Geography Methods Supported by Spatial Indexes
Under certain conditions, spatial indexes support the following set-oriented geography methods: STIntersects(),STEquals(), and STDistance(). To be supported by a spatial index, these methods must be used within the WHERE clause of a query, and they must occur within a predicate of the following general form:
To return a non-null result, geography1 and geography2 must have the same Spatial Reference Identifier (SRID). Otherwise, the method returns NULL.
Spatial indexes support the following predicate forms:
geography1.STDistance(geography2) < number
geography1.STDistance(geography2) <= number
In either form of STDistance() predicate, at least one of geography arguments (geography1 or geography2) must specify a geography instance that represents a point. Otherwise, the method returns NULL.