我有 2 个关于订购数据的查询:
查询 1:
SELECT * FROM (
SELECT idprovince, COUNT(*) total
FROM cities
JOIN persons USE INDEX (index_5) USING (idcity)
WHERE is_tutor = 'Y'
GROUP BY idprovince
) A
ORDER BY total DESC
查询 2:
SELECT idprovince, COUNT(*) total
FROM cities
JOIN persons USE INDEX (index_5) USING (idcity)
WHERE is_tutor = 'Y'
GROUP BY idprovince
ORDER BY total DESC
查询 1 返回数据的速度比查询 2 快得多，我的问题是使用查询排序和在子查询中使用排序有什么区别？
注意:我的数据库版本是 mysql-5.0.96-x64。人口数据约为 40 万，城市数据约为 500。
更新: mysql 解释命令的输出:
查询 1:
mysql> EXPLAIN
-> SELECT *
-> FROM (
-> SELECT idprovince, COUNT(*) total
-> FROM cities
-> JOIN persons USE INDEX (index_5) USING (idcity)
-> WHERE is_tutor = 'Y'
-> GROUP BY idprovince
-> ) A
-> ORDER BY total DESC
-> ;
+----+-------------+------------+--------+---------------+---------+---------+------------------------------------+--------+----------------------------------------------+
| id | select_type | table | type | possible_keys | key | key_len | ref | rows | Extra |
+----+-------------+------------+--------+---------------+---------+---------+------------------------------------+--------+----------------------------------------------+
| 1 | PRIMARY | <derived2> | ALL | NULL | NULL | NULL | NULL | 34 | Using filesort |
| 2 | DERIVED | persons | ref | index_5 | index_5 | 2 | | 163316 | Using where; Using temporary; Using filesort |
| 2 | DERIVED | cities | eq_ref | PRIMARY | PRIMARY | 4 | _myproject_lesaja_2.persons.idcity | 1 | |
+----+-------------+------------+--------+---------------+---------+---------+------------------------------------+--------+----------------------------------------------+
3 rows in set (1.22 sec)
查询 2:
mysql> EXPLAIN
-> SELECT idprovince, COUNT(*) total
-> FROM cities
-> JOIN persons USE INDEX (index_5) USING (idcity)
-> WHERE is_tutor = 'Y'
-> GROUP BY idprovince
-> ORDER BY total DESC;
+----+-------------+---------+-------+---------------+-------------+---------+-------+--------+----------------------------------------------+
| id | select_type | table | type | possible_keys | key | key_len | ref | rows | Extra |
+----+-------------+---------+-------+---------------+-------------+---------+-------+--------+----------------------------------------------+
| 1 | SIMPLE | cities | index | PRIMARY | FK_cities_1 | 4 | NULL | 4 | Using index; Using temporary; Using filesort |
| 1 | SIMPLE | persons | ref | index_5 | index_5 | 2 | const | 163316 | Using where |
+----+-------------+---------+-------+---------------+-------------+---------+-------+--------+----------------------------------------------+
2 rows in set (0.00 sec)
结果查询 1:
mysql> SELECT *
-> FROM (
-> SELECT idprovince, COUNT(*) total
-> FROM cities
-> JOIN persons USE INDEX (index_5) USING (idcity)
-> WHERE is_tutor = 'Y'
-> GROUP BY idprovince
-> ) A
-> ORDER BY total DESC
-> ;
+------------+-------+
| idprovince | total |
+------------+-------+
| 35 | 15797 |
......................
......................
......................
| 76 | 2091 |
| 65 | 2018 |
+------------+-------+
34 rows in set (0.78 sec)
结果查询 2:
mysql> SELECT idprovince, COUNT(*) total
-> FROM cities
-> JOIN persons USE INDEX (index_5) USING (idcity)
-> WHERE is_tutor = 'Y'
-> GROUP BY idprovince
-> ORDER BY total DESC;
+------------+-------+
| idprovince | total |
+------------+-------+
| 35 | 15797 |
| 33 | 14413 |
| 12 | 13683 |
......................
......................
......................
| 34 | 2135 |
| 76 | 2091 |
| 65 | 2018 |
+------------+-------+
34 rows in set (8 min 25.80 sec)
显示配置文件输出: 查询 1:
+----------------------+----------+
| Status | Duration |
+----------------------+----------+
| starting | 0.000240 |
| Opening tables | 0.000043 |
| System lock | 0.000004 |
| Table lock | 0.000392 |
| optimizing | 0.000084 |
| statistics | 0.004455 |
| preparing | 0.000026 |
| Creating tmp table | 0.000221 |
| executing | 0.000002 |
| Copying to tmp table | 0.913722 |
| Sorting result | 0.000065 |
| Sending data | 0.000020 |
| removing tmp table | 0.000145 |
| Sending data | 0.000008 |
| init | 0.000017 |
| optimizing | 0.000002 |
| statistics | 0.000038 |
| preparing | 0.000007 |
| executing | 0.000001 |
| Sorting result | 0.000012 |
| Sending data | 0.000337 |
| end | 0.000002 |
| end | 0.000002 |
| query end | 0.000002 |
| freeing items | 0.000020 |
| closing tables | 0.000001 |
| removing tmp table | 0.000074 |
| closing tables | 0.000003 |
| logging slow query | 0.000001 |
| cleaning up | 0.000003 |
+----------------------+----------+
查询 2:
+----------------------+------------+
| Status | Duration |
+----------------------+------------+
| starting | 0.000195 |
| Opening tables | 0.000029 |
| System lock | 0.000004 |
| Table lock | 0.000011 |
| init | 0.000078 |
| optimizing | 0.000021 |
| statistics | 0.003399 |
| preparing | 0.000025 |
| Creating tmp table | 0.000259 |
| Sorting for group | 0.000007 |
| executing | 0.000001 |
| Copying to tmp table | 506.711308 |
| Sorting result | 0.000049 |
| Sending data | 0.000298 |
| end | 0.000004 |
| removing tmp table | 0.000150 |
| end | 0.000002 |
| end | 0.000002 |
| query end | 0.000002 |
| freeing items | 0.000013 |
| closing tables | 0.000003 |
| logging slow query | 0.000001 |
| logging slow query | 0.000042 |
| cleaning up | 0.000003 |
+----------------------+------------+
创建语句
CREATE TABLE persons (
idperson INT(10) UNSIGNED NOT NULL AUTO_INCREMENT,
is_tutor ENUM('Y','N') NULL DEFAULT 'N',
name VARCHAR(64) NOT NULL,
...
idcity INT(10) UNSIGNED NOT NULL,
...
PRIMARY KEY (idperson),
UNIQUE INDEX index_3 (name) USING BTREE,
UNIQUE INDEX index_4 (email) USING BTREE,
INDEX index_5 (is_tutor),
...
CONSTRAINT FK_persons_1 FOREIGN KEY (idcity) REFERENCES cities (idcity)
)
ENGINE=InnoDB
AUTO_INCREMENT=414738;
CREATE TABLE cities (
idcity INT(10) UNSIGNED NOT NULL,
idprovince INT(10) UNSIGNED NOT NULL,
city VARCHAR(64) NOT NULL,
PRIMARY KEY (idcity),
UNIQUE INDEX index_3 (city),
INDEX FK_cities_1 (idprovince),
CONSTRAINT FK_cities_1 FOREIGN KEY (idprovince) REFERENCES provinces (idprovince)
)
ENGINE=InnoDB;
最佳答案
诚然，我不是这方面的专家，但正在查看 MySQL Documentation在 ORDER BY Optimization 上，您在第 2 条查询中不仅有一个而且有两个未优化的 ORDER BY 使用:
SELECT idprovince, COUNT(*) total
FROM cities
JOIN persons USE INDEX (index_5) USING (idcity)
WHERE is_tutor = 'Y'
GROUP BY idprovince
ORDER BY total DESC
第一个:
用于获取行的键
WHERE is_tutor = 'Y'
与 ORDER BY 中使用的不同:
ORDER BY total DESC
第二个:
您有不同的 ORDER BY 和 GROUP BY 表达式。
GROUP BY idprovince
ORDER BY total DESC
在上述两种情况下，MySQL 不会使用索引来解析 ORDER BY，尽管它可以使用索引来搜索与 WHERE 子句匹配的行。
另一方面，您的查询 1 遵循优化形式的 ORDER BY，尽管 ORDER BY 在子查询之外使用。
这可能是查询 2 比查询 1 慢得多的原因。
此外，在这两种情况下，Index (idCity) 在解析 ORDER BY 时几乎毫无用处，因为索引使用 idCity 而 ORDER BY 子句使用 Total，这是一个聚合结果。
参见讨论 here还有。
关于查询与子查询中聚合顺序之间的 MySQL 差异，我们在Stack Overflow上找到一个类似的问题： https://stackoverflow.com/questions/19737736/