3. -- CARDINALITY:表示不重复数据总和,数据为采样
    5. mysql> show create table dept_emp\G
    6. *************************** 1. row ***************************
    7.        Table: dept_emp
    8. Create Table: CREATE TABLE `dept_emp` (
    9.   `emp_no` int(11) NOT NULL,
    10.   `dept_no` char(4) NOT NULL,
    11.   `from_date` date NOT NULL,
    12.   `to_date` date NOT NULL,
    13.   PRIMARY KEY (`emp_no`,`dept_no`),
    14.   KEY `dept_no` (`dept_no`),
    15.   CONSTRAINT `dept_emp_ibfk_1` FOREIGN KEY (`emp_no`) REFERENCES `employees` (`emp_no`) ON DELETE CASCADE,
    16.   CONSTRAINT `dept_emp_ibfk_2` FOREIGN KEY (`dept_no`) REFERENCES `departments` (`dept_no`) ON DELETE CASCADE
    17. ) ENGINE=InnoDB DEFAULT CHARSET=utf8
    18. 1 row in set (0.00 sec)
    21. mysql> select * from information_schema.STATISTICS where TABLE_NAME='dept_emp'  \G
    22. *************************** 1. row ***************************
    23. TABLE_CATALOG: def
    24.  TABLE_SCHEMA: employees
    25.    TABLE_NAME: dept_emp
    26.    NON_UNIQUE: 0
    27.  INDEX_SCHEMA: employees
    28.    INDEX_NAME: PRIMARY        -- 主键
    29.  SEQ_IN_INDEX: 1                    -- 字段在索引的位置
    30.   COLUMN_NAME: emp_no
    31.     COLLATION: A
    32.   CARDINALITY: 299600            -- 第一列的基数
    33.      SUB_PART: NULL
    34.        PACKED: NULL
    35.      NULLABLE: 
    36.    INDEX_TYPE: BTREE
    37.       COMMENT: 
    38. INDEX_COMMENT: 
    39. *************************** 2. row ***************************
    40. TABLE_CATALOG: def
    41.  TABLE_SCHEMA: employees
    42.    TABLE_NAME: dept_emp
    43.    NON_UNIQUE: 0
    44.  INDEX_SCHEMA: employees
    45.    INDEX_NAME: PRIMARY
    46.  SEQ_IN_INDEX: 2
    47.   COLUMN_NAME: dept_no
    48.     COLLATION: A
    49.   CARDINALITY: 331143            -- 两列加起来后的基数
    50.      SUB_PART: NULL
    51.        PACKED: NULL
    52.      NULLABLE: 
    53.    INDEX_TYPE: BTREE
    54.       COMMENT: 
    55. INDEX_COMMENT: 
    56. *************************** 3. row ***************************
    57. TABLE_CATALOG: def
    58.  TABLE_SCHEMA: employees
    59.    TABLE_NAME: dept_emp
    60.    NON_UNIQUE: 1
    61.  INDEX_SCHEMA: employees
    62.    INDEX_NAME: dept_no            -- 索引名称
    63.  SEQ_IN_INDEX: 1
    64.   COLUMN_NAME: dept_no
    65.     COLLATION: A
    66.   CARDINALITY: 8                        -- 部门的基数,只有8,并不适合创建索引
    67.      SUB_PART: NULL
    68.        PACKED: NULL
    69.      NULLABLE: 
    70.    INDEX_TYPE: BTREE
    71.       COMMENT: 
    72. INDEX_COMMENT: 
    73. 3 rows in set (0.00 sec)
    75. -- emp表的相关信息,均为采样,只能参考
    76. mysql> select * from information_schema.tables where TABLE_NAME='dept_emp'  \G
    77. *************************** 1. row ***************************
    78.   TABLE_CATALOG: def
    79.    TABLE_SCHEMA: employees
    80.      TABLE_NAME: dept_emp
    81.      TABLE_TYPE: BASE TABLE
    82.          ENGINE: InnoDB
    83.         VERSION: 10
    84.      ROW_FORMAT: Dynamic
    85.      TABLE_ROWS: 331143                    -- 采样的总行数
    86.  AVG_ROW_LENGTH: 36
    87.     DATA_LENGTH: 12075008                -- 数据大小(字节)
    88. MAX_DATA_LENGTH: 0
    89.    INDEX_LENGTH: 5783552                -- 索引大小(字节)
    90.       DATA_FREE: 4194304
    91.  AUTO_INCREMENT: NULL
    92.     CREATE_TIME: 2020-02-04 10:31:12
    93.     UPDATE_TIME: NULL
    94.      CHECK_TIME: NULL
    95. TABLE_COLLATION: utf8_general_ci
    96.        CHECKSUM: NULL
    97.  CREATE_OPTIONS: 
    98.   TABLE_COMMENT: 
    99. 1 row in set (0.00 sec)