SELECT count(*) FROM propositions p, output o WHERE p.verb_id=o.verb_id AND p.tag=o.tag AND (p.stop!=o.stop OR p.start!=o.start);
I don't think this will be much help and is very implementation specific, I expect, but....
Your first two where clauses are effectively a "JOIN USING verb_id,tag", which has a much better chance of using any built in optimizations for cross indexing.
Hopefully someone much more knowledgable than I will pipe up, but I hope this helps a little.
michael
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