mysql> explain select web_page_object.web_page_object_id,
 -> web_page_object.object_id,
 -> web_div_name,web_page_object.position_sort,web_page_object.end_time,om1.label,om1.file,jump_url,om2.label as label1,om2.file as file1
 -> from web_page_div,web_page_object,object_media as om1,object_media as om2
 -> where web_page_div.id=web_page_object.web_page_div_id
 -> and web_page_object.object_media_id=om1.object_media_id
 -> and web_page_div.web_page_id=1200
 -> and if(web_page_object.object_media_id1=0,
 -> web_page_object.object_media_id=om2.object_media_id,
 -> web_page_object.object_media_id1=om2.object_media_id)
 -> 
 -> and '2014-05-01 15:09:49'>=start_time
 -> and '2014-05-01 15:09:49'<= end_time
 -> 
 -> and object_status=0
 -> order by web_page_div.id,web_page_object.position_sort;
+----+-------------+-----------------+--------+-----------------------+---------+---------+-------------------------------------------+-------+----------------------------------------------------+
| id | select_type | table | type | possible_keys | key | key_len | ref | rows | Extra |
+----+-------------+-----------------+--------+-----------------------+---------+---------+-------------------------------------------+-------+----------------------------------------------------+
| 1 | SIMPLE | web_page_object | ALL | object_media_id_index | NULL | NULL | NULL | 51165 | Using where; Using temporary; Using filesort |
| 1 | SIMPLE | web_page_div | eq_ref | PRIMARY,idx | PRIMARY | 4 | db_jiapin.web_page_object.web_page_div_id | 1 | Using where |
| 1 | SIMPLE | om1 | eq_ref | PRIMARY | PRIMARY | 4 | db_jiapin.web_page_object.object_media_id | 1 | Using where |
| 1 | SIMPLE | om2 | ALL | NULL | NULL | NULL | NULL | 74759 | Using where; Using join buffer (Block Nested Loop) |
+----+-------------+-----------------+--------+-----------------------+---------+---------+-------------------------------------------+-------+----------------------------------------------------+
Using join buffer (Block Nested Loop)

(Block Nested Loop)的案例是 join后面的on條件里面有or判斷。

也會引起Block Nested Loop，導致數據庫負載過高。