Most efficient way to find if a polygon falls with in the boundary of another polygon using postgis

Question

I have a polygon layer that contains some polygons inside of polygons. I need to be able to identify the polygons inside of polygons and discard the polygons inside of the polygons. What is the most efficient way of doing that?

Answer 1

If the polygons need to be completely within larger polygons you can use ST_Contains, if they simply need to be touching at some point you can use ST_Intersects.

If they're completely within your union is just going to return the outer boundary of the larger polygon. Do you need to doughnut out the smaller polygons from the larger ones?