索引可能因为删除，或者页分裂等原因，导致数据页有空洞，重建索引的过程会创建一个新的索引，把数据按顺序插入，这样页面的利用率最高，也就是索引更紧凑、更省空间。

    重建索引
    alter table t drop index id;
    alter table t add index (id);

    如果是主键，下面重建是错误的
    alter table T drop primary key; ~~
    alter table T add primary key(id);~~

    不论是删除主键还是创建主键，都会将整个表重建。所以连着执行这两个语句的话，第一个语句就白做了。
    这两个语句，你可以用这个语句代替 ： alter table T engine=InnoDB。（也相当于把普通索引重建）

    查看索引数据碎片率: DATA_FREE单位字节

    1. [root@localhost][employees]: select * from information_schema.tables where table_name='employees'\G
    2. *************************** 1. row ***************************
    3.   TABLE_CATALOG: def
    4.    TABLE_SCHEMA: employees
    5.      TABLE_NAME: employees
    6.      TABLE_TYPE: BASE TABLE
    7.          ENGINE: InnoDB
    8.         VERSION: 10
    9.      ROW_FORMAT: Dynamic
    10.      TABLE_ROWS: 290137
    11.  AVG_ROW_LENGTH: 54
    12.     DATA_LENGTH: 15777792
    13. MAX_DATA_LENGTH: 0
    14.    INDEX_LENGTH: 9977856
    15.       DATA_FREE: 7340032
    16.  AUTO_INCREMENT: NULL
    17.     CREATE_TIME: 2019-10-14 07:40:27
    18.     UPDATE_TIME: NULL
    19.      CHECK_TIME: NULL
    20. TABLE_COLLATION: utf8mb4_general_ci
    21.        CHECKSUM: NULL
    22.  CREATE_OPTIONS: 
    23.   TABLE_COMMENT: 
    24. 1 row in set (0.16 sec)
    26. [root@localhost][employees]: alter table employees engine=InnoDB;
    27. Query OK, 0 rows affected (27.10 sec)
    28. Records: 0  Duplicates: 0  Warnings: 0
    30. [root@localhost][employees]: select * from information_schema.tables where table_name='employees'\G
    31. *************************** 1. row ***************************
    32.   TABLE_CATALOG: def
    33.    TABLE_SCHEMA: employees
    34.      TABLE_NAME: employees
    35.      TABLE_TYPE: BASE TABLE
    36.          ENGINE: InnoDB
    37.         VERSION: 10
    38.      ROW_FORMAT: Dynamic
    39.      TABLE_ROWS: 299329
    40.  AVG_ROW_LENGTH: 57
    41.     DATA_LENGTH: 17317888
    42. MAX_DATA_LENGTH: 0
    43.    INDEX_LENGTH: 5783552
    44.       DATA_FREE: 2097152
    45.  AUTO_INCREMENT: NULL
    46.     CREATE_TIME: 2021-07-28 10:39:41
    47.     UPDATE_TIME: NULL
    48.      CHECK_TIME: NULL
    49. TABLE_COLLATION: utf8mb4_general_ci
    50.        CHECKSUM: NULL
    51.  CREATE_OPTIONS: 
    52.   TABLE_COMMENT: 
    53. 1 row in set (0.01 sec)