15.10 - Tessellation Overview - Teradata Database

Teradata Database SQL Geospatial Types

Teradata Database
Programming Reference

The SELECT request shown in “Methods Specific to the Subtype the ST_Geometry Type Represents” on page 17 specified a join condition of streetShape.ST_Within(cityShape) = 1 to find all of the streets within a table of cities. To analyze the join without the aid of geospatial indexes, Teradata Database would need to check every street against every city, which is a cross product. This can be costly for geospatial types, especially since they can be complex and irregular in shape. A lot of comparisons will be done even if the geospatial objects are not close to each other.

Tessellation is a technique where the object universe is described as a grid with a specified grid size. The tiles within the grid are referred to as cells. Grids can be multilevel, with each level a finer granularity.

Here is an example of a single-level grid (of unspecified size) that contains 16 cells.

To use tessellation, the geospatial objects (for example, streets and cities) of the join are converted into minimum bounding rectangles (MBRs) and placed within this grid (multilevel if desired). During a join operation, instead of comparing every street to every city, only the streets and cities that lie within the same grid cells are compared. The query can use a tessellation method on the geospatial objects to return the cell ids and grid level where the geospatial objects lie.

The following sections of this appendix describe the tessellation functions and methods available in Teradata Database.