Explain中的filtered列

Eg::创建表

create table t1 (

id int not null auto_increment,

filter char(200),

PRIMARY KEY(ID)

);

插入语句:

CREATE DEFINER=`root`@`localhost` FUNCTION `rand_string`(n INT) RETURNS varchar(255) CHARSET utf8

BEGIN

  declare chars_str varchar(100) DEFAULT 'abvdefsf';

  declare return_str varchar(255) default '';

  declare i int default 0;

  while i< n do

     set return_str=concat(return_str,substring(chars_str,floor(1+rand()*52),1)); #52个字母中随机找出一个

     set i=i+1;

  end while;

  return return_str;

END

CREATE DEFINER=`root`@`localhost` PROCEDURE `insert1`()

begin

set @a=1;

while <20000 do

set @b=rand_string(5);

insert into t1 (filter) values (@b);

set @a=@a+1;

end while;

end

call insert1();

explain select * from t1 where id <500

id, select_type, table, partitions, type, possible_keys, key, key_len, ref, rows, filtered, Extra

1, SIMPLE,       t1, null ,    range, PRIMARY, PRIMARY,,null, 4, , 499, 100.00, Using where

符合某条件的记录数百分比。