Creating indes makes no difference in SELECT statement

[MockY]

I have a table with almost 800,000 rows. Without any indexes (just an auto incremented Id), a simple SELECT statement takes almost 2 seconds.

SELECT Id, Number FROM clients WHERE Number=8596;

 

Creating an index on Number makes no difference. The query takes just as long. Why on earth is that? I figured that if you create an index on the field you have in the WHERE clause, that the query should become so much faster.

The Number field is a VARCHAR(11) by the way.

 

EDIT: For some reason I tried to enclode the number in the where clause in single quotes, and then the query took 0.00 seconds. I've been trying to figure this out all day as I am creating a report file with just 17,000 records, but it has been taken hours and it's not even half done. I made this single quote change and the file was created in a few seconds.

 

Now begs the question, why this enormous change?

 

EDIT2:

I did an explain on the last query and ended up with this

+----+-------------+-------+------+---------------+------+---------+------+------+-----------------------------------------------------+
| id | select_type | table | type | possible_keys | key  | key_len | ref  | rows | Extra                                               |
+----+-------------+-------+------+---------------+------+---------+------+------+-----------------------------------------------------+
|  1 | SIMPLE      | NULL  | NULL | NULL          | NULL | NULL    | NULL | NULL | Impossible WHERE noticed after reading const tables |
+----+-------------+-------+------+---------------+------+---------+------+------+-----------------------------------------------------+

 

Without the quotes, I get this

+----+-------------+----------+------+---------------+------+---------+------+--------+-------------+
| id | select_type | table    | type | possible_keys | key  | key_len | ref  | rows   | Extra       |
+----+-------------+----------+------+---------------+------+---------+------+--------+-------------+
|  1 | SIMPLE      | clients  | ALL  | number_index | NULL | NULL    | NULL | 725713 | Using where |
+----+-------------+----------+------+---------------+------+---------+------+--------+-------------+

 

Even though the quoted query is efficient as heck and creates the results I want, the explain command sure came back with some odd data that makes me kind of confused.

Any help with shedding some light on my problem would be much appreciated.

 

[fenway]

Post the table structure, and the query.