On Tue, May 13, 2014 at 5:08 PM, Tim Kane <tim.kane@xxxxxxxxx> wrote: > Hi all, > > So I was thinking about the following, after experimenting with constraint > exclusion. > > I thought I would see what happens when I do this: > > SELECT * FROM ONLY table_a UNION SELECT * FROM table_b; > > > I noticed that despite table_a still having no data in it, the planner has > already decided that it needs to insert a chain of ‘append->sort->unique’ > nodes into the plan. > > That’s fairly reasonable. > While I understand that we can’t readily know about wether a given node will > return anything or not - would it be possible to have the execution engine > branch off in the event that a given node returns nothing at all? > > I guess there are probably a lot of considerations, and I suspect it would > considerably increase planning time, though maybe it also presents an > opportunity for some interesting approaches to adaptive query execution. What's the point, in the context of this example? The sort-unique still has to be performed even if you didn't have data in one side, since the other could still have duplicates.
