MySQL不使用带JOIN,WHERE和ORDER的索引

Question

我们有两个类似于简单标记记录结构的表格如下(实际上它要复杂得多,但这是问题的本质):

tag (A.a) | recordId (A.b)
1         | 1
2         | 1
2         | 2
3         | 2
....

和

recordId (B.b) | recordData (B.c)
1              | 123
2              | 666
3              | 1246

问题是获取具有特定标记的有序记录.显而易见的方法是使用简单的连接和索引(PK)(Aa,Ab),(Ab),(PK)(Bb),(Bb,Bc),如下所示:

select A.a, A.b, B.c from A join B on A.b = B.b where a = 44 order by c;

但是,这会给文件排序带来不愉快的结果:

+----+-------------+-------+------+---------------+---------+---------+-----------+------+----------------------------------------------+
| id | select_type | table | type | possible_keys | key     | key_len | ref       | rows | Extra                                        |
+----+-------------+-------+------+---------------+---------+---------+-----------+------+----------------------------------------------+
|  1 | SIMPLE      | A     | ref  | PRIMARY,b     | PRIMARY | 4       | const     |   94 | Using index; Using temporary; Using filesort | 
|  1 | SIMPLE      | B     | ref  | PRIMARY,b     | b       | 4       | booli.A.b |    1 | Using index                                  | 
+----+-------------+-------+------+---------------+---------+---------+-----------+------+----------------------------------------------+

使用巨大且极其冗余的"物化视图",我们可以获得相当不错的性能,但这是以使业务逻辑复杂化为代价的,这是我们想要避免的,特别是因为A和B表已经是MV:s(并且是其他查询需要,以及使用UNION的相同查询.

create temporary table C engine=innodb as (select A.a, A.b, B.c from A join B on A.b = B.b);
explain select a, b, c from C where a = 44 order by c;

使情况进一步复杂化的是我们在B表上有条件,例如范围过滤器.

select A.a, A.b, B.c from A join B on A.b = B.b where a = 44 AND B.c > 678 order by c;

但我们相信,如果文件出口问题消失,我们可以解决这个问题.

有谁知道为什么上面的代码块3中的简单连接不会使用索引进行排序,如果我们可以在不创建新MV的情况下以某种方式解决问题？

下面是我们用于测试的完整SQL列表.

DROP TABLE IF EXISTS A;
DROP TABLE IF EXISTS B;
DROP TABLE IF EXISTS C;
CREATE TEMPORARY TABLE A (a INT NOT NULL, b INT NOT NULL, PRIMARY KEY(a, b), INDEX idx_A_b (b)) ENGINE=INNODB;
CREATE TEMPORARY TABLE B (b INT NOT NULL, c INT NOT NULL, d VARCHAR(5000) NOT NULL DEFAULT '', PRIMARY KEY(b), INDEX idx_B_c (c), INDEX idx_B_b (b, c)) ENGINE=INNODB;

DELIMITER $$
CREATE PROCEDURE prc_filler(cnt INT)
BEGIN
        DECLARE _cnt INT;
        SET _cnt = 1;
        WHILE _cnt <= cnt DO
                INSERT IGNORE INTO A SELECT RAND()*100, RAND()*10000;
                INSERT IGNORE INTO B SELECT RAND()*10000, RAND()*1000, '';
                SET _cnt = _cnt + 1;
        END WHILE;
END
$$
DELIMITER ;

START TRANSACTION;
CALL prc_filler(100000);
COMMIT;
DROP PROCEDURE prc_filler;

CREATE TEMPORARY TABLE C ENGINE=INNODB AS (SELECT A.a, A.b, B.c FROM A JOIN B ON A.b = B.b);
ALTER TABLE C ADD (PRIMARY KEY(a, b), INDEX idx_C_a_c (a, c));

EXPLAIN EXTENDED SELECT A.a, A.b, B.c FROM A JOIN B ON A.b = B.b WHERE A.a = 44;
EXPLAIN EXTENDED SELECT A.a, A.b, B.c FROM A JOIN B ON A.b = B.b WHERE 1 ORDER BY B.c;
EXPLAIN EXTENDED SELECT A.a, A.b, B.c FROM A JOIN B ON A.b = B.b where A.a = 44 ORDER BY B.c;
EXPLAIN EXTENDED SELECT a, b, c FROM C WHERE a = 44 ORDER BY c;
-- Added after Quassnois comments
EXPLAIN EXTENDED SELECT A.a, A.b, B.c FROM  B FORCE INDEX (idx_B_c) JOIN A ON A.b = B.b WHERE A.a = 44 ORDER BY B.c;
EXPLAIN EXTENDED SELECT A.a, A.b, B.c FROM A JOIN B ON A.b = B.b WHERE A.a = 44 ORDER BY B.c LIMIT 10;
EXPLAIN EXTENDED SELECT A.a, A.b, B.c FROM  B FORCE INDEX (idx_B_c) JOIN A ON A.b = B.b WHERE A.a = 44 ORDER BY B.c LIMIT 10;

Answer 1

当我尝试使用您的脚本重现此查询时:

SELECT  A.a, A.b, B.c
FROM    A
JOIN    B
ON      A.b = B.b
WHERE   a = 44
ORDER BY
        c

,它0.0043 seconds(立即)完成,返回930行并产生这个计划:

1, 'SIMPLE', 'A', 'ref', 'PRIMARY', 'PRIMARY', '4', 'const', 1610, 'Using index; Using temporary; Using filesort'
1, 'SIMPLE', 'B', 'eq_ref', 'PRIMARY', 'PRIMARY', '4', 'test.A.b', 1, ''

这种查询非常有效.

对于此类查询,您不能使用单个索引进行筛选和排序.

有关更详细的说明,请参阅我的博客中的这篇文章:

• 选择索引

如果您希望查询返回少量记录,则应使用索引A进行过滤,然后使用filesort进行排序(如上面的查询那样).

如果您希望它返回许多记录(和LIMIT它们),您需要使用索引进行排序,然后过滤:

CREATE INDEX ix_a_b ON a (b);
CREATE INDEX ix_b_c ON b (c)

SELECT  *
FROM    B FORCE INDEX (ix_b_c)
JOIN    A
ON      A.b = B.b
ORDER BY
        b.c
LIMIT 10;

1, 'SIMPLE', 'B', 'index', '', 'ix_b_c', '4', '', 2, 'Using index'
1, 'SIMPLE', 'A', 'ref', 'ix_a_b', 'ix_a_b', '4', 'test.B.b', 4, 'Using index'