Database Software

I'm trying to speed up a query against a geometry lookup and I don't know where to put the index. What is happening is I'm creating a polygon and based on its centroid, selecting a value (G.ID) to insert into a column. The sample of my case statement is:

WHEN :new.ZONE = 'ETMA1' THEN

                         SELECT G.ID into :new.WU

                         FROM GRID_ETMA1 G

                         WHERE

                         (sde.st_intersects (st_centroid(:new.shape), G.shape) = 1)

               AND.  (sde.st_within (st_centroid(:new.shape), G.shape) = 1);

Any advise or rewrite is appreciated.

Jeff