Create a functional index in PostGIS

It possible to set an index on the return value of a function.

Say you’re casting a geometry to geography a lot. You have an GIST on the geometry, but that index is not utilized when you cast the geometry to geography.

What you can do to circumvent that without creating a geography column and index that, is to create the index on the return of CAST(your_geometry AS geography) or geography(your_geometry_column.

create index idx_cast_geography_on_your_geometry_column 
on your_table 
using GIST (
	geography(your_geometry_column)
);

source: gis.stackexchange.com