Question

My table (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

As you may have noticed, this is hierarchical data using the nested set model. Tree pretty-printed:

1
 2
 3
4
 5
6
7

I want to select all sub projects under project 1 and 4. I can do this with:

SELECT p.id
FROM projects AS p, projects AS ps
WHERE (ps.id = 1 OR ps.id = 4)
AND p.lft BETWEEN ps.lft AND ps.rgt

However, this is very slow with a large table, when running EXPLAIN (Query) i get:

+----+-------------+-------+-------+------------------------+---------+---------+------+------+-------------------------------------------------+
| id | select_type | table | type  | possible_keys          | key     | key_len | ref  | rows | Extra                                           |
+----+-------------+-------+-------+------------------------+---------+---------+------+------+-------------------------------------------------+
|  1 | SIMPLE      | ps    | range | PRIMARY,lft,rgt,lftRgt | PRIMARY | 4       | NULL |    2 | Using where                                     | 
|  1 | SIMPLE      | p     | ALL   | lft,lftRgt             | NULL    | NULL    | NULL | 7040 | Range checked for each record (index map: 0x12) | 
+----+-------------+-------+-------+------------------------+---------+---------+------+------+-------------------------------------------------+

(The project table has indexes on lft, rgt, and lft-rgt. As you can see, mysql does not use any index, and loops through the 7040 records)

I have found that if I only select for one of the super project, mysql manages to use the indexes:

SELECT p.id
FROM projects AS p, projects AS ps
WHERE ps.id = 1
AND p.lft BETWEEN ps.lft AND ps.rgt

EXPLAINs to:

+----+-------------+-------+-------+------------------------+---------+---------+-------+------+-------------+
| id | select_type | table | type  | possible_keys          | key     | key_len | ref   | rows | Extra       |
+----+-------------+-------+-------+------------------------+---------+---------+-------+------+-------------+
|  1 | SIMPLE      | ps    | const | PRIMARY,lft,rgt,lftRgt | PRIMARY | 4       | const |    1 |             | 
|  1 | SIMPLE      | p     | range | lft,lftRgt             | lft     | 4       | NULL  |    7 | Using where | 
+----+-------------+-------+-------+------------------------+---------+---------+-------+------+-------------+

FINALLY, my question: I there any way i can SELECT rows matching multiple ranges, and still benefit from indexes?

Answer 1

From 7.2.5.1. The Range Access Method for Single-Part Indexes in MySQL reference manual:

Currently, MySQL does not support merging multiple ranges for the range access method for spatial indexes. To work around this limitation, you can use a UNION with identical SELECT statements, except that you put each spatial predicate in a different SELECT.

So you need to have a union of two different selects.