我的MySQL查询出了什么问题？

Question

I'm not getting any errors as such just a minor performance issue.

EXPLAIN
SELECT
a.nid,
a.title,
a.uid,
b.parent,
b.weight,
c.name,
d.value
FROM table1 AS a INNER JOIN table2 AS b ON a.vid = b.vid AND a.status = 1
INNER JOIN table3 AS c ON c.uid = a.uid
INNER JOIN table4 AS d ON d.content_id = a.nid AND d.value_type = 'percent' AND d.function = 'average'

When I look at which tables are being referenced, everything is fine, but from table4 where it should only be selecting the "value" field, I'm getting an ALL being called...

id  select_type     table   type      possible_keys                                   key     key_len   ref                   rows  Extra
1   SIMPLE          a     ref     PRIMARY,vid,status,uid,node_status_type,nid   status  4         const                 1    
1   SIMPLE          b     eq_ref    PRIMARY                                         PRIMARY 4         databasename.a.vid    1    
1   SIMPLE          c     eq_ref    PRIMARY                                         PRIMARY 4         databasename.a.uid    1   Using where
1   SIMPLE          d     ALL     NULL                                          NULL      NULL      NULL                  2     Using where

As you can see, it's selecting * from the final table (d). Why is it doing this when I only need ONE field selected from it? Can anyone help me out?

Answer 1

ALL means all rows, not all columns. Since it says there are no possible keys, I'd guess that you don't have an index on d.content_id or d.value_type or d.function.

If you wanted to be fancy, you could put an index across all 3 of those columns.