Author: Tom Lane <tgl@sss.pgh.pa.us>
Date:   Tue Jul 8 14:03:32 2014 -0400

    While the x output of "select x from t group by x" can be presumed unique,
    this does not hold for "select x, generate_series(1,10) from t group by x",
    because we may expand the set-returning function after the grouping step.
    (Perhaps that should be re-thought; but considering all the other oddities
    involved with SRFs in targetlists, it seems unlikely we'll change it.)
    Put a check in query_is_distinct_for() so it's not fooled by such cases.

    Back-patch to all supported branches.

    David Rowley

(cherry picked from commit 2e7469dc8b3bac4fe0f9bd042aaf802132efde85)