Precede any valid SELECT statement with the word "EXPLAIN" and MySQL will explain to you how it would process that SELECT - how many rows it would have returned, etc, which is a great way to spot problems. The faster of the two queries from above, when used with EXPLAIN, produces the following output:
+--------+------+---------------+------+---------+------+--------+------------+
| table | type | possible_keys | key | key_len | re f | rows | Extra |
+--------+------+---------------+------+---------+------+--------+------------+
| mydata | ALL | NULL | NULL | NULL | NULL | 199999 | where used |
+--------+------+---------------+------+---------+------+--------+------------+
The important parts are "possible_keys", "rows", and "Extra". Possible keys shows "NULL", which means MySQL cannot run the query using indexes - we will be solving that later. Rows shows how many rows MySQL would have had to search through, and "Extra" shows what extra functionality it would have used to perform the search - here we can see it would have filtered the search using the where clause.
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