How to optimize sql of left join?

Question

I have two table phone and phone_area

phone columns:

1. id: pk

2. phone: unique index

phone data:

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

phone_area columns:

1.id: pk

2.phone: unique index

3.area: varchar(20)

phone_area data:

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

My sql following:

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

It is very slowly.

When explain the sql， it shows the type as "ALL" and Using where; Using join buffer (Block Nested Loop)

How to improve my sql?

Answer 1

Follow this steps:

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

And then :

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