## Correspondences between SQL Functions and DE-9IM Intersection Matrices

Quick recall:- The standard Spatial SQL model defines several functions (as e.g.
**ST_Equals()**,**ST_Intersects()**,**ST_Touches()**and alike) intended to test if a given spatial relationship exists between a pair of Geometries. - All such SQL functions are simply convenience wrappers based on the most general DE-9IM model.
- An alternative (more flexible, more generic and more efficient) approach for evaluating spatial relationships could be based on the following two
SQL functions:*advanced***ST_Relate (***g1*Geometry ,*g2*Geometry**) :***intersection_matrix*Text

This first function will evaluate both Geometries, then returning a Text string corresponding to a nine digits serialized DE-9IM**intersection_matrix**as e.g.**'0F1FF0FF2'**or**'212101212'**with the following interpretation:**F**: this stands for**FALSE**, i.e. the corresponding intersection doesn't occurs.**0**: the corresponding intersection is a Point, i.e. a zero-dimensions Geometry.**1**: the corresponding intersection is a Linestring, i.e. a one-dimension Geometry.**2**: the corresponding intersection is a Polygon, i.e. a two-dimensions Geometry.

**ST_RelateMatch (***intersection_matrix*Text ,*reference_pattern*Text**) :***true_or_false*Boolean

This second function is intended to evaluate an**intersection matrix**returned by a previous call to**ST_Relate()**against a given**reference_pattern**, then returning**TRUE**or**FALSE**.

A**reference_pattern**as e.g.**'T*F**FFF*'**or**'0********'**simply is yet another nine digits serialized DE-9IM matrix adopting a slightly changed syntax:**F**,**0**,**1**and**2**: same interpretation as before.**T**: this stands for**TRUE**, i.e. the corresponding intersection is expected to occur, but its specific nature doesn't matters.*****: an asterisk is a**mask sign**and always meansthis matrix cell.*ignore / don't care / skip*

__Note__:**ST_Relate()**is a computationally heavy function (exactly as**ST_Equals()**,**ST_Crosses()**and alike are); on the other hand**ST_RelateMatch()**is a very light and quick function.

So when you have to check for more than a single relationship at the same time, calling**ST_Relate()**just once and then evaluating each spatial relationship by calling**ST_RelateMatch()**multiple times will always ensure noticeably faster performances.__Conclusion__: using the quirky and awkward DE-9IM model surely is neither easy or intuitive.

The following table is specifically intended to make the usage of both**ST_Relate()**and**ST_RelateMatch()**easier, in a reasonably painless way.

SQL function | Reference Pattern |
---|---|

ST_Equals( g1, g2 ) | T*F**FFF* |

ST_Disjoint( g1, g2 ) | FF*FF**** |

ST_Touches( g1, g2 ) | FT******* |

F**T***** | |

F***T**** | |

ST_Within( g1, g2 ) | T*F**F*** |

ST_Overlaps( g1, g2 ) | T*T***T** |

1*T***T** | |

ST_Crosses( g1, g2 ) | T*T****** |

T*****T** | |

0******** | |

ST_Intersects( g1, g2 ) | T******** |

*T******* | |

***T***** | |

****T**** | |

ST_Contains( g1, g2 ) | T*****FF* |

ST_Covers( g1, g2 ) | T*****FF* |

*T****FF* | |

***T**FF* | |

****T*FF* | |

ST_CoveredBy( g1, g2 ) | T*F**F*** |

*TF**F*** | |

**FT*F*** | |

**F*TF*** |

Further useful readings:

- a general introduction to DE-9IM
- a well-written documentation from a Proprietary SW Vendor

### A practical example (and related objective timings)

SELECT m.name AS municipality, p.name AS province, ST_Disjoint(m.geom, p.geom) AS disjoint, ST_Touches(m.geom, p.geom) AS touches, ST_Within(m.geom, p.geom) AS within, ST_Intersects(m.geom, p.geom) AS intersects, ST_Overlaps(m.geom, p.geom) AS overlaps, ST_CoveredBy(m.geom, p.geom) AS covered_by FROM municipalities AS m, provinces AS p;In this first test I've followed the most

**classical approach**to determine several spatial relationships intercurring between:

- the
**municipalities**table (MultiPolygons), containing the**276**Municipalites of Tuscany. - and the
**provinces**table (MultiPolygons), containing the**10**Provinces of Tuscany. - this is an unsophisticated query, and the resulting
**cartesian product**of both tables contains**2.760**rows.

__Measured timing__:

**67 secs**

CREATE TEMPORARY TABLE tmp_relate AS SELECT m.name AS municipality, p.name AS province, ST_Relate(m.geom, p.geom) AS matrix FROM municipalities AS m, provinces AS p; SELECT municipality, province, matrix, ST_RelateMatch(matrix, 'FF*FF****') AS disjoint, ST_RelateMatch(matrix, 'FT*******') OR ST_RelateMatch(matrix, 'F**T*****') OR ST_RelateMatch(matrix, 'F***T****') AS touches, ST_RelateMatch(matrix, 'T*F**F***') AS within, ST_RelateMatch(matrix, 'T********') OR ST_RelateMatch(matrix, '*T*******') OR ST_RelateMatch(matrix, '***T*****') OR ST_RelateMatch(matrix, '****T****') AS intersects, ST_RelateMatch(matrix, 'T*T***T**') AS overlaps, ST_RelateMatch(matrix, 'T*F**F***') OR ST_RelateMatch(matrix, '*TF**F***') OR ST_RelateMatch(matrix, '**FT*F***') OR ST_RelateMatch(matrix, '**F*TF***') AS covered_by FROM tmp_relate;Using the same tables as before in this second test, but this time I've adopted the alternative approach based on

**ST_Relate()**and

**ST_RelateMatch()**.

After some preliminary tests, it quickly emerged that the SQLite optimizer doesn't like the mixture of an inner query together with function calls on the same overall query (resulting in awful timings), so I duly switched to an indirect

**two-steps approach**:

**step #1**: computing**ST_Related()**and saving the whole resultset into a**temporary table**.**step #2**: completing the task by calling**ST_RelateMatch**on the above temporary table.

__Measured timing__:

**22 secs**

__Note__: the second query (ST_RelateMatch) only required a few milliseconds to complete; the real computational load was entirely confined within the first query (ST_Relate).

__Final conclusion__: directly using the awkward DE-9IM model is in someways difficult, but it does ensure an astonishing performance boost.

It's usage is highly recommended when you are required to check more than a single spatial relationship between the same pair of Geometries.