If you need to compute the minimum Cartesian distance between two points, you can use the PostGIS ST_Distance function. This function accepts two-point geometries as input parameters and these geometries must be specified in the same spatial reference system.
If the two input geometries are using different spatial references, you can use the ST_Transform function on one or both of them to make them consistent with a single spatial reference system.
To get better results, you should consider the earth's curvature, which is mandatory when measuring large distances, and use the ST_Distance_Sphere or the ST_Distance_Spheroid functions. Alternatively, use ST_Distance, but cast the input geometries to the geography spatial data ...
