Using an index vs. full table scan, character vs. integers.

[kevin]

I've got what are surprising (to me!) results from a very simple query. I'm hoping that some of the experts around here could confirm that my understanding of things is correct.

Given the table:
mysql> desc my_table;
+-----------+---------+------+-----+---------+-------+
| Field | Type | Null | Key | Default | Extra |
+-----------+---------+------+-----+---------+-------+
| year | char(4) | | PRI | | |
| id | char(5) | | PRI | | |
+-----------+---------+------+-----+---------+-------+
2 rows in set (0.00 sec)

which has a unique index on both fields:

create unique index my_index_1 on my_table (year, id);

and about 38,000 rows in it.

and given these two queries against the table:

mysql> explain select * from my_table where year = 2003 and id = 38590 \G
*************************** 1. row ***************************
table: my_table
type: index
possible_keys: my_index_1
key: my_index_1
key_len: 9
ref: NULL
rows: 38487
Extra: Using where; Using index
1 row in set (0.00 sec)

mysql> explain select * from my_table where year = '2003' and id = '38590' \G
*************************** 1. row ***************************
table: my_table
type: const
possible_keys: my_index_1
key: my_index_1
key_len: 9
ref: const,const
rows: 1
Extra:
1 row in set (0.00 sec)

Both queries have a "key: my_index_1" which makes me think that my index is being used, but the "rows: 38487" for the first query makes me wonder if the first query is acutally doing a full table scan, because of some sort of integer to character conversion. could someone explain?

Thanks!
Kevin