NULL used as index key instead of possible key in MySQL db

Question

I am running the following query:

SELECT p.val1, p.val2, p.val3, p.val4, p.val5, p.val6, p.val7, p.val8
FROM db1.tbl1 AS p
INNER JOIN db2.tbl2 vp ON p.pid = vp.pid
INNER JOIN db2.tbl1 AS vs ON vp.vid = vs.vid
INNER JOIN db3.tbl1 AS sa ON vs.sid = sa.sid
LEFT JOIN db4.tbl1 AS fs ON p.aid = fs.aid
WHERE sa.id =  '11594'
AND fs.aid IS NULL 
ORDER BY IF( (
ISNULL( egl ) 
OR egl =  '' ) , 1, 0
), egl DESC 
LIMIT 15 
OFFSET 0

Unfortunately, it just hangs when run. Running an EXPLAIN nets me this info:

|  id | select_type | table | type  | possible_keys                     | key        | key_len | ref        |  rows  | Extra 
|*---*|*-----------*|*-----*|*-----*|*---------------------------------*|*----------*|*-------*|*----------*|*------*|*--------------
|  1  | SIMPLE      |   sa  | const | PRIMARY,s_key,p_key,n_key,ignored |   PRIMARY  |    4    | const      |   1    | Using filesort
|  1  | SIMPLE      |   p   |  ALL  | PRIMARY, pid                      |    NULL    |  NULL   | NULL       | 744704 |   
|  1  | SIMPLE      |   vp  |  ref  | PRIMARY,pid                       |   pid      |   130   | db1.p.pid  |   1    | Using index
|  1  | SIMPLE      |   vs  |  ref  | vid                               |   vid      |   130   | db2.vp.vid |   1    | Using where
|  1  | SIMPLE      |   fs  |  ref  | a_key                             |   a_key    |   97    | func       |   1    | Using where; Using index

If I and USE INDEX or FORCE INDEX after the FROM db1.tbl1 AS p, it does not change a thing.

My assumption is the problem is that table p isn't using any of the indexes. Is this assumption correct? What are some reasons this query wouldn't use one of the possible keys?

Answer 1

The problem was with the ORDER BY clause. The dbms was attempting to apply it to db1.tbl1 before the joins (apparently). Wrapping the query in a select and putting the ORDER BY outside made the dbms work as expected.

SELECT * FROM 
(SELECT p.val1, p.val2, p.val3, p.val4, p.val5, p.val6, p.val7, p.val8
FROM db1.tbl1 AS p
INNER JOIN db2.tbl2 vp ON p.pid = vp.pid
INNER JOIN db2.tbl1 AS vs ON vp.vid = vs.vid
INNER JOIN db3.tbl1 AS sa ON vs.sid = sa.sid
LEFT JOIN db4.tbl1 AS fs ON p.aid = fs.aid
WHERE sa.id =  '11594'
AND fs.aid IS NULL) AS tmp
ORDER BY IF( (
ISNULL( egl ) 
OR egl =  '' ) , 1, 0
), egl DESC 
LIMIT 15 
OFFSET 0