MySQL查询优化左联接索引存在

Question

I'm struggling with the optimization of a MySQL query.

select * 
from course as t1 
left join semester as t3 on t1.semester=t3.id 
where t1.visibleFrom <= '1364621522' 
and '1364621522' <= t1.visibleTo 
order by t3.begin desc, t1.name;

This is reported by EXPLAIN to use an ALL-query on the course table. The table has several different indexes (visibleFrom, visibleTo, combination of both, name, semester). name is a varchar column, begin , visibleFrom and visibleTo are integers that can also be null.

If I leave out the join with t3 and force the use of the index name it works somehow.

Any ideas why this query does not make use of indexes?

Answer 1

I think the important index for the range restriction in the WHERE is the index on the two columns visibleFrom and visibleTo, ie

ALTER TABLE `course` ADD INDEX ( `visibleFrom` , `visibleTo` );

If MySQL doesn't use it in your query, my guess would be that the course table has less than ~ 100 rows. Therefore, the MySQL query optimizer decides not to use the index because of the overhead. As soon as the course table contains several hundreds of rows, the index should be used.