Re: Optimizing this query slows it down

[KimSeong Loh]

With a LEFT JOIN, the left table must always being read first, so the order of the table will be o then m.

When it read table o, what index can it used to filter the rows from the WHERE, the OR causes it to read all rows since mysql version 4 can only use 1 index to read 1 table. MySQL 5.0 is able to use index merge using multiple index and merge the result.
However, your
or ( o.loc=0 and m.code like '%##any:%' )
may not work here since it also contain condition from another table.