On Wed, Jan 18, 2017 at 6:19 PM, Andres Freund <and...@anarazel.de> wrote: >> SELECT x, CASE WHEN x > 0 THEN generate_series(1, 5) ELSE 0 END FROM tab; >> >> It might seem that this should produce five repetitions of input rows >> that have x > 0, and a single repetition of those that do not; but >> actually it will produce five repetitions of every input row. This is >> because generate_series() is run first, and then the CASE expression is >> applied to its result rows. The behavior is thus comparable to >> >> SELECT x, CASE WHEN x > 0 THEN g ELSE 0 END >> FROM tab, LATERAL generate_series(1,5) AS g; >> >> It would be exactly the same, except that in this specific example, the >> planner could choose to put g on the outside of the nestloop join, since >> g has no actual lateral dependency on tab. That would result in a >> different output row order. Set-returning functions in the select list >> are always evaluated as though they are on the inside of a nestloop join >> with the rest of the FROM clause, so that the function(s) are run to >> completion before the next row from the FROM clause is considered. >> >> So is this too ugly to live, or shall we put up with it? > > I'm very tentatively in favor of living with it.
So, one of the big reasons I use CASE is to avoid evaluating expressions in cases where they might throw an ERROR. Like, you know: CASE WHEN d != 0 THEN n / d ELSE NULL END I guess it's not the end of the world if that only works for non-set-returning functions, but it's something to think about. -- Robert Haas EnterpriseDB: http://www.enterprisedb.com The Enterprise PostgreSQL Company -- Sent via pgsql-hackers mailing list (pgsql-hackers@postgresql.org) To make changes to your subscription: http://www.postgresql.org/mailpref/pgsql-hackers
