Re: Costing elided SubqueryScans more nearly correctly

I wrote:
> I instrumented the code in setrefs.c, and found that during the
> core regression tests this patch estimates correctly in 2103
> places while guessing wrongly in 54, so that seems like a pretty
> good step forward.

On second thought, that's not a terribly helpful summary.  Breaking
things down to the next level, there were

   1088 places where we correctly guessed a subquery isn't trivial
        (so no change from current behavior, which is correct)

   1015 places where we correctly guessed a subquery is trivial
        (hence, improving the cost estimate from before)

     40 places where we incorrectly guessed a subquery isn't trivial
        (so no change from current behavior, although that's wrong)

     14 places where we incorrectly guessed a subquery is trivial
        (hence, incorrectly charging zero for the SubqueryScan)

1015 improvements to 14 disimprovements isn't a bad score.  I'm
a bit surprised there are that many removable SubqueryScans TBH;
maybe that's an artifact of all the "SELECT *" queries.