About update performance in mysql

Question

I have two tables.

tb_users 
 id (pk)
 login_name

tb_test
 id (pk)
 user_id
 user_code

The sample date is like below

tb_users (id, login_name)    
11e8, tom
11e3, jerry
11e1, peter
   ...  49316 more items

tb_test (id,user_id,user_code)
ff3f10b01, ,tom
ff3f10b02, ,tom
ff3f10b03, ,tom 
ff3f10b04, ,peter
  ...  70000 more items

Now I need to update field 'user_id' in tb_test with 'id' in tb_user. I wrote the sql.

update tb_test a
   set a.user_id = (select b.id
                      from tb_user b
                     where a.user_code = b.login_name ) 

It takes more than 1 minute! I wrote the select sql, it runs very fast. How should I improve the update efficiency?

select a.*, b.id
 from tb_test a, tb_user b
where a.user_code = b.login_name 

The result should be like this

tb_test (id,user_id,user_code)
ff3f10b01, 11e8 ,tom
ff3f10b02, 11e8 ,tom
ff3f10b03, 11e8 ,tom 
ff3f10b04, 11e1 ,peter
  ...  70000 more items

Answer 1

First, I would recommend writing this as a join :

update tb_test t join
       tb_user u
       on t.user_code = u.login_name
   set t.user_id = u.id; 

Then, for performance, you want an index on tb_user(login_name, id) .

That said, you are updating almost all rows. In many cases, it is cheaper just to recreate the table:

create table temp_tb_test as
    select t.id, u.id as user_id, t.user_code
    from tb_test t left join
         tb_user u
         on t.user_code = u.login_name;

truncate table tb_test;  -- be very cautious here!

insert into tb_test(id, user_id, user_code)
    select id, user_id, user_code
    from temp_tb_test;