Question

I am using MySQl database.

I know if I create a index for a column, it will be fast to query data from a table by using that column index. But, I still have the following questions:

(suppose I have a table named cars, there is a column named country, and I have created index for country column)

1. I know for example the query SELECT * FROM cars WHERE country='japan'will use the index on column country to query data which is fast. How about != operation? will SELECT * FROM cars WHERE country!='japan'; also use index to query data?

2. Does WHERE ... IN ... operation use index to query data? For example SELECT * FROM cars WHERE country IN ('japan','usa','sweden');

Answer 1

You can use EXPLAIN SELECT to find out if your query uses an index or not.

For example:

EXPLAIN SELECT * 
FROM A 
WHERE foo NOT IN (1,4,5,6);

Might yield:

+----+-------------+-------+------+---------------
| id | select_type | table | type | possible_keys 
+----+-------------+-------+------+---------------
|  1 | SIMPLE      | A     | ALL  | NULL          
+----+-------------+-------+------+---------------

+------+---------+------+------+-------------+
| key  | key_len | ref  | rows | Extra       |
+------+---------+------+------+-------------+
| NULL | NULL    | NULL |    2 | Using where |
+------+---------+------+------+-------------+

In this case, the query had no possible_keys and therefore used no key to do the query. It’s the key column you’d be interested in.

More information here:

• http://dev.mysql.com/doc/refman/5.0/en/explain.html
• http://dev.mysql.com/doc/refman/5.0/en/optimization-indexes.html