I have a query to select from another sub-query select. While the two queries look almost the same the second query (in this sample) runs much slower:

```
SELECT
   user.id
  ,user.first_name
  -- user.*
  FROM user
  WHERE
    user.id IN (SELECT ref_id 
                  FROM education 
                 WHERE ref_type='user' 
                   AND education.institute_id='58' 
                   AND education.institute_type='1'
                );

```

This query takes 1.2s Explain on this query results:

```
 id select_type table   type    possible_keys   key key_len ref rows    Extra
 1  PRIMARY         user   index    first_name  152 141192  Using where; Using index
 2  DEPENDENT SUBQUERY  education   index_subquery  ref_type,ref_id,institute_id,institute_type,ref_type_2  ref_id  4   func    1   Using where

```

The second query:

```
SELECT
  -- user.id
  -- user.first_name
  user.*
  FROM user
  WHERE
    user.id IN (SELECT ref_id 
                  FROM education 
                 WHERE ref_type='user' 
                   AND education.institute_id='58' 
                   AND education.institute_type='1'
                );

```

Takes 45sec to run, with explain:

```
 id select_type table   type    possible_keys   key key_len ref rows    Extra
 1  PRIMARY user    ALL                 141192  Using where
 2  DEPENDENT SUBQUERY  education   index_subquery  ref_type,ref_id,institute_id,institute_type,ref_type_2  ref_id  4   func    1   Using where

```

Why is it slower if i query only by index fields?
Why both queries scans the full length of the user table?
Any ideas how to improve?

Thanks.