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
....
和
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;