Indices do not seem to work for OR queries

[Henning Schulzrinne]

Both email columns have the same format (varchar) and are indexed 
individually:

 describe SELECT person FROM person WHERE email1='foo' OR email2='foo';
+--------+------+---------------+------+---------+------+-------+-------------+
| table  | type | possible_keys | key  | key_len | ref  | rows  | Extra 
      |
+--------+------+---------------+------+---------+------+-------+-------------+
| person | ALL  | email1,email2 | NULL |    NULL | NULL | 57051 | Using 
where |
+--------+------+---------------+------+---------+------+-------+-------------+

mysql> describe SELECT person FROM person WHERE email1='foo' and 
email2='foo';
+--------+------+---------------+--------+---------+-------+------+-------------+
| table  | type | possible_keys | key    | key_len | ref   | rows | 
Extra       |
+--------+------+---------------+--------+---------+-------+------+-------------+
| person | ref  | email1,email2 | email1 |     101 | const |    1 | 
Using where |
+--------+------+---------------+--------+---------+-------+------+-------------+

I'm curious why this is. (In general, I have been surprised by how often 
indices are not used even if they exist.)

Henning Schulzrinne



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