Well, I found and tried the EXPLAIN command on the two-column query. Here's what it gave me:

+-------+--------+---------------+---------+---------+----------------
+----+------+--------------------------+
| table | type   | possible_keys | key     | key_len | ref            
+    | rows | Extra                    |
+-------+--------+---------------+---------+---------+----------------
+----+------+--------------------------+
| B     | ref    | PRIMARY       | PRIMARY |       2 | const          
+    | 3007 | Using where; Using index |
| A     | eq_ref | PRIMARY       | PRIMARY |       4 | const,B.respond
+ent |    1 | Using where; Using index |
+-------+--------+---------------+---------+---------+----------------
+----+------+--------------------------+
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Unfortunately I have no idea what this means. More reading...

Edit by castaway - swapped pre tags for code tags