So I started re-reading this thread in preparation for looking at the
patch, and this bit in your initial message jumped out at me:

> In all of our join algorithms in the executor, if the join type is SEMI,
> we skip to the next outer row once we find a matching inner row. This is
> because we don't want to allow duplicate rows in the inner side to
> duplicate outer rows in the result set. Obviously this is required per SQL
> spec. I believe we can also skip to the next outer row in this case when
> we've managed to prove that no other row can possibly exist that matches
> the current outer row, due to a unique index or group by/distinct clause
> (for subqueries).

I wondered why, instead of inventing an extra semantics-modifying flag,
we couldn't just change the jointype to *be* JOIN_SEMI when we've
discovered that the inner side is unique.

Now of course this only works if the join type was INNER to start with.
If it was a LEFT join, you'd need an "outer semi join" jointype which
we haven't got at the moment.  But I wonder whether inventing that
jointype wouldn't let us arrive at a less messy handling of things in
the executor and EXPLAIN.  I'm not very enamored of plastering this
"match_first_tuple_only" flag on every join, in part because it doesn't
appear to have sensible semantics for other jointypes such as JOIN_RIGHT.
And I'd really be happier to see the information reflected by join type
than a new line in EXPLAIN, also.

                        regards, tom lane

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