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Section 10.5 didn't say explicitly that multiple UNIONs are resolved pairwise. Since the resolution algorithm is described as taking any number of inputs, readers might well think that a query like "select x union select y union select z" would be resolved by considering x, y, and z in one resolution step. But that's not what happens (and I think that behavior is per SQL spec). Add an example clarifying this point. Per bug #15129 from Philippe Beaudoin. Discussion: https://postgr.es/m/152196085023.32649.9916472370480121694@wrigleys.postgresql.org