Exploring Spatial Relationships: MBRTouches in MariaDB

• MBRTouches Function
  

  • This function takes two geometries (g1 and g2) as input.
  • It analyzes the MBRs of these geometries.
  • It returns 1 (TRUE) if the MBRs touch but their interiors don't intersect.
  • It returns 0 (FALSE) if the MBRs don't touch or their interiors intersect.

• MBR (Minimum Bounding Rectangle)
  Imagine a rectangle that tightly encloses a geometric object. This rectangle captures the object's extent but might not perfectly match its shape. MBRTouches operates on these rectangles, not the actual geometries themselves.

Here are some additional points to consider:

• It's useful for initial spatial filtering or coarse-grained analysis.
• MBRTouches is a simpler and faster operation compared to functions that check for more complex spatial relationships (like intersection or overlap) because it only deals with MBRs.

Example

This example from the MariaDB Knowledge Base demonstrates MBRTouches in action:

SET @g1 = GeomFromText('Polygon((0 0,0 3,3 3,3 0,0 0))');  -- Square with coordinates
SET @g2 = GeomFromText('Polygon((3 3,3 6,6 6,6 3,3 3))');  -- Another square

-- Check if they touch (corner to edge)
SELECT mbrtouches(@g1,@g2);

-- Check if they don't touch (no contact)
SET @g2 = GeomFromText('Polygon((4 4,4 7,7 7,7 4,4 4))');
SELECT mbrtouches(@g1,@g2);

---

Example 1: Combining MBRTouches with MBRIntersects

This example checks for both touching and intersecting geometries:

SET @g1 = GeomFromText('Polygon((0 0,0 3,3 3,3 0,0 0))');  -- Square
SET @g2 = GeomFromText('Polygon((1 1,1 4,4 4,4 1,1 1))');  -- Square touching and intersecting

-- Check if touches (1) or intersects (2)
SELECT 
  CASE 
    WHEN MBRTouches(@g1, @g2) THEN 1
    WHEN MBRIntersects(@g1, @g2) THEN 2
    ELSE 0
  END AS relationship;

• The case statement checks the results of MBRTouches and MBRIntersects:
  • If MBRTouches returns 1, it sets the relationship to 1 (touching).
  • If MBRIntersects returns 1, it sets the relationship to 2 (intersecting).
  • Otherwise, it sets the relationship to 0 (no relation).
• @g2 is positioned to both touch and intersect @g1.
• We define two squares: @g1 and @g2.

Example 2: Using MBRTouches for Spatial Filtering

This example finds points near a rectangle but avoids points within the rectangle:

SET @boundary = GeomFromText('Polygon((0 0,0 10,10 10,10 0,0 0))');  -- Rectangular boundary
SET @point_table = (
  SELECT id, GeomFromText(POINT(X, Y)) AS point FROM point_data
);

-- Find points that touch the boundary but aren't inside
SELECT * FROM @point_table
WHERE MBRTouches(point, @boundary);

• The WHERE clause selects points where MBRTouches between the point and the boundary returns 1 (touching). This eliminates points that fall within the rectangle.
• We filter the points using a temporary table (@point_table).
• We have a table point_data containing points with IDs and location data.
• We define a rectangular boundary (@boundary).

---

1. MBRIntersects
   This function checks if the Minimum Bounding Rectangles (MBRs) of two geometries intersect. It's a broader check compared to MBRTouches and returns TRUE if any part of the MBRs overlap, including complete enclosure.

2. ST_Intersects
   This is a more robust function that analyzes the actual geometries, not just their MBRs. It returns TRUE if the interiors of the geometries have any overlap. This provides a more accurate picture of the spatial relationship.

3. ST_Within
   This function checks if one geometry is completely inside another. It returns TRUE if the geometry being queried is entirely within the other geometry.

4. ST_Contains
   This is the opposite of ST_Within. It returns TRUE if the geometry being queried completely contains the other geometry.

Function	Description
MBRTouches	Checks if MBRs touch (not intersect)
MBRIntersects	Checks if MBRs intersect (may overlap)
ST_Intersects	Checks if geometries themselves intersect
ST_Within	Checks if one geometry is completely inside another
ST_Contains	Checks if one geometry completely contains another

Choosing the best alternative depends on your specific needs:

• For accurate spatial relationship checks
  ST_Intersects, ST_Within, or ST_Contains provide more precise results by analyzing the actual geometries.
• For initial filtering or coarse-grained analysis
  MBRTouches or MBRIntersects can be efficient due to their reliance on MBRs.

-- Two squares touching at a corner
SET @g1 = GeomFromText('Polygon((0 0,0 2,2 2,2 0,0 0))');
SET @g2 = GeomFromText('Polygon((2 0,2 2,4 2,4 0,2 0))');

-- Check with different functions
SELECT MBRTouches(@g1, @g2);  -- Returns 1 (TRUE) - Touching MBRs
SELECT MBRIntersects(@g1, @g2); -- Returns 1 (TRUE) - Intersecting MBRs
SELECT ST_Intersects(@g1, @g2); -- Returns 0 (FALSE) - No intersection