Re: Planner question

Tom Raney <raneyt@cecs.pdx.edu> writes:
> Why does the planner consider both input variations of each symmetric merge join?  The README says "there is not a
lotof difference" between the two options.  When are there any differences?
 

The righthand side needs to support mark/restore, the left doesn't;
so depending on plan types one way might need a helper Materialize
node that the other way doesn't.  Also, duplicated values are a bit
cheaper to process on the left than the right.
        regards, tom lane