On Thu, Mar 15, 2018 at 8:57 PM, Tom Lane <t...@sss.pgh.pa.us> wrote: > We've long made fun of Oracle(TM) for the fact that if you just want > to evaluate some expressions, you have to write "select ... from dual" > rather than just "select ...". But I've realized recently that there's > a bit of method in that madness after all. Specifically, having to > cope with FromExprs that contain no base relation is fairly problematic > in the planner. prepjointree.c is basically unable to cope with > flattening a subquery that looks that way, although we've inserted a > lot of overly-baroque logic to handle some subsets of the case (cf > is_simple_subquery(), around line 1500). If memory serves, there are > other places that are complicated by the case. > > Suppose that, either in the rewriter or early in the planner, we were > to replace such cases with nonempty FromExprs, by adding a dummy RTE > representing a table with no columns and one row. This would in turn > give rise to an ordinary Path that converts to a Result plan, so that > the case is handled without any special contortions later. Then there > is no case where we don't have a nonempty relids set identifying a > subquery, so that all that special-case hackery in prepjointree.c > goes away, and we can simplify whatever else is having a hard time > with it. >
The idea looks neat. Since table in the dummy FROM clause returns one row without any column, I guess, there will be at least one row in the output. I am curious how would we handle cases which do not return any row like create function set_ret_func() returns setof record as $$select * from pg_class where oid = 0;$$ language sql; select set_ret_func(); set_ret_func -------------- (0 rows) -- Best Wishes, Ashutosh Bapat EnterpriseDB Corporation The Postgres Database Company
