Get Lon Lat values from a GEOGRAPHY data type in PostGIS

The ST_X(point) function only supports the geometry (as for now).

One workaround is to cast the geog to geom using ::geometry.

Hense your query should be like this:

SELECT id, geog, ST_X(geog::geometry), ST_Y(geog::geometry) FROM locations; 

And since it's geog the SRID would be 4326.

Also here's a good source for the geog type.