AS DB, c.TABLE_NAME AS TBL, c.COLUMN_NAME AS COL, c.CHARACTER_OCTET_LENGTH AS COL_LEN_BYTES, s.INDEX_NAME,
    s.SUB_PART * c.CHARACTER_OCTET_LENGTH/c.CHARACTER_MAXIMUM_LENGTH AS SUB_PART_LEN
    FROM 
    ( select c.TABLE_SCHEMA, c.TABLE_NAME, c.COLUMN_NAME ,c.CHARACTER_OCTET_LENGTH ,c.CHARACTER_MAXIMUM_LENGTH , c.DATA_TYPE
    from 
    information_schema.COLUMNS c 
    WHERE c.TABLE_SCHEMA not in ('information_schema','performance_schema','mysql','sys', 'test') 
    AND c.DATA_TYPE IN ("varchar", "char", "text", "blob") 
    limit 10000000000
    ) c 
    INNER JOIN 
    (
    select s.TABLE_SCHEMA, s.TABLE_NAME, s.COLUMN_NAME ,s.SUB_PART,s.INDEX_NAME
    from 
    information_schema.STATISTICS s 
    WHERE s.TABLE_SCHEMA not in ('information_schema','performance_schema','mysql','sys', 'test')
    limit 10000000000 
    
    
    )s USING(TABLE_SCHEMA, TABLE_NAME, COLUMN_NAME)
    INNER JOIN information_schema.TABLES t USING(TABLE_SCHEMA, TABLE_NAME) 
    WHERE c.TABLE_SCHEMA not in ('information_schema','performance_schema','mysql','sys', 'test') 
    AND c.DATA_TYPE IN ("varchar", "char", "text", "blob") 
    AND ((c.CHARACTER_OCTET_LENGTH > 50 and s.SUB_PART is null) or
    s.SUB_PART * c.CHARACTER_OCTET_LENGTH/c.CHARACTER_MAXIMUM_LENGTH > 50)
    AND t.TABLE_ROWS > 10000
    ORDER BY COL_LEN_BYTES DESC;
    
    
    
    
    *************************** 1. row ***************************
               id: 1
      select_type: PRIMARY
            table: t
       partitions: NULL
             type: ALL
    possible_keys: NULL
              key: NULL
          key_len: NULL
              ref: NULL
             rows: NULL
         filtered: NULL
            Extra: Using where; Open_full_table; Scanned all databases; Using temporary; Using filesort
    *************************** 2. row ***************************
               id: 1
      select_type: PRIMARY
            table: <derived2>
       partitions: NULL
             type: ref
    possible_keys: <auto_key0>
              key: <auto_key0>
          key_len: 388
              ref: information_schema.t.TABLE_SCHEMA,information_schema.t.TABLE_NAME
             rows: 2
         filtered: 50.00
            Extra: Using where
    *************************** 3. row ***************************
               id: 1
      select_type: PRIMARY
            table: <derived3>
       partitions: NULL
             type: ref
    possible_keys: <auto_key0>
              key: <auto_key0>
          key_len: 582
              ref: information_schema.t.TABLE_SCHEMA,information_schema.t.TABLE_NAME,c.COLUMN_NAME
             rows: 2
         filtered: 100.00
            Extra: Using where
    *************************** 4. row ***************************
               id: 3
      select_type: DERIVED
            table: s
       partitions: NULL
             type: ALL
    possible_keys: NULL
              key: NULL
          key_len: NULL
              ref: NULL
             rows: NULL
         filtered: NULL
            Extra: Using where; Open_frm_only; Scanned all databases
    *************************** 5. row ***************************
               id: 2
      select_type: DERIVED
            table: c
       partitions: NULL
             type: ALL
    possible_keys: NULL
              key: NULL
          key_len: NULL
              ref: NULL
             rows: NULL
         filtered: NULL
            Extra: Using where; Open_frm_only; Scanned all databases
    5 rows in set, 1 warning (0.01 sec)

结果来了 2463 rows in set, 417 warnings (23.39 sec)

但是经过几次运行之后 有时候是40多秒有时候甚至达到了166s 非常不稳定！

那分析下上面这个SQL的问题在哪里？

问题就是生成的AUTO KEY的量相对来说非常大！因为没有进行任何过滤

那现在的思路就是 对生成的AUTOKEY的量 进行减少

我们通过相对小的表TABLES 表生成autokey 之后 STATISTICS ,COLUMNS

表分别跟 TABLES 表进行JOIN 然后减少数据量 达到减少生成AUOKEY 的量 减少 达到优化目的 ，具体的方法如下

select count(1) 
    from 
    (
    select s.TABLE_SCHEMA, s.TABLE_NAME, s.COLUMN_NAME ,s.SUB_PART,s.INDEX_NAME
    from 
    information_schema.STATISTICS s 
    WHERE s.TABLE_SCHEMA not in ('information_schema','performance_schema','mysql','sys', 'test') 
    )s straight_join 
    (
    select t.TABLE_SCHEMA, t.TABLE_NAME
    from 
     information_schema.TABLES t  
    WHERE t.TABLE_SCHEMA not in ('information_schema','performance_schema','mysql','sys', 'test')  
    AND t.TABLE_ROWS > 10000
    limit 10000000000
    ) t on s.TABLE_SCHEMA=t.TABLE_SCHEMA and  s.TABLE_NAME =t.TABLE_NAME
    
    
    *************************** 1. row ***************************
               id: 1
      select_type: PRIMARY
            table: s
       partitions: NULL
             type: ALL
    possible_keys: NULL
              key: NULL
          key_len: NULL
              ref: NULL
             rows: NULL
         filtered: NULL
            Extra: Using where; Open_frm_only; Scanned all databases
    *************************** 2. row ***************************
               id: 1
      select_type: PRIMARY
            table: <derived3>
       partitions: NULL
             type: ref
    possible_keys: <auto_key0>
              key: <auto_key0>
          key_len: 388
              ref: information_schema.s.TABLE_SCHEMA,information_schema.s.TABLE_NAME
             rows: 2
         filtered: 100.00
            Extra: Using index
    *************************** 3. row ***************************
               id: 3
      select_type: DERIVED
            table: t
       partitions: NULL
             type: ALL
    possible_keys: NULL
              key: NULL
          key_len: NULL
              ref: NULL
             rows: NULL
         filtered: NULL
            Extra: Using where; Open_full_table; Scanned all databases
    3 rows in set, 1 warning (0.00 sec)
    
    
    
    
    +----------+
    | count(1) |
    +----------+
    |     7478 |
    +----------+
    1 row in set, 40 warnings (7.52 sec)
    
    
    
    
    select count(1) 
    from 
    ( select c.TABLE_SCHEMA, c.TABLE_NAME, c.COLUMN_NAME ,c.CHARACTER_OCTET_LENGTH ,c.CHARACTER_MAXIMUM_LENGTH , c.DATA_TYPE
    from 
    information_schema.COLUMNS c 
    WHERE c.TABLE_SCHEMA not in ('information_schema','performance_schema','mysql','sys', 'test') 
    AND c.DATA_TYPE IN ("varchar", "char", "text", "blob")  
    ) c straight_join 
    (
    select t.TABLE_SCHEMA, t.TABLE_NAME
    from 
     information_schema.TABLES t  
    WHERE t.TABLE_SCHEMA not in ('information_schema','performance_schema','mysql','sys', 'test')  
    AND t.TABLE_ROWS > 10000
    limit 10000000000
    ) t on c.TABLE_SCHEMA=t.TABLE_SCHEMA and  c.TABLE_NAME =t.TABLE_NAME
    
    
    *************************** 1. row ***************************
               id: 1
      select_type: PRIMARY
            table: c
       partitions: NULL
             type: ALL
    possible_keys: NULL
              key: NULL
          key_len: NULL
              ref: NULL
             rows: NULL
         filtered: NULL
            Extra: Using where; Open_frm_only; Scanned all databases
    *************************** 2. row ***************************
               id: 1
      select_type: PRIMARY
            table: <derived3>
       partitions: NULL
             type: ref
    possible_keys: <auto_key0>
              key: <auto_key0>
          key_len: 388
              ref: information_schema.c.TABLE_SCHEMA,information_schema.c.TABLE_NAME
             rows: 2
         filtered: 100.00
            Extra: Using index
    *************************** 3. row ***************************
               id: 3
      select_type: DERIVED
            table: t
       partitions: NULL
             type: ALL
    possible_keys: NULL
              key: NULL
          key_len: NULL
              ref: NULL
             rows: NULL
         filtered: NULL
            Extra: Using where; Open_full_table; Scanned all databases
    3 rows in set, 1 warning (0.00 sec)
    
    
    +----------+
    | count(1) |
    +----------+
    |     8106 |
    +----------+
    1 row in set, 417 warnings (8.62 sec)

最终SQL 如下

SELECT c.TABLE_SCHEMA AS DB, c.TABLE_NAME AS TBL, c.COLUMN_NAME AS COL, c.CHARACTER_OCTET_LENGTH AS COL_LEN_BYTES, s.INDEX_NAME,
    s.SUB_PART * c.CHARACTER_OCTET_LENGTH/c.CHARACTER_MAXIMUM_LENGTH AS SUB_PART_LEN
    from 
    ( select c.TABLE_SCHEMA, c.TABLE_NAME, c.COLUMN_NAME ,c.CHARACTER_OCTET_LENGTH ,c.CHARACTER_MAXIMUM_LENGTH , c.DATA_TYPE
    from 
    information_schema.COLUMNS c 
    WHERE c.TABLE_SCHEMA not in ('information_schema','performance_schema','mysql','sys', 'test') 
    AND c.DATA_TYPE IN ("varchar", "char", "text", "blob")  
    ) c straight_join 
    (
    select t.TABLE_SCHEMA, t.TABLE_NAME
    from 
     information_schema.TABLES t  
    WHERE t.TABLE_SCHEMA not in ('information_schema','performance_schema','mysql','sys', 'test')  
    AND t.TABLE_ROWS > 10000
    limit 10000000000
    ) t on c.TABLE_SCHEMA=t.TABLE_SCHEMA and  c.TABLE_NAME =t.TABLE_NAME
    
    
    straight_join
    (
    select s.* 
    from 
    (
    select s.TABLE_SCHEMA, s.TABLE_NAME, s.COLUMN_NAME ,s.SUB_PART,s.INDEX_NAME
    from 
    information_schema.STATISTICS s 
    WHERE s.TABLE_SCHEMA not in ('information_schema','performance_schema','mysql','sys', 'test') 
    )s straight_join 
    (
    select t.TABLE_SCHEMA, t.TABLE_NAME
    from 
     information_schema.TABLES t  
    WHERE t.TABLE_SCHEMA not in ('information_schema','performance_schema','mysql','sys', 'test')  
    AND t.TABLE_ROWS > 10000
    limit 10000000000
    ) t on s.TABLE_SCHEMA=t.TABLE_SCHEMA and  s.TABLE_NAME =t.TABLE_NAME
    limit 10000000000
    ) s 
    on c.TABLE_SCHEMA=s.TABLE_SCHEMA and  c.TABLE_NAME=s.TABLE_NAME and c.COLUMN_NAME =s.COLUMN_NAME
    where 
      ((c.CHARACTER_OCTET_LENGTH > 50 and s.SUB_PART is null) or
    s.SUB_PART * c.CHARACTER_OCTET_LENGTH/c.CHARACTER_MAXIMUM_LENGTH > 50)
    
    
    
    
    *************************** 1. row ***************************
               id: 1
      select_type: PRIMARY
            table: c
       partitions: NULL
             type: ALL
    possible_keys: NULL
              key: NULL
          key_len: NULL
              ref: NULL
             rows: NULL
         filtered: NULL
            Extra: Using where; Open_frm_only; Scanned all databases
    *************************** 2. row ***************************
               id: 1
      select_type: PRIMARY
            table: <derived3>
       partitions: NULL
             type: ref
    possible_keys: <auto_key0>
              key: <auto_key0>
          key_len: 388
              ref: information_schema.c.TABLE_SCHEMA,information_schema.c.TABLE_NAME
             rows: 2
         filtered: 100.00
            Extra: Using index
    *************************** 3. row ***************************
               id: 1
      select_type: PRIMARY
            table: <derived4>
       partitions: NULL
             type: ref
    possible_keys: <auto_key0>
              key: <auto_key0>
          key_len: 582
              ref: information_schema.c.TABLE_SCHEMA,information_schema.c.TABLE_NAME,information_schema.c.COLUMN_NAME
             rows: 2
         filtered: 100.00
            Extra: Using where
    *************************** 4. row ***************************
               id: 4
      select_type: DERIVED
            table: s
       partitions: NULL
             type: ALL
    possible_keys: NULL
              key: NULL
          key_len: NULL
              ref: NULL
             rows: NULL
         filtered: NULL
            Extra: Using where; Open_frm_only; Scanned all databases
    *************************** 5. row ***************************
               id: 4
      select_type: DERIVED
            table: <derived6>
       partitions: NULL
             type: ref
    possible_keys: <auto_key0>
              key: <auto_key0>
          key_len: 388
              ref: information_schema.s.TABLE_SCHEMA,information_schema.s.TABLE_NAME
             rows: 2
         filtered: 100.00
            Extra: Using index
    *************************** 6. row ***************************
               id: 6
      select_type: DERIVED
            table: t
       partitions: NULL
             type: ALL
    possible_keys: NULL
              key: NULL
          key_len: NULL
              ref: NULL
             rows: NULL
         filtered: NULL
            Extra: Using where; Open_full_table; Scanned all databases
    *************************** 7. row ***************************
               id: 3
      select_type: DERIVED
            table: t
       partitions: NULL
             type: ALL
    possible_keys: NULL
              key: NULL
          key_len: NULL
              ref: NULL
             rows: NULL
         filtered: NULL
            Extra: Using where; Open_full_table; Scanned all databases
    7 rows in set, 1 warning (0.00 sec)

看起来稳定了，跑了几次，都没超过15秒

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