Question

SELECT COALESCE(tb2.number_new, tb1.number_original) as number
FROM
tb1
LEFT JOIN tb2 ON tb2.id = tb1.id
WHERE 
tb1.number_original = <PARAM> OR tb2.number_new = <PARAM>

以上查询生成全表扫描。如何优化它？

说明：

+----+-------------+-------+-------+----------------+----------------+---------+------+----------+----------------------------------------------------+
| id | select_type | table | type  | possible_keys  | key            | key_len | ref  | rows     | Extra                                              |
+----+-------------+-------+-------+----------------+----------------+---------+------+----------+----------------------------------------------------+
|  1 | SIMPLE      | tb1   | index | number_original| number_original| 5       | NULL | 11683843 | Using index                                        |
|  1 | SIMPLE      | tb2   | ALL   | PRIMARY        | NULL           | NULL    | NULL |        2 | Using where; Using join buffer (Block Nested Loop) |
+----+-------------+-------+-------+----------------+----------------+---------+------+----------+----------------------------------------------------+

我需要解决的问题是：

鉴于tb1是一个包含字段number_original的表，我想创建表tb2来存储此字段的新值，（我称之为number_new）。我这样做是因为我无法改变number_original值，我不喜欢在表number_new上创建字段tb1，因为它在95％+的11mi记录中都是NULL。

Answer 1

您需要tb1.number_original和tb2.number_new的索引，也可能需要tb1.id和tb2.id的索引。然后你需要MySQL来利用你的索引。我不知道是否会。

通常，当没有索引回答查询的搜索参数时，DBMS将扫描表。

Answer 2

这是您的查询：

SELECT COALESCE(tb2.number_new, tb1.number_original) as number
FROM tb1 LEFT JOIN
     tb2
     ON tb2.id = tb1.id
WHERE  tb1.number_original = <PARAM> OR tb2.number_new = <PARAM>;

这是一个复杂的问题。首先，尝试两个索引tb1(number_original, id)和tb2(number_new, id)。

您可以将查询重写为：

SELECT COALESCE(tb2.number_new, tb1.number_original) as number
FROM tb1 LEFT JOIN
     tb2 
     ON tb2.id = tb1.id
WHERE tb1.number_original = <PARAM>
UNION
SELECT COALESCE(tb2.number_new, tb1.number_original) as number
FROM tb1 LEFT JOIN
     tb2 
     ON tb2.id = tb1.id
WHERE tb2.number_new = <PARAM>;

每个子查询都应使用其中一个索引。这确实会产生union的开销（重复删除），但这可能比全表扫描更好。

LEFT JOIN与WHERE在第一个表的字段或第二个表的字段中

2 个答案: