问执行MySQL查询127秒

EN

SELECT * FROM magnitudes t1 INNER JOIN value_magnitudes t2 ON t1.id = t2.magnitude_id WHERE t1.floor_id = 42 AND t2.reading_date = (SELECT Max(reading_date) FROM value_magnitudes WHERE t2.magnitude_id = t1.id);

CREATE TABLE magnitudes (
  id INTEGER UNSIGNED  NOT NULL   AUTO_INCREMENT,
  name VARCHAR(255)  NULL  ,
  sdi_id VARCHAR(255)  NULL  ,
  sdi_id_floor VARCHAR(255)  NULL  ,
  visible TINYINT(1) UNSIGNED  NULL  ,
  history TINYINT(1) UNSIGNED  NULL  ,
  created_at DATETIME  NULL  ,
  updated_at DATETIME  NULL  ,
  floor_id INT(11)  NULL  ,
  unit VARCHAR(255)  NULL  ,
  kind_id INT(11)  NULL  ,
  sample_time INT(11)  NULL  ,
  history_from DATETIME  NULL DEFAULT 2011-02-22 11:18:07   ,
PRIMARY KEY(id));

CREATE TABLE value_magnitudes (
  id INTEGER UNSIGNED  NOT NULL   AUTO_INCREMENT,
  value FLOAT  NULL  ,
  magnitude_id INT(11)  NULL  ,
  sdi_belongs_id VARCHAR(255)  NULL  ,
  reading_date DATETIME  NULL  ,
  created_at DATETIME  NULL  ,
  updated_at DATETIME  NULL    ,
PRIMARY KEY(id));