Re: foreign key locks

Here's version 28 of this patch.  The only substantive change here from
v26 is that I've made GetTupleForTrigger() use either LockTupleExclusive
or LockTupleNoKeyExclusive, depending on whether the key columns are
being modified by the update.  This needs to go through EvalPlanQual, so
that function is now getting the lock mode as a parameter instead of
hardcoded LockTupleExclusive.  (All other users of GetTupleForTrigger
still use LockTupleExclusive, so there's no change for anybody other
than FOR EACH ROW BEFORE UPDATE triggers).

At this point, I think I've done all I wanted to do in connection with
this patch, and I intend to commit.  I think this is a good time to get
it committed, so that people can play with it more extensively and
report any breakage I might have caused before we even get into beta.

-- 
Álvaro Herrera                http://www.2ndQuadrant.com/
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