mysql - 如何优化left join的sql？

Question

我有两个表phone和phone_area

电话列:

1. id: pk

2. phone: unique index

电话数据:

|id| phone       |
+--+-------------+
|1 | 1882601xxxx |
+--+-------------+
|2 | 1882602xxxx |
+--+-------------+
|2 | 1882602xxxx |
+--+-------------+
|2 | 1882603xxxx  | 
+--+-------------+

phone_area 列:

1.id: pk

2.phone: unique index

3.area: varchar(20)

phone_area数据:

|id| phone  |  area    |  
+--+--------+----------+
|1 | 1882601|  area_one|
+--+--------+----------+
|2 | 1882602|  area_two|
+--+--------+----------+ 
|2 | 1882603|area_three|
+--+--------+----------+

我的 sql 如下:

SELECT t1.phone,t2.area FROM phone t1
LEFT JOIN phone_area t2 ON substr(t1.phone, 1, 7)= t2.phone

它很慢。

在解释sql时，显示type为"ALL"和Using where;使用连接缓冲区( block 嵌套循环)

如何改进我的sql？

Answer 1

按照以下步骤:

ALTER TABLE phone ADD phone_cut int; --add a new column

UPDATE phone SET phone_cut = substr(phone, 1, 7); --store the cropped value of phone

ALTER TABLE phone ADD INDEX ind_name (phone_cut); -- add an index on that column

然后:

SELECT t1.phone,t2.area FROM phone t1
LEFT JOIN phone_area t2
 ON t1.phone_cut= t2.phone