Well, when slony gets around to the cases where it needs to alter tables,
it'll be a problem if it's fighting for exclusive locks against the
requests for lesser locks by the read-oriented connections.

But the crucial thing about the LOCK SET is that it forcibly prevents data
from going into the replicated tables so that the queue of updates in
sl_log_{1|2} are able to flush with the guarantee that no more data goes in.

So I'd say the reads aren't 100% unaffected; they're 95% unaffected.  (And
the 5% represents "I WANT EXCLUSIVE LOCKS, NOW!!!  Gotta finish this
upgrade!")

Hopefully that helps?  Knowing what the 5% is represents the battle!  :-)
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