Table scan in join on primary keys??

[Nick Arnett]

I'm updating one table with data from another and finding that the 
server is doing a table scan on the second table, even though it's a 
simple join on the primary keys.  This doesn't seem right.  The query 
looks liket this:

UPDATE a SET a.y TO b.a WHERE a.primary_key = b.primary_key
and if I do the equivalent SELECT:
EXPLAIN SELECT * FROM a, b WHERE a.primary_key = b.primary_key
it shows that it's doing a table scan on table b.
Is this normal?  Is it because primary keys are unique?  I had imagined 
that this would be a very fast operation, but it's not, it's 
surprisingly slow.

Nick
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