Question

I’m having a little trouble getting MySQL to put an INDEX() on the right fields. I have a query that would look, for example, something like this:

SELECT m.id as mid,p.id as pid,po.* FROM m 
LEFT OUTER JOIN p ON p.mid = m.id AND p.uid = 2 
LEFT OUTER JOIN po ON po.mid = m.id AND po.uid = 2 
ORDER BY m.date,m.time

And I have the following indexes:

• m.date
• m.time
• p.mid
• p.uid
• po.mid
• po.uid

However, when I run an EXPLAIN query, none of the keys are being picked up. I tried to put FORCE INDEX statements in, but MySQL keeps rejecting them (saying there is a syntax error in the query). Any ideas?

EDIT

Here’s the EXPLAIN:

id  select_type table   type    possible_keys   key key_len ref rows    Extra
1   SIMPLE  m   ALL NULL    NULL    NULL    NULL    31  Using filesort
1   SIMPLE  p   ref mid,uid mid 5   db.m.id 1   
1   SIMPLE  po  ref uid,mid uid 5   const   1   

Note that p and po are being handle fine now, but m still isn’t. I just added an INDEX(date, time) for m, and still got the same.

Answer 1

FORCE INDEX is usually used after the table name, i.e.

... FROM <table_name> FORCE INDEX (<index_name>)

But I don’t recommend using it. If you’d like it to be ordered using index, you may create compound index IDX_date_time on (m.date, m.time).