The fact that the estimator knows that the LIMIT is pointless because there
are less rows in the subselect than the LIMIT will return is not something we
want to count on; sometimes the estimator has innaccurate information. The
UNIQUE index makes this more certain, except that I'm not sure that the
planner distinguishes between actual UNIQUE indexes and columns which are
estimated unique (per the pg_stats). And I think you can see in your case
that there's quite a difference between a column we're CERTAIN is unique,
versus a column we THINK is unique.
I think a UNIQUE constraint can permit several 'different' NULL values... better say "UNIQUE NOT NULL" ?
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