mysql - 如何优化跨三个表、40k 行且仅返回 22 个结果的慢速 "select distinct"查询

Question

所以我有其他人写的这个查询，我正在尝试重构，它为一个项目(通常是鞋子)提取了一些特性/ Material 。

有很多产品，因此有很多连接表条目，但只有少数几个功能可供它们使用。我认为必须有一种方法来减少触及“大”项目列表的需要，以获得这些功能，我听说要避免不同，但我没有可以替换此处的“不同”选项的语句。

根据我的日志，我的结果时间很慢:

Query_time: 7 Lock_time: 0 Rows_sent: 32 Rows_examined: 5362862

Query_time: 8 Lock_time: 0 Rows_sent: 22 Rows_examined: 6581994

正如消息所说，有时需要 7 或 8 秒，有时或每次查询超过 500 万行。

这可能是由于同时发生了其他负载，因为这里是直接从 mysql 命令行在数据库上运行的选择:

mysql> SELECT DISTINCT features.FeatureId, features.Name
       FROM features, itemsfeatures, items
       WHERE items.FlagStatus != 'U'
         AND items.TypeId = '13'
         AND features.Type = 'Material'
         AND features.FeatureId = itemsfeatures.FeatureId
       ORDER BY features.Name;
+-----------+--------------------+
| FeatureId | Name               |
+-----------+--------------------+
|        40 | Alligator          |
|        41 | Burnished Calfskin |
|        42 | Calfskin           |
|        59 | Canvas             |
|        43 | Chromexcel         |
|        44 | Cordovan           |
|        57 | Cotton             |
|        45 | Crocodile          |
|        58 | Deerskin           |
|        61 | Eel                |
|        46 | Italian Leather    |
|        47 | Lizard             |
|        48 | Nappa              |
|        49 | NuBuck             |
|        50 | Ostrich            |
|        51 | Patent Leather     |
|        60 | Rubber             |
|        52 | Sharkskin          |
|        53 | Silk               |
|        54 | Suede              |
|        56 | Veal               |
|        55 | Woven              |
+-----------+--------------------+
22 rows in set (0.00 sec)

mysql> select count(*) from features;
+----------+
| count(*) |
+----------+
|      122 |
+----------+
1 row in set (0.00 sec)

mysql> select count(*) from itemsfeatures;
+----------+
| count(*) |
+----------+
|    38569 |
+----------+
1 row in set (0.00 sec)

mysql> select count(*) from items;
+----------+
| count(*) |
+----------+
|     8656 |
+----------+
1 row in set (0.00 sec)

explain SELECT DISTINCT features.FeatureId, features.Name  FROM features, itemsfeatures, items    WHERE items.FlagStatus != 'U'  AND items.TypeId = '13'  AND features.Type = 'Material' AND features.FeatureId = itemsfeatures.FeatureId  ORDER BY features.Name;
+----+-------------+---------------+------+-------------------+-----------+---------+---------------------------------+------+----------------------------------------------+
| id | select_type | table         | type | possible_keys     | key       | key_len | ref                             | rows | Extra                                        |
+----+-------------+---------------+------+-------------------+-----------+---------+---------------------------------+------+----------------------------------------------+
|  1 | SIMPLE      | features      | ref  | PRIMARY,Type      | Type      | 33      | const                           |   21 | Using where; Using temporary; Using filesort |
|  1 | SIMPLE      | itemsfeatures | ref  | FeatureId         | FeatureId | 4       | sherman_live.features.FeatureId |  324 | Using index; Distinct                        |
|  1 | SIMPLE      | items         | ALL  | TypeId,FlagStatus | NULL      | NULL    | NULL                            | 8656 | Using where; Distinct; Using join buffer     |
+----+-------------+---------------+------+-------------------+-----------+---------+---------------------------------+------+----------------------------------------------+
3 rows in set (0.04 sec)

编辑:

这里是没有不同的示例结果，(但有一个限制，否则它只会挂起)用于比较:

SELECT features.FeatureId, features.Name        FROM features, itemsfeatures, items        WHERE items.FlagStatus != 'U'          AND items.TypeId = '13'          AND features.Type = 'Material'          AND features.FeatureId = itemsfeatures.FeatureId        ORDER BY features.Name limit 10;
+-----------+-----------+
| FeatureId | Name      |
+-----------+-----------+
|        40 | Alligator |
|        40 | Alligator |
|        40 | Alligator |
|        40 | Alligator |
|        40 | Alligator |
|        40 | Alligator |
|        40 | Alligator |
|        40 | Alligator |
|        40 | Alligator |
|        40 | Alligator |
+-----------+-----------+
10 rows in set (23.30 sec)

这里使用 group by 而不是 select distinct:

SELECT features.FeatureId, features.Name        FROM features, itemsfeatures, items        WHERE items.FlagStatus != 'U'          AND items.TypeId = '13'          AND features.Type = 'Material'          AND features.FeatureId = itemsfeatures.FeatureId        group by features.name ORDER BY features.Name;
+-----------+--------------------+
| FeatureId | Name               |
+-----------+--------------------+
|        40 | Alligator          |
|        41 | Burnished Calfskin |
|        42 | Calfskin           |
|        59 | Canvas             |
|        43 | Chromexcel         |
|        44 | Cordovan           |
|        57 | Cotton             |
|        45 | Crocodile          |
|        58 | Deerskin           |
|        61 | Eel                |
|        46 | Italian Leather    |
|        47 | Lizard             |
|        48 | Nappa              |
|        49 | NuBuck             |
|        50 | Ostrich            |
|        51 | Patent Leather     |
|        60 | Rubber             |
|        52 | Sharkskin          |
|        53 | Silk               |
|        54 | Suede              |
|        56 | Veal               |
|        55 | Woven              |
+-----------+--------------------+
22 rows in set (13.28 sec)

编辑:增加赏金

...因为我试图理解这个一般问题，除了这个查询特别容易导致的缓慢之外，如何替换一般的错误选择不同查询。

我想知道选择 distinct 的替换是否通常是 group by(尽管在这种情况下这不是一个全面的解决方案，因为它仍然很慢)？

Answer 1

您似乎缺少将 itemsfeatures 链接到 items 的 JOIN 条件。如果您使用显式 JOIN 操作编写查询，则更明显。

SELECT DISTINCT f.FeatureId, f.Name  
    FROM features f
        INNER JOIN itemsfeatures ifx
            ON f.FeatureID = ifx.FeatureID
        INNER JOIN items i
            ON ifx.ItemID = i.ItemID /* This is the part you're missing */
    WHERE i.FlagStatus != 'U'  
        AND i.TypeId = '13'  
        AND f.Type = 'Material' 
    ORDER BY f.Name;