Re: "SELECT ... FROM DUAL" is not quite as silly as it appears

On Thu, Mar 15, 2018 at 11:27 AM, Tom Lane <tgl@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.

Good idea.

-- 
Robert Haas
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