Adaptive query execution

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.

I don’t know so much about this, though I’m sure there are all kinds of
research papers discussing it.
Is this something that has been considered before?

Tim