Oracle spatial: Point cluster vs MultiPoint?

Question

I'm reading Oracle's documentation and i can't see the difference between Point cluster and MultiPoint element type.

The following example code is from the documentation

INSERT INTO t1 (i, d, g)
VALUES (
 16,
 'Point cluster',
 sdo_geometry (2005, null, null, sdo_elem_info_array (1,1,3), 
 sdo_ordinate_array (50,5, 55,7, 60,5))
);
INSERT INTO t1 (i, d, g)
VALUES (
 17,
 'Multipoint',
 sdo_geometry (2005, null, null, sdo_elem_info_array (1,1,1, 3,1,1, 5,1,1), 
 sdo_ordinate_array (65,5, 70,7, 75,5))
);

It seems that:

Point cluster is one element (of many points -in that example 3 points: 50,5, 55,7, 60,5) as it needs one triplet in sdo_elem_info_array

MultiPoint is many elements ( and each element is a Point -in that example 3 points: 65,5, 70,7, 75,5) as it needs three triplets in sdo_elem_info_array

And further down documentation refers to them as the same thing(?):

2.5.8 Several Geometry Types
Example 2-13 creates a table and inserts various geometries,
including multipoints (point clusters), multipolygons, and collections...

What is this mess? What's the difference? The sdo_geometry requires different sdo_elem_info_array parameters so there must be a difference between those types?

Answer 1

You are creating the same MultiPoint geometry with different constructor parameters for SDO_ELEM_INFO_ARRAY. You are defining the GTYPE of your geometry with the first parameter of the constructor (2005) where the 5 says that your geometry is a MultiPoint!

You can also check the GTYPE of your geometry like this:

SELECT x.g.GET_GTYPE() AS geometry_type FROM t1 x WHERE x.i = 16;
-- returns 5
SELECT x.g.GET_GTYPE() AS geometry_type FROM t1 x WHERE x.i = 17;
-- returns 5

The difference is just in the creation of the SDO_ELEM_INFO_ARRAY which takes attributes of the type SDO_ELEM_INFO. Each SDO_ELEM_INFO is a triple of numbers where the digits mean:

1. SDO_STARTING_OFFSET: The index in SDO_ORDINATE_ARRAY where this (sub-)geometry begins.
2. SDO_ETYPE: The geometry (element) type of the (sub-)geometry (where 1 means point!)
3. SDO_INTERPRETATION: The specification of the element type (which relies on the SDO_ETYPE). For points
   • 1 means a single point -> SFA: Point
   • n> 1 means a collection of n points -> SFA:MultiPoint

Your first point (id 16) is created using a value of 3 for SDO_INTERPRETATION which means that your ordinates are interpreted as a collection of points.

The second point (id 17) is created using a value of 1 for SDO_INTERPRETATION which means that your ordinates are interpreted as single points. This is why the SDO_STARTING_OFFSET has to specify the start index for each of your points (1, 3, 5).