查询mysql数据库表的信息（表大小、数据大小、索引大小）

select * from information_schema.TABLES 
  where information_schema.TABLES.TABLE_SCHEMA='databasename'
  and information_schema.TABLES.TABLE_NAME='tablename'\G

返回结果:

*************************** 1. row ***************************
  TABLE_CATALOG: def
   TABLE_SCHEMA: databasename
     TABLE_NAME: tablename
     TABLE_TYPE: BASE TABLE
         ENGINE: MyISAM
        VERSION: 10
     ROW_FORMAT: Dynamic
     TABLE_ROWS: 6422930
 AVG_ROW_LENGTH: 213
    DATA_LENGTH: 1370884700
MAX_DATA_LENGTH: 281474976710655
   INDEX_LENGTH: 412930048
      DATA_FREE: 0
 AUTO_INCREMENT: 6422931
    CREATE_TIME: 2012-05-11 05:00:02
    UPDATE_TIME: 2012-05-22 15:12:06
     CHECK_TIME: 2012-05-11 09:58:52
TABLE_COLLATION: utf8_general_ci
       CHECKSUM: NULL
 CREATE_OPTIONS: 
  TABLE_COMMENT: 'table comment'
1 row in set (0.00 sec)

其中：
information_schema.TABLES.TABLE_SCHEMA = ‘databasename’ and information_schema.TABLES.TABLE_NAME = ‘tablename’

信息存储在information_schema.TABLES这个表中，TABLE_SCHEMA 对应数据库名，TABLE_NAME 对应表名。
# TABLE_ROWS 代表拥有的数据行数。
# 总大小 = DATA_LENGTH(数据大小) + INDEX_LENGTH(索引大小)

结果以字节为单位，除1024为K，除1048576(=1024*1024)为M。