子查詢與join效能差異

原理分析參見：http://blog.csdn.net/wudongxu/article/details/6966052

這裡展示下另外一種情況:子查詢不是實體表，而是查詢結果集

# 表結構
DROP TABLE IF EXISTS `t1`;
CREATE TABLE `t1` (
  `id` INT NOT NULL AUTO_INCREMENT,
  `c1` INT NOT NULL,
  PRIMARY KEY (`id`)
) ENGINE=INNODB;

# 資料
INSERT INTO `t1`(`c1`) VALUES ('0'),('1'),('2'),('3'),('4');

# SLEEP函式返回值為0
# join 
SELECT SQL_NO_CACHE t1.* FROM `t1` INNER JOIN (SELECT SLEEP(1) AS val ) AS t2 ON t1.`c1`=t2.val;
# 子查詢:被執行了5次(t1表符合條件的行數)
SELECT SQL_NO_CACHE t1.* FROM `t1` WHERE t1.`c1`= (SELECT SLEEP(1) AS val ) ;

效能測試結果