MySQL是否將ORDER BY子句短路？

Question

鑑於這種SQL：

SELECT * FROM mytable ORDER BY mycolumn, RAND() 

假設mycolumn恰好只包含唯一的值（因此，包含足夠的信息來執行ORDER BY），確實MySQL的短路操作並跳過評估休息嗎？

Answer 1

我認爲這是答案。 Mysql使用不同的計劃，不能執行懶惰評估（o「hort-circuit」）。

mysql> explain select * from avatar order by id; 
+----+-------------+--------+-------+---------------+---------+---------+------+-------+-------+ 
| id | select_type | table | type | possible_keys | key  | key_len | ref | rows | Extra | 
+----+-------------+--------+-------+---------------+---------+---------+------+-------+-------+ 
| 1 | SIMPLE  | avatar | index | NULL   | PRIMARY | 8  | NULL | 28777 |  | 
+----+-------------+--------+-------+---------------+---------+---------+------+-------+-------+ 
1 row in set (0.00 sec) 

mysql> explain select * from avatar order by id, name; 
+----+-------------+--------+------+---------------+------+---------+------+-------+----------------+ 
| id | select_type | table | type | possible_keys | key | key_len | ref | rows | Extra   | 
+----+-------------+--------+------+---------------+------+---------+------+-------+----------------+ 
| 1 | SIMPLE  | avatar | ALL | NULL   | NULL | NULL | NULL | 28777 | Using filesort | 
+----+-------------+--------+------+---------------+------+---------+------+-------+----------------+ 
1 row in set (0.00 sec) 
mysql> explain select * from avatar order by id, RAND(); 
+----+-------------+--------+------+---------------+------+---------+------+-------+---------------------------------+ 
| id | select_type | table | type | possible_keys | key | key_len | ref | rows | Extra       | 
+----+-------------+--------+------+---------------+------+---------+------+-------+---------------------------------+ 
| 1 | SIMPLE  | avatar | ALL | NULL   | NULL | NULL | NULL | 28782 | Using temporary; Using filesort | 
+----+-------------+--------+------+---------------+------+---------+------+-------+---------------------------------+ 
1 row in set (0.00 sec) 

Answer 2

經驗表明它不會，即使mycolumn是主鍵。