7

我们有两个类似于简单标签记录结构的表,如下所示(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
....

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

问题是获取具有特定标签的有序记录。最明显的方法是在 (PK)(Aa, Ab), (Ab), (PK)(Bb), (Bb,Bc) 上进行简单的连接和索引:

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,但这会以业务逻辑复杂化为代价,这是我们希望避免的,特别是因为 A 和 B 表已经是 MV:s(并且是其他查询需要,并且实际上使用 UNION 进行相同的查询)。

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;

使情况更加复杂的是我们在 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;

但是如果文件排序问题消失,我们有信心可以处理这个问题。

有谁知道为什么上面代码块 3 中的简单连接不会使用索引进行排序,以及我们是否可以在不创建新 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;
4

2 回答 2

10

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, ''

这样的查询非常有效。

对于这样的查询,您不能同时使用单个索引进行过滤和排序。

请参阅我的博客中的这篇文章以获得更详细的解释:

如果您希望您的查询返回很少的记录,您应该使用索引A进行过滤,然后使用 filesort 进行排序(就像上面的查询一样)。

如果您希望它返回许多记录(以及LIMIT它们),则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'
于 2009-08-04T13:50:19.727 回答
1

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

如果给列加上别名,这有帮助吗?例子:

 SELECT 
 T1.a AS colA, 
 T2.b AS colB, 
 T2.c AS colC 
 FROM A AS T1 
 JOIN B AS T2 
 ON (T1.b = T2.b) 
 WHERE 
 T1.a = 44 
 ORDER BY colC;

我所做的唯一更改是:

  • 我将连接条件放在括号中
  • 连接条件和 where 条件基于表列
  • ORDER BY 条件基于结果表列
  • 我为结果表列和查询表设置了别名,以(希望)在我使用其中一个或另一个时更清楚(并且对服务器更清楚。您忽略了在原始查询中的两个位置引用您的列)。

我知道您的真实数据更复杂,但我假设您提供了一个简单版本的查询,因为问题就在那个简单的级别。

于 2009-08-04T14:15:33.773 回答