Key reads are slower. When you read a key, the MERGE storage engine needs to issue a read on all underlying tables to check which one most closely matches the given key. To read the next key, the MERGE storage engine needs to search the read buffers to find the next key. Only when one key buffer is used up does the storage engine need to read the next key block. This makes MERGE keys much slower on eq_ref searches, but not much slower on ref searches. See Section 12.3.2, “EXPLAIN Syntax”, for more information about eq_ref and ref.
http://dev.mysql.com/doc/refman/5.0/...ge-engine.html
http://dev.mysql.com/doc/refman/5.0/...ointype_eq_ref

ma sei certo di avere la necessita' di accedere a 1 giga di messaggi ogni volta?