我有一些层次结构的数据,采用嵌套集模型存储(表名:projects):
我的表结构如下(表名:projects):
id, lft, rgt
1, 1, 6
2, 2, 3
3, 4, 5
4, 7, 10
5, 8, 9
6, 11, 12
7, 13, 14
...
美化后的:
1
2
3
4
5
6
7
为了找到节点3(已知其lft值)的最近超级节点,我可以执行以下操作:
explain
SELECT projects.*
FROM projects
WHERE 4 BETWEEN projects.lft AND projects.rgt
我得到了一份从根节点到节点3的项目列表。然后通过分组并查找结果中的MAX(projects.lft),我获得了最近的超级节点。然而,我似乎无法使这个查询快速运行,它不会使用我定义的索引。EXPLAIN 说明如下:
+----+-------------+----------+-------+----------------+----------+---------+------+------+--------------------------+
| id | select_type | table | type | possible_keys | key | key_len | ref | rows | Extra |
+----+-------------+----------+-------+----------------+----------+---------+------+------+--------------------------+
| 1 | SIMPLE | projects | index | lft,rgt,lftRgt | idLftRgt | 12 | NULL | 10 | Using where; Using index |
+----+-------------+----------+-------+----------------+----------+---------+------+------+--------------------------+
虽然Mysql知道要使用哪个索引,但仍需要循环遍历所有10行(或实际表中的100k行)。
我如何让MySql正确优化此查询? 我在下面附上一个测试脚本。
DROP TABLE IF EXISTS projects;
CREATE TABLE projects (
id INT NOT NULL ,
lft INT NOT NULL ,
rgt INT NOT NULL ,
PRIMARY KEY ( id )
) ENGINE = MYISAM ;
ALTER TABLE projects ADD INDEX lft (lft);
ALTER TABLE projects ADD INDEX rgt (rgt);
ALTER TABLE projects ADD INDEX lftRgt (lft, rgt);
ALTER TABLE projects ADD INDEX idLftRgt (id, lft, rgt);
INSERT INTO projects (id,lft,rgt) VALUES (1,1,6);
INSERT INTO projects (id,lft,rgt) VALUES (2,2,3);
INSERT INTO projects (id,lft,rgt) VALUES (3,4,5);
INSERT INTO projects (id,lft,rgt) VALUES (4,7,10);
INSERT INTO projects (id,lft,rgt) VALUES (5,8,9);
INSERT INTO projects (id,lft,rgt) VALUES (6,11,12);
INSERT INTO projects (id,lft,rgt) VALUES (7,13,14);
INSERT INTO projects (id,lft,rgt) VALUES (8,15,16);
INSERT INTO projects (id,lft,rgt) VALUES (9,17,18);
INSERT INTO projects (id,lft,rgt) VALUES (10,19,20);
explain
SELECT projects.*
FROM projects
WHERE 4 BETWEEN projects.lft AND projects.rgt