11. Spatial Extensions in MySQL

MySQL 4.1 introduces spatial extensions to allow the generation, storage, and analysis of geographic features. This chapter covers the following topics:

• The basis of these spatial extensions in the OpenGIS geometry model
• Data formats for representing spatial data
• How to use spatial data in MySQL
• Use of indexing for spatial data
• MySQL differences from the OpenGIS specification

11.1 Introduction

MySQL implements spatial extensions following the specification of the Open GIS Consortium (OGC). This is an international consortium of more than 250 companies, agencies, and universities participating in the development of publicly available conceptual solutions that can be useful with all kinds of applications that manage spatial data. The OGC maintains a web site at http://www.opengis.org/.

In 1997, the Open GIS Consortium published the OpenGIS (R) Simple Features Specifications For SQL, a document that proposes several conceptual ways for extending an SQL RDBMS to support spatial data. This specification is available from the Open GIS web site at http://www.opengis.org/techno/implementation.htm. It contains additional information relevant to this chapter.

MySQL implements a subset of the SQL with Geometry Types environment proposed by OGC. This term refers to an SQL environment that has been extended with a set of geometry types. A geometry-valued SQL column is implemented as a column that has a geometry type. The specifications describe a set of SQL geometry types, as well as functions on those types to create and analyse geometry values.

A geographic feature is anything in the world that has a location. A feature can be:

• An entity. For example, a mountain, a pond, a city.
• A space. For example, a postcode area, the tropics.
• A definable location. For example, a crossroad, as a particular place where two streets intersect.

You can also find documents that use term geospatial feature to refer to geographic features.

Geometry is another word that denotes a geographic feature. The original meaning of the word geometry denotes a branch of mathematics. Another meaning comes from cartography, referring to the geometric features that cartographers use to map the world.

This chapter uses all of these terms synonymously: geographic feature, geospatial feature, feature, or geometry. The term most commonly used here is geometry.

Let's define a geometry as a point or an aggregate of points representing anything in the world that has a location.

11.2 The OpenGIS Geometry Model

The set of geometry types proposed by OGC's SQL with Geometry Types environment is based on the OpenGIS Geometry Model. In this model, each geometric object has the following general properties:

• It is associated with a Spatial Reference System, which describes the coordinate space in which the object is defined.
• It belongs to some geometry class.

11.2.1 The Geometry Class Hierarchy

The geometry classes define a hierarchy as follows:

• Geometry (non-instantiable)
  • Point (instantiable)
  • Curve (non-instantiable)
    • LineString (instantiable)
      • Line
      • LinearRing
  • Surface (non-instantiable)
    • Polygon (instantiable)
  • GeometryCollection (instantiable)
    • MultiPoint (instantiable)
    • MultiCurve (non-instantiable)
      • MultiLineString (instantiable)
    • MultiSurface (non-instantiable)
      • MultiPolygon (instantiable)

Some of these classes are abstract (non-instantiable). That is, it is not possible to create an object of these classes. Other classes are instantiable and objects may be created of them. Each class has properties and instantiable classes may have assertions (rules that define valid class instances).

Geometry is the base class. It's an abstract class. The instantiable subclasses of Geometry are restricted to zero-, one-, and two-dimensional geometric objects that exist in two-dimensional coordinate space. All instantiable geometry classes are defined so that valid instances of a geometry class are topologically closed (that is, all defined geometries include their boundary).

The base Geometry class has subclasses for Point, Curve, Surface and GeometryCollection:

• Point represents zero-dimensional objects.
• Curve represents one-dimensional objects, and has subclass LineString, with sub-subclasses Line and LinearRing.
• Surface is designed for two-dimensional objects and has subclass Polygon.
• GeometryCollection has specialised zero-, one-, and two-dimensional collection classes named MultiPoint, MultiLineString, and MultiPolygon for modelling geometries corresponding to collections of Points, LineStrings, and Polygons, respectively. MultiCurve and MultiSurface are introduced as abstract superclasses that generalise the collection interfaces to handle Curves and Surfaces.

Geometry, Curve, Surface, MultiCurve, and MultiSurface are defined as non-instantiable classes. They define a common set of methods for their subclasses and are included for the reason of extensibility.

Point, LineString, Polygon, GeometryCollection, MultiPoint, MultiLineString, and MultiPolygon are instantiable classes.

11.2.2 Class Geometry

Geometry is the root class of the hierarchy. It is a non-instantiable class but has a number of properties that are common to all geometry values created from any of the Geometry subclasses. These properties are described in the following list. (Particular subclasses have their own specific properties, described later.)

Geometry properties

A geometry value has the following properties:

• Its type. Each geometry belongs to one of the instantiable classes in the hierarchy.
• Its SRID, or Spatial Reference Identifier. This value identifies the geometry's associated Spatial Reference System that describes the coordinate space in which the geometry object is defined.
• Its coordinates in its Spatial Reference System, represented as double-precision (8-byte) numbers. All non-empty geometries include at least one pair of (X,Y) coordinates. Empty geometries contain no coordinates.

  Coordinates are related to the SRID. For example, in different coordinate systems, the distance between two objects may differ even when objects have the same coordinates, because the distance on the planar coordinate system and the distance on the geocentric system (coordinates on the Earth's surface) are different things.

• Its interior, boundary, and exterior. All geometries occupy some position in space. The exterior of a geometry is all space not occupied by the geometry. The interior is the space occupied by the geometry. The boundary is the interface between geometry's interior and exterior.
• Its MBR (Minimum Bounding Rectangle), or Envelope. This is the bounding geometry, formed by the minimum and maximum (X,Y) coordinates:

  ((MINX MINY, MAXX MINY, MAXX MAXY, MINX MAXY, MINX MINY))

• The quality of being simple or non-simple. Geometry values of some types (LineString, MultiPoint, MultiLineString) are either simple of non-simple. Each type determines its own assertions for being simple or non-simple.
• The quality of being closed or not closed. Geometry values of some types (LineString, MultiString) are either closed or not closed. Each type determines its own assertions for being closed or not closed.
• The quality of being empty or not empty A geometry is empty if it does not have any points. Exterior, interior and boundary of an empty geometry are not defined (that is, they are represented by a NULL value). An empty geometry is defined to be always simple and has an area of 0.
• Its dimension. A geometry can have a dimension of -1, 0, 1, or 2:
  • -1 stands for empty geometries.
  • 0 stands for geometries with no length and no area.
  • 1 stands for geometries with non-zero length and zero area.
  • 2 stands for geometries with non-zero area.

  Point objects have a dimension of zero. LineString objects have a dimension of 1. Polygon objects have a dimension of 2. The dimensions of MultiPoint, MultiLineString, and MultiPolygon objects are the same as the dimensions of the elements they consist of.

11.2.3 Class Point

A Point is a geometry that represents a single location in coordinate space.

Point Examples

• Imagine a large-scale map of the world with many cities. A point could represent each city.
• On a city map, a Point could represent a bus stop.

Point Properties

• X-coordinate value.
• Y-coordinate value.
• Point is defined as a zero-dimensional geometry.
• The boundary of a Point is the empty set.

11.2.4 Class Curve

A Curve is a one-dimensional geometry, usually represented by a sequence of points. Particular subclasses of Curve define the type of interpolation between points. Curve is a non-instantiable class.

Curve Properties

• The coordinates of its points.
• Curve is defined as one-dimensional geometry.
• A Curve is simple if it does not pass through the same point twice.
• A Curve is closed if its start point is equal to its end point.
• The boundary of a closed Curve is empty.
• The boundary of a non-closed Curve consists of its two end points.
• A Curve that is simple and closed is a LinearRing.

11.2.5 Class LineString

A LineString is a Curve with linear interpolation between points.

LineString Examples

• On a world map, LineString objects could represent rivers.
• In a city map, LineString objects could represent streets.

LineString Properties

• Coordinates of LineString segments, defined by each consecutive pair of points.
• A LineString is a Line if it consists of exactly two points.
• A LineString is a LinearRing if it's both closed and simple.

11.2.6 Class Surface

A Surface is a two-dimensional geometry. It is a non-instantiable class. Its only instantiable subclass is Polygon.

Surface Properties

• A Surface is defined as a two-dimensional geometry.
• The OpenGIS specification defines a simple Surface as a geometry that consists of a single "patch" that is associated with a single exterior boundary and zero or more interior boundaries.
• The boundary of a simple Surface is the set of closed curves corresponding to its exterior and interior boundaries.

11.2.7 Class Polygon

A Polygon is a planar Surface representing a multisided geometry. It is defined by a single exterior boundary and zero or more interior boundaries, where each interior boundary defines a hole in the Polygon.

Polygon Examples

• On a region map, Polygon objects could represent forests, districts, etc.

Polygon Assertions

• The boundary of a Polygon consists of a set of LinearRings (that is, LineStrings that are both simple and closed) that make up its exterior and interior boundaries.
• No two rings in the boundary cross. The rings in the boundary of a Polygon may intersect at a Point, but only as a tangent.
• A Polygon may not have cut lines, spikes, or punctures.
• The interior of every Polygon is a connected point set.
• The exterior of a Polygon with one or more holes is not connected. Each hole defines a connected component of the exterior.

In the above assertions, polygons are simple geometries. These assertions make a Polygon a simple geometry.

11.2.8 Class GeometryCollection

A GeometryCollection is a geometry that is a collection of one or more geometries of any class.

All the elements in a GeometryCollection must be in the same Spatial Reference System (that is, in the same coordinate system). GeometryCollection places no other constraints on its elements, although the subclasses of GeometryCollection described in the following sections may restrict membership. Retrictions may be based on:

• Element type (for example, a MultiPoint may contain only Point elements)
• Dimension
• Constraints on the degree of spatial overlap between elements

11.2.9 Class MultiPoint

A MultiPoint is a geometry collection composed of Point elements. The points are not connected or ordered in any way.

MultiPoint Examples

• On a world map, a Multipoint could represent a chain of small islands.
• On a city map, a Multipoint could represent the outlets for a ticket office.

MultiPoint Properties

• MultiPoint is defined as a zero-dimensional geometry.
• A MultiPoint is simple if no two of its Point values are equal (have identical coordinate values).
• The boundary of a MultiPoint is the empty set.

11.2.10 Class MultiCurve

A MultiCurve is a geometry collection composed of Curve elements. MultiCurve is a non-instantiable class.

MultiCurve Properties

• MultiCurve is defined as a one-dimensional geometry.
• A MultiCurve is simple if and only if all of its elements are simple, the only intersections between any two elements occur at points that are on the boundaries of both elements.
• The boundary of a MultiCurve is obtained by applying the "mod 2 union rule" (also known as the odd-even rule): A point is in the boundary of a MultiCurve if it is in the boundaries of an odd number of MultiCurve elements.
• A MultiCurve is closed if all of its elements are closed.
• The boundary of a closed MultiCurve is always empty.

11.2.11 Class MultiLineString

A MultiLineString is a MultiCurve geometry collection composed of LineString elements.

MultiLineString Examples

• On a region map, a MultiLineString could represent a river system or a highway system.

11.2.12 Class MultiSurface

A MultiSurface is a geometry collection composed of surface elements. MultiSurface is a non-instantiable class. Its only instantiable subclass is MultiPolygon.

MultiSurface Assertions

• The interiors of any two surfaces in a MultiSurface may not intersect.
• The boundaries of any two elements in a MultiSurface may intersect at most at a finite number of points.

11.2.13 Class MultiPolygon

A MultiPolygon is a MultiSurface object composed of Polygon elements.

MultiPolygon Examples

• On a region map, a MultiPolygon could represent a system of lakes.

MultiPolygon Assertions

• The interiors of two Polygon values that are elements of a MultiPolygon may not intersect.
• The boundaries of any two Polygon values that are elements of a MultiPolygon may not cross and may touch at only a finite number of points. (Crossing is also forbidden by the preceding assertion.)
• A MultiPolygon may not have cut lines, spikes or punctures. A MultiPolygon is a regular, closed point set.
• The interior of a MultiPolygon composed of more than one Polygon is not connected. The number of connected components of the interior of a MultiPolygon is equal to the number of Polygon values in the MultiPolygon.

MultiPolygon Properties

• A MultiPolygon is defined as a two-dimensional geometry.
• The boundary of a MultiPolygon is a set of closed curves (LineString values) corresponding to the boundaries of its Polygon elements.
• Each Curve in the boundary of the MultiPolygon is in the boundary of exactly one element Polygon.
• Every Curve in the boundary of an element Polygon is in the boundary of the MultiPolygon.

11.3 Supported Spatial Data Formats

This section describes the standard spatial data formats that are used to represent geometry objects in queries. They are:

• Well-Known Text (WKT) format
• Well-Known Binary (WKB) format

Internally, MySQL stores geometry values in a format that is not identical to either WKT or WKB format.

11.3.1 Well-Known Text (WKT) Format

The Well-Known Text (WKT) representation of Geometry is designed to exchange geometry data in ASCII form.

Examples of WKT representations of geometry objects are:

• A Point:

  POINT(15 20)

  Note that point coordinates are specified with no separating comma.

• A LineString with four points:

  LINESTRING(0 0, 10 10, 20 25, 50 60)

• A Polygon with one exterior ring and one interior ring:

  POLYGON((0 0,10 0,10 10,0 10,0 0),(5 5,7 5,7 7,5 7, 5 5))

• A MultiPoint with three Point values:

  MULTIPOINT(0 0, 20 20, 60 60)

• A MultiLineString with two LineString values:

  MULTILINESTRING((10 10, 20 20), (15 15, 30 15))

• A MultiPolygon with two Polygon values:

  MULTIPOLYGON(((0 0,10 0,10 10,0 10,0 0)),((5 5,7 5,7 7,5 7, 5 5)))

• A GeometryCollection consisting of two Point values and one LineString:

  GEOMETRYCOLLECTION(POINT(10 10), POINT(30 30), LINESTRING(15 15, 20 20))

A Backus-Naur grammer that specifies the formal production rules for writing WKT values may be found in the OGC specification document referenced near the beginning of this chapter.

11.3.2 Well-Known Binary (WKB) Format

The Well-Known Binary (WKB) representation for geometric values is defined by the OpenGIS specifications. It is also defined in the ISO "SQL/MM Part 3: Spatial" standard.

WKB is used to exchange geometry data as binary streams represented by BLOB values containing geometric WKB information.

WKB uses 1-byte unsigned integers, 4-byte unsigned integers, and 8-byte double-precision numbers (IEEE 754 format). A byte is 8 bits.

For example, a WKB value that corresponds to POINT(1 1) consists of this sequence of 21 bytes (each represented here by two hex digits):

0101000000000000000000F03F000000000000F03F

The sequence may be broken down into these components:

Byte order : 01
WKB type   : 01000000
X          : 000000000000F03F
Y          : 000000000000F03F

Component representation is as follows:

• The byte order may be either 0 or 1 to indicate little-endian or big-endian storage. The little-endian and big-endian byte orders are also known as Network Data Representation (NDR) and External Data Representation (XDR), respectively.
• The WKB type is a code that indicates the geometry type. Values from 1 through 7 indicate Point, LineString, Polygon, MultiPoint, MultiLineString, MultiPolygon, and GeometryCollection.
• A Point value has X and Y coordinates, each represented as a double-precision value.

WKB values for more complex geometry values are represented by more complex data structures, as detailed in the OpenGIS specification.

11.4 Creating a Spatially Enabled MySQL Database

This section describes the datatypes you can use for representing spatial data in MySQL, and the functions available for creating and retrieving spatial values.

11.4.1 MySQL Spatial Datatypes

MySQL provides a set of datatypes that correspond to classes in the class hierarchy of the OpenGIS Geometry Model. Some of these types hold single geometry values:

• GEOMETRY
• POINT
• LINESTRING
• POLYGON

GEOMETRY is the most general of these single-value types; it can store geometry values of any type. The others restrict their values to a particular geometry type.

The other datatypes hold collections of values:

• MULTIPOINT
• MULTILINESTRING
• MULTIPOLYGON
• GEOMETRYCOLLECTION

GEOMETRYCOLLECTION can store a collection of objects of any type. The other collection types restrict collection members to those having a particular geometry type.

11.4.2 Creating Spatial Values

This section describes how to create spatial values using Well-Known Text and Well-Known Binary functions that are defined in the OpenGIS standard, and using MySQL-specific functions.

11.4.2.1 Creating Geometry Values Using WKT Functions

MySQL provides a number of functions that take as input parameters a Well-Known Text representation (and, optionally, a spatial reference system identifier (SRID)), and return the corresponding geometry.

GeomFromText() accepts a WKT of any geometry type as its first argument. An implementation also provides type-specific construction functions for construction of geometry values of each geometry type.

GeomFromText(wkt[,srid])

GeometryFromText(wkt[,srid])

Constructs a geometry value of any type using its WKT representation and SRID.

PointFromText(wkt[,srid])

Constructs a POINT value using its WKT representation and SRID.

LineFromText(wkt[,srid])

LineStringFromText(wkt[,srid])

Constructs a LINESTRING value using its WKT representation and SRID.

PolyFromText(wkt[,srid])

PolygonFromText(wkt[,srid])

Constructs a POLYGON value using its WKT representation and SRID.

MPointFromText(wkt[,srid])

MultiPointFromText(wkt[,srid])

Constructs a MULTIPOINT value using its WKT representation and SRID.

MLineFromText(wkt[,srid])

MultiLineStringFromText(wkt[,srid])

Constructs a MULTILINESTRING value using its WKT representation and SRID.

MPolyFromText(wkt[,srid])

MultiPolygonFromText(wkt[,srid])

Constructs a MULTIPOLYGON value using its WKT representation and SRID.

GeomCollFromText(wkt[,srid])

GeometryCollectionFromText(wkt[,srid])

Constructs a GEOMETRYCOLLECTION value using its WKT representation and SRID.

The OpenGIS specification also describes optional functions for constructing Polygon or MultiPolygon values based on the WKT representation of a collection of rings or closed LineString values. These values may intersect. MySQL does not yet implement these functions:

BdPolyFromText(wkt,srid)

Constructs a Polygon value from a MultiLineString value in WKT format containing an arbitrary collection of closed LineString values.

BdMPolyFromText(wkt,srid)

Constructs a MultiPolygon value from a MultiLineString value in WKT format containing an arbitrary collection of closed LineString values.

11.4.2.2 Creating Geometry Values Using WKB Functions

MySQL provides a number of functions that take as input parameters a BLOB containing a Well-Known Binary representation (and, optionally, a spatial reference system identifier (SRID)), and return the corresponding geometry.

GeomFromWKT() accepts a WKB of any geometry type as its first argument. An implementation also provides type-specific construction functions for construction of geometry values of each geometry type.

GeomFromWKB(wkb[,srid])

GeometryFromWKB(wkt[,srid])

Constructs a geometry value of any type using its WKB representation and SRID.

PointFromWKB(wkb[,srid])

Constructs a POINT value using its WKB representation and SRID.

LineFromWKB(wkb[,srid])

LineStringFromWKB(wkb[,srid])

Constructs a LINESTRING value using its WKB representation and SRID.

PolyFromWKB(wkb[,srid])

PolygonFromWKB(wkb[,srid])

Constructs a POLYGON value using its WKB representation and SRID.

MPointFromWKB(wkb[,srid])

MultiPointFromWKB(wkb[,srid])

Constructs a MULTIPOINT value using its WKB representation and SRID.

MLineFromWKB(wkb[,srid])

MultiLineStringFromWKB(wkb[,srid])

Constructs a MULTILINESTRING value using its WKB representation and SRID.

MPolyFromWKB(wkb[,srid])

MultiPolygonFromWKB(wkb[,srid])

Constructs a MULTIPOLYGON value using its WKB representation and SRID.

GeomCollFromWKB(wkb[,srid])

GeometryCollectionFromWKB(wkt[,srid])

Constructs a GEOMETRYCOLLECTION value using its WKB representation and SRID.

The OpenGIS specification also describes optional functions for constructing Polygon or MultiPolygon values based on the WKB representation of a collection of rings or closed LineString values. These values may intersect. MySQL does not yet implement these functions:

BdPolyFromWKB(wkb,srid)

Constructs a Polygon value from a MultiLineString value in WKB format containing an arbitrary collection of closed LineString values.

BdMPolyFromWKB(wkb,srid)

Constructs a MultiPolygon value from a MultiLineString value in WKB format containing an arbitrary collection of closed LineString values.

11.4.2.3 Creating Geometry Values Using MySQL-Specific Functions

Note: MySQL does not yet implement the functions listed in this section.

MySQL provides a set of useful functions for creating geometry WKB representations. The functions described in this section are MySQL extensions to the OpenGIS specifications. The results of these functions are BLOB values containing WKB representations of geometry values with no SRID. The results of these functions can be substituted as the first argument for any function in the GeomFromWKB() function family.

Point(x,y)

Constructs a WKB Point using its coordinates.

MultiPoint(pt1,pt2,...)

Constructs a WKB MultiPoint value using WKB Point arguments. If any argument is not a WKBPoint, the return value is NULL.

LineString(pt1,pt2,...)

Constructs a WKB LineString valeu from a number of WKB Point arguments. If any argument is not a WKB Point, the return value is NULL. If the number of Point arguments is less than two, the return value is NULL.

MultiLineString(ls1,ls2,...)

Constructs a WKB MultiLineString value using using WBK LineString arguments. If any argument is not a LineString, the return value is NULL.

Polygon(ls1,ls2,...)

Constructs a WKB Polygon value from a number of WKB LineString arguments. If any argument does not represent the WKB of a LinearRing (that is, not a closed and simple LineString) the return value is NULL.

MultiPolygon(poly1,poly2,...)

Constructs a WKB MultiPolygon value from a set of WKB Polygon arguments. If any argument is not a WKB Polygon, the rerurn value is NULL.

GeometryCollection(g1,g2,...)

Constucts a WKB GeometryCollection. If any argument is not a well-formed WKB representation of a geometry, the return value is NULL.

11.4.3 Creating Spatial Columns

MySQL provides a standard way of creating spatial columns for geometry types, for example, with CREATE TABLE or ALTER TABLE. Currently, spatial columns are supported only for MyISAM tables.

• Use the CREATE TABLE statement to create a table with a spatial column:

  mysql> CREATE TABLE geom (g GEOMETRY); Query OK, 0 rows affected (0.02 sec)

• Use the ALTER TABLE statement to add or drop a spatial column to or from an existing table:

  mysql> ALTER TABLE geom ADD pt POINT; Query OK, 0 rows affected (0.00 sec) Records: 0 Duplicates: 0 Warnings: 0 mysql> ALTER TABLE geom DROP pt; Query OK, 0 rows affected (0.00 sec) Records: 0 Duplicates: 0 Warnings: 0

11.4.4 Populating Spatial Columns

After you have created spatial columns, you can populate them with spatial data.

Values should be stored in internal geometry format, but you can convert them to that format from either Well-Known Text (WKT) or Well-Known Binary (WKB) format. The following examples demonstrate how to insert geometry values into a table by converting WKT values into internal geometry format.

You can perform the conversion directly in the INSERT statement:

INSERT INTO geom VALUES (GeomFromText('POINT(1 1)'));

SET @g = 'POINT(1 1)';
INSERT INTO geom VALUES (GeomFromText(@g));

Or conversion can take place prior to the INSERT:

SET @g = GeomFromText('POINT(1 1)');
INSERT INTO geom VALUES (@g);

The following examples insert more complex geometries into the table:

SET @g = 'LINESTRING(0 0,1 1,2 2)';
INSERT INTO geom VALUES (GeomFromText(@g));

SET @g = 'POLYGON((0 0,10 0,10 10,0 10,0 0),(5 5,7 5,7 7,5 7, 5 5))';
INSERT INTO geom VALUES (GeomFromText(@g));

SET @g = 'GEOMETRYCOLLECTION(POINT(1 1),LINESTRING(0 0,1 1,2 2,3 3,4 4))';
INSERT INTO geom VALUES (GeomFromText(@g));

The preceding examples all use GeomFromText() to create geometry values. You can also use type-specific functions:

SET @g = 'POINT(1 1)';
INSERT INTO geom VALUES (PointFromText(@g));

SET @g = 'LINESTRING(0 0,1 1,2 2)';
INSERT INTO geom VALUES (LineStringFromText(@g));

SET @g = 'POLYGON((0 0,10 0,10 10,0 10,0 0),(5 5,7 5,7 7,5 7, 5 5))';
INSERT INTO geom VALUES (PolygonFromText(@g));

SET @g = 'GEOMETRYCOLLECTION(POINT(1 1),LINESTRING(0 0,1 1,2 2,3 3,4 4))';
INSERT INTO geom VALUES (GeomCollFromText(@g));

Note that if a client application program wants to use WKB representations of geometry values, it is responsible for sending correctly formed WKB in queries to the server. However, there are several ways of satisfying this requirement. For example:

• Inserting a POINT(1 1) value with hex literal syntax:

  mysql> INSERT INTO geom VALUES -> (GeomFromWKB(0x0101000000000000000000F03F000000000000F03F));

• An ODBC application can send a WKB representation, binding it to a placeholder using an argument of BLOB type:

  INSERT INTO geom VALUES (GeomFromWKB(?))

  Other programming interfaces may support a similar placeholder mechanism.

• In a C program, you can escape a binary value using mysql_real_escape_string() and include the result in a query string that is sent to the server. See section mysql_real_escape_string().

11.4.5 Fetching Spatial Data

Geometry values stored in a table can be fetched with conversion in internal format. You can also convert them into WKT or WKB format.

11.4.5.1 Fetching Spatial Data in Internal Format

Fetching geometry values using internal format can be useful in table-to-table transfers:

CREATE TABLE geom2 (g GEOMETRY) SELECT g FROM geom;

11.4.5.2 Fetching Spatial Data in WKT Format

The AsText() function provides textual access to geometry values. It converts a geometry from internal format into a WKT string.

mysql> SELECT AsText(g) FROM geom;
+-------------------------+
| AsText(p1)              |
+-------------------------+
| POINT(1 1)              |
| LINESTRING(0 0,1 1,2 2) |
+-------------------------+

11.4.5.3 Fetching Spatial Data in WKB Format

The AsBinary() function provides binary access to geometry values. It converts a geometry from internal format into a BLOB containing the WKB value.

SELECT AsBinary(g) FROM geom;

11.5 Analysing Spatial Information

After populating spatial columns with values, you are ready to query and analyse them. MySQL provides a set of functions to perform various operations on spatial data. These functions can be grouped into four major categories according to the type of operation they perform:

• Functions that convert geometries between various formats
• Functions that provide access to qualitative or quantitative properties of a geometry
• Functions that describe relations between two geometries
• Functions that create new geometries from existing ones

Spatial analysis functions can be used in many contexts, such as:

• Any interactive SQL program, like mysql or MySQLCC
• Application programs written in any language that supports a MySQL client API

11.5.1 Functions To Convert Geometries Between Formats

MySQL supports the following functions for converting geometry values between internal format and either WKT or WKB format:

GeomFromText(wkt[,srid])

Converts a string value from its WKT representation into internal geometry format and returns the result. A number of type-specific functions are also supported, such as PointFromText() and LineFromText(); see Creating Geometry Values Using WKT Functions.

GeomFromWKB(wkb[,srid])

Converts a binary value from its WKB representation into internal geometry format and returns the result. A number of type-specific functions are also supported, such as PointFromWKB() and LineFromWKB(); see Creating Geometry Values Using WKB Functions.

AsText(g)

Converts a value in internal geometry format to its WKT representation and returns the resulting string.

mysql> SET @g = 'LineString(1 1,2 2,3 3)'; mysql> SELECT AsText(GeomFromText(@g)); +--------------------------+ | AsText(GeomFromText(@G)) | +--------------------------+ | LINESTRING(1 1,2 2,3 3) | +--------------------------+

AsBinary(g)

Converts a value in internal geometry format to its WKB representation and returns the resulting binary value.

11.5.2 Geometry Property Analysis Functions

Each function that belongs to this group takes a geometry value as its argument and returns some quantitive or qualitive property of the geometry. Some functions restrict their argument type. Such functions return NULL if the argument is of an incorrect geometry type. For example, Area() returns NULL if the object type is neither Polygon nor MultiPolygon.

11.5.2.1 General Geometry Property Analysis Functions

The functions listed in this ssection do not restrict their argument and accept a geometry value of any type.

GeometryType(g)

Returns as a string the name of the geometry type of which the geometry instance g is a member. The name will correspond to one of the instantiable Geometry subclasses.

mysql> SELECT GeometryType(GeomFromText('POINT(1 1)')); +------------------------------------------+ | GeometryType(GeomFromText('POINT(1 1)')) | +------------------------------------------+ | POINT | +------------------------------------------+

Dimension(g)

Returns the inherent dimension of the geometry value g. The result can be -1, 0, 1, or 2. (The meaning of these values is given in Class Geometry.)

mysql> SELECT Dimension(GeomFromText('LineString(1 1,2 2)')); +------------------------------------------------+ | Dimension(GeomFromText('LineString(1 1,2 2)')) | +------------------------------------------------+ | 1 | +------------------------------------------------+

SRID(g)

Returns an integer indicating the Spatial Reference System ID for the geometry value g.

mysql> SELECT SRID(GeomFromText('LineString(1 1,2 2)',101)); +-----------------------------------------------+ | SRID(GeomFromText('LineString(1 1,2 2)',101)) | +-----------------------------------------------+ | 101 | +-----------------------------------------------+

Envelope(g)

Returns the Minimum Bounding Rectangle (MBR) for the geometry value g. The result is returned as a polygon value.

mysql> SELECT AsText(Envelope(GeomFromText('LineString(1 1,2 2)'))); +-------------------------------------------------------+ | AsText(Envelope(GeomFromText('LineString(1 1,2 2)'))) | +-------------------------------------------------------+ | POLYGON((1 1,2 1,2 2,1 2,1 1)) | +-------------------------------------------------------+

The polygon is defined by the corner points of the bounding box:

POLYGON((MINX MINY, MAXX MINY, MAXX MAXY, MINX MAXY, MINX MINY))

The OpenGIS specification also defines the following functions, which MySQL does not yet implement:

Boundary(g)

Returns a geometry that is the closure of the combinatorial boundary of the geometry value g.

IsEmpty(g)

Returns 1 if the geomtry value g is the empty geometry, 0 if it is not empty, and -1 if the argument is NULL. If the geometry is empty, it represents the empty point set.

IsSimple(g)

Returns 1 if the geometry value g has no anomalous geometric points, such as self intersection or self tangency. IsSimple() returns 0 if the argument is not simple, and -1 if it is NULL.

The description of each instantiable geometric class given earlier in the chapter includes the specific conditions that cause an instance of that class to be classified as not simple.

11.5.2.2 Point Property Analysis Functions

A Point consists of its X and Y coordinates, which may be obtained using the following functions:

X(p)

Returns the X-coordinate value for the point p as a double-precision number.

mysql> SELECT X(GeomFromText('Point(56.7 53.34)')); +--------------------------------------+ | X(GeomFromText('Point(56.7 53.34)')) | +--------------------------------------+ | 56.7 | +--------------------------------------+

Y(p)

Returns the Y-coordinate value for the point p as a double-precision number.

mysql> SELECT Y(GeomFromText('Point(56.7 53.34)')); +--------------------------------------+ | Y(GeomFromText('Point(56.7 53.34)')) | +--------------------------------------+ | 53.34 | +--------------------------------------+

11.5.2.3 LineString Property Analysis Functions

A LineString consists of Point values. You can extract particular points of a LineString, count the number of points that it contains, or obtain its length.

EndPoint(ls)

Returns the Point that is the end point of the LineString value ls.

mysql> SELECT AsText(EndPoint(GeomFromText('LineString(1 1,2 2,3 3)'))); +------------------------------------------------------------+ | AsText(EndPoint(GeomFromText('LineString(1 1,2 2,3 3)'))) | +------------------------------------------------------------+ | POINT(3 3) | +------------------------------------------------------------+

GLength(ls)

Returns as a double-precision number the length of the LineString value ls in its associated spatial reference.

mysql> SELECT GLength(GeomFromText('LineString(1 1,2 2,3 3)')); +--------------------------------------------------+ | GLength(GeomFromText('LineString(1 1,2 2,3 3)')) | +--------------------------------------------------+ | 2.8284271247462 | +--------------------------------------------------+

IsClosed(ls)

Returns 1 if the LineString value ls is closed (that is, it sStartPoint() and EndPoint() values are the same). Returns 0 if ls is not closed, and -1 if it is NULL.

mysql> SELECT IsClosed(GeomFromText('LineString(1 1,2 2,3 3)')); +---------------------------------------------------+ | IsClosed(GeomFromText('LineString(1 1,2 2,3 3)')) | +---------------------------------------------------+ | 0 | +---------------------------------------------------+

NumPoints(ls)

Returns the number of points in the LineString value ls.

mysql> SELECT NumPoints(GeomFromText('LineString(1 1,2 2,3 3)')); +----------------------------------------------------+ | NumPoints(GeomFromText('LineString(1 1,2 2,3 3)')) | +----------------------------------------------------+ | 3 | +----------------------------------------------------+

PointN(ls,n)

Returns the n-th point in the Linestring value ls. Point numbers begin at 1.

mysql> SELECT AsText(PointN(GeomFromText('LineString(1 1,2 2,3 3)'),2)); +-----------------------------------------------------------+ | AsText(PointN(GeomFromText('LineString(1 1,2 2,3 3)'),2)) | +-----------------------------------------------------------+ | POINT(2 2) | +-----------------------------------------------------------+

StartPoint(ls)

Returns the Point that is the start point of the LineString value ls.

mysql> SELECT AsText(StartPoint(GeomFromText('LineString(1 1,2 2,3 3)'))); +-------------------------------------------------------------+ | AsText(StartPoint(GeomFromText('LineString(1 1,2 2,3 3)'))) | +-------------------------------------------------------------+ | POINT(1 1) | +-------------------------------------------------------------+

The OpenGIS specification also defines the following function, which MySQL does not yet implement:

IsRing(ls)

Returns 1 if the LineString value ls is closed (thatis, its StartPoint() and EndPoint() values are the same) and is simple (does not pass through the same point more than once). Returns 0 if ls is not a ring, and -1 if it is NULL.

11.5.2.4 MultiLineString Property Analysis Functions

GLength(mls)

Returns as a double-precision number the length of the MultiLineString value mls. The length of mls is equal to the sum of the lengths of its elements.

mysql> SELECT GLength(GeomFromText('MultiLineString((1 1,2 2,3 3),(4 4,5 5))')); +-------------------------------------------------------------------+ | GLength(GeomFromText('MultiLineString((1 1,2 2,3 3),(4 4,5 5))')) | +-------------------------------------------------------------------+ | 4.2426406871193 | +-------------------------------------------------------------------+

IsClosed(mls)

Returns 1 if the MultiLineString value mls is closed (that is, the StartPoint() and EndPoint() values are the same for each LineString in mls). Returns 0 if mls is not closed, and -1 if it is NULL.

mysql> SELECT IsClosed(GeomFromText('MultiLineString((1 1,2 2,3 3),(4 4,5 5))')); +--------------------------------------------------------------------+ | IsClosed(GeomFromText('MultiLineString((1 1,2 2,3 3),(4 4,5 5))')) | +--------------------------------------------------------------------+ | 0 | +--------------------------------------------------------------------+

11.5.2.5 Polygon Property Analysis Functions

Area(poly)

Returns as a double-precision number the area of the Polygon value poly, as measured in its spatial reference system.

mysql> SELECT Area(GeomFromText('Polygon((0 0,0 3,3 3,3 0,0 0),(1 1,1 2,2 2,2 1,1 1))')); +----------------------------------------------------------------------------+ | Area(GeomFromText('Polygon((0 0,0 3,3 3,3 0,0 0),(1 1,1 2,2 2,2 1,1 1))')) | +----------------------------------------------------------------------------+ | 8 | +----------------------------------------------------------------------------+

NumInteriorRings(poly)

Returns the number of interior rings in the Polygon value poly.

mysql> SELECT NumInteriorRings(GeomFromText('Polygon((0 0,0 3,3 3,3 0,0 0),(1 1,1 2,2 2,2 1,1 1))')); +----------------------------------------------------------------------------------------+ | NumInteriorRings(GeomFromText('Polygon((0 0,0 3,3 3,3 0,0 0),(1 1,1 2,2 2,2 1,1 1))')) | +----------------------------------------------------------------------------------------+ | 1 | +----------------------------------------------------------------------------------------+

ExteriorRing(poly)

Returns the exterior ring of the Polygon value poly as a LineString.

mysql> SELECT AsText(ExteriorRing(GeomFromText('Polygon((0 0,0 3,3 3,3 0,0 0),(1 1,1 2,2 2,2 1,1 1))'))); +--------------------------------------------------------------------------------------------+ | AsText(ExteriorRing(GeomFromText('Polygon((0 0,0 3,3 3,3 0,0 0),(1 1,1 2,2 2,2 1,1 1))'))) | +--------------------------------------------------------------------------------------------+ | LINESTRING(0 0,0 3,3 3,3 0,0 0) | +--------------------------------------------------------------------------------------------+

InteriorRingN(poly,n)

Returns the n-th interior ring for the Polygon value poly as a LineString. Ring numbers begin at 1.

mysql> SELECT AsText(InteriorRingN(GeomFromText('Polygon((0 0,0 3,3 3,3 0,0 0),(1 1,1 2,2 2,2 1,1 1))'),1)); +-----------------------------------------------------------------------------------------------+ | AsText(InteriorRingN(GeomFromText('Polygon((0 0,0 3,3 3,3 0,0 0),(1 1,1 2,2 2,2 1,1 1))'),1)) | +-----------------------------------------------------------------------------------------------+ | LINESTRING(1 1,1 2,2 2,2 1,1 1) | +-----------------------------------------------------------------------------------------------+

The OpenGIS specification also defines the following functions, which MySQL does not yet implement:

Centroid(poly)

Returns the mathematical centroid for the Polygon value poly as a Point. The result is not guaranteed to be on the polygon.

PointOnSurface(poly)

Returns a Point value that is guaranteed to be on the Polygon value poly.

11.5.2.6 MultiPolygon Property Analysis Functions

Area(mpoly)

Returns as a double-precision number the area of the MultiPolygon value mpoly, as measured in its spatial reference system.

mysql> SELECT Area(GeomFromText('MultiPolygon(((0 0,0 3,3 3,3 0,0 0),(1 1,1 2,2 2,2 1,1 1)))')); +-----------------------------------------------------------------------------------+ | Area(GeomFromText('MultiPolygon(((0 0,0 3,3 3,3 0,0 0),(1 1,1 2,2 2,2 1,1 1)))')) | +-----------------------------------------------------------------------------------+ | 8 | +-----------------------------------------------------------------------------------+

The OpenGIS specification also defines the following functions, which MySQL does not yet implement:

Centroid(mpoly)

Returns the mathematical centroid for the MultiPolygon value mpoly as a Point. The result is not guaranteed to be on the MultiPolygon.

PointOnSurface(mpoly)

Returns a Point value that is guaranteed to be on the MultiPolygon value mpoly.

11.5.2.7 GeometryCollection Property Analysis Functions

NumGeometries(gc)

Returns the number of geometries in the GeometryCollection value gc.

mysql> SELECT NumGeometries(GeomFromText('GeometryCollection(Point(1 1),LineString(2 2, 3 3))')); +------------------------------------------------------------------------------------+ | NumGeometries(GeomFromText('GeometryCollection(Point(1 1),LineString(2 2, 3 3))')) | +------------------------------------------------------------------------------------+ | 2 | +------------------------------------------------------------------------------------+

GeometryN(gc,n)

Returns the n-th geometry in the GeometryCollection value gc. Geometry numbers begin at 1.

mysql> SELECT AsText(GeometryN(GeomFromText('GeometryCollection(Point(1 1),LineString(2 2, 3 3))'),1)); +------------------------------------------------------------------------------------------+ | AsText(GeometryN(GeomFromText('GeometryCollection(Point(1 1),LineString(2 2, 3 3))'),1)) | +------------------------------------------------------------------------------------------+ | POINT(1 1) | +------------------------------------------------------------------------------------------+

11.5.3 Functions That Create New Geometries From Existing Ones

11.5.3.1 Geometry Functions That Produce New Geometries

In the section Geometry Property Analysis Functions, we've already discussed some functions that can construct new geometries from the existing ones:

• Envelope(g)
• StartPoint(ls)
• EndPoint(ls)
• PointN(ls,n)
• ExteriorRing(poly)
• InteriorRingN(poly,n)
• GeometryN(gc,n)

11.5.3.2 Spatial Operators

OpenGIS proposes a number of other functions that can produce geometries. They are designed to implement Spatial Operators.

These functions are not yet implemented in MySQL. They should appear in future releases.

Intersection(g1,g2)

Returns a geometry that represents the point set intersection of the geometry values g1 with g2.

Union(g1,g2)

Returns a geometry that represents the point set union of the geometry values g1 and g2.

Difference(g1,g2)

Returns a geometry that represents the point set difference of the geometry value g1 with g2.

SymDifference(g1,g2)

Returns a geometry that represents the point set symmetric difference of the geometry value g1 with g2.

Buffer(g,d)

Returns a geometry that represents all points whose distance from the geometry value g is less than or equal to a distance of d.

ConvexHull(g)

Returns a geometry that represents the convex hull of the geometry value g.

11.5.4 Functions For Testing Spatial Relations Between Geometric Objects

The functions described in these sections take two geometries as input parameters and return a qualitive or quantitive relation between them.

11.5.5 Relations On Geometry Minimal Bounding Rectangles (MBRs)

MySQL provides some functions that can test relations between mininal bounding rectangles of two geometries g1 and g2. They include:

MBRContains(g1,g2)

Returns 1 or 0 to indicate whether or not the Minimum Bounding Rectangle of g1 contains the Minimum Bounding Rectangle of g2.

mysql> SET @g1 = GeomFromText('Polygon((0 0,0 3,3 3,3 0,0 0))'); mysql> SET @g2 = GeomFromText('Point(1 1)'); mysql> SELECT MBRContains(@g1,@g2), MBRContains(@g2,@g1); ----------------------+----------------------+ | MBRContains(@g1,@g2) | MBRContains(@g2,@g1) | +----------------------+----------------------+ | 1 | 0 | +----------------------+----------------------+

MBRWithin(g1,g2)

Returns 1 or 0 to indicate whether or not the Minimum Bounding Rectangle of g1 is within the Minimum Bounding Rectangle of g2.

mysql> SET @g1 = GeomFromText('Polygon((0 0,0 3,3 3,3 0,0 0))'); mysql> SET @g2 = GeomFromText('Polygon((0 0,0 5,5 5,5 0,0 0))'); mysql> SELECT MBRWithin(@g1,@g2), MBRWithin(@g2,@g1); +--------------------+--------------------+ | MBRWithin(@g1,@g2) | MBRWithin(@g2,@g1) | +--------------------+--------------------+ | 1 | 0 | +--------------------+--------------------+

MBRDisjoint(g1,g2)

Returns 1 or 0 to indicate whether or not the Minimum Bounding Rectangles of the two geometries g1 and g2 are disjoint (do not intersect).

MBREquals(g1,g2)

Returns 1 or 0 to indicate whether or not the Minimum Bounding Rectangles of the two geometries g1 and g2 are the same.

MBRIntersects(g1,g2)

Returns 1 or 0 to indicate whether or not the Minimum Bounding Rectangles of the two geometries g1 and g2 intersect.

MBROverlaps(g1,g2)

Returns 1 or 0 to indicate whether or not the Minimum Bounding Rectangles of the two geometries g1 and g2 overlap.

MBRTouches(g1,g2)

Returns 1 or 0 to indicate whether or not the Minimum Bounding Rectangles of the two geometries g1 and g2 touch.

11.5.6 Functions That Test Spatial Relationships Between Geometries

The OpenGIS specification defines the following functions, which MySQL does not yet implement. They should appear in future releases. When implemented, they will provide full support for spatial analysis, not just MBR-based support.

The functions operate on two geometry values g1 and g2.

Contains(g1,g2)

Returns 1 or 0 to indicate whether or not g1 completely contains g2.

Crosses(g1,g2)

Returns 1 if g1 spatially crosses g2. Returns NULL if g1 is a Polygon or a MultiPolygon, or if g2 is a Point or a MultiPoint. Otherwise, returns 0.

The term spatially crosses denotes a spatial relation between two given geometries that has the following properties:

• The two geometries intersect
• Their intersection results in a geometry that has a dimension that is one less than the maximum dimension of the two given geometries
• Their intersection is not equal to either of the two given geometries

Disjoint(g1,g2)

Returns 1 or 0 to indicate whether or not g1 is spatially disjoint from (does not intersect) g2.

Equals(g1,g2)

Returns 1 or 0 to indicate whether or not g1 is spatially equal to g2.

Intersects(g1,g2)

Returns 1 or 0 to indicate whether or not g1 spatially intersects g2.

Overlaps(g1,g2)

Returns 1 or 0 to indicate whether or not g1 spatially overlaps g2. The term spatially overlaps is used if two geometries intersect and their intersection results in a geometry of the same dimension but not equal to either of the given geometries.

Touches(g1,g2)

Returns 1 or 0 to indicate whether or not g1 spatially touches g2. Two geometries spatially touch if the interiors of the geometries do not intersect, but the boundary of one of the geometries intersects either the boundary or the interior of the other.

Within(g1,g2)

Returns 1 or 0 to indicate whether or not g1 is spatially within g2.

Distance(g1,g2)

Returns as a double-precision number the shortest distance between any two points in the two geometries.

Related(g1,g2,pattern_matrix)

Returns 1 or 0 to indicate whether or not the spatial relationship specified by pattern_matrix exists between g1 and g2. Returns -1 if the arguments are NULL. The pattern matrix is a string. Its specification will be noted here when this function is implemented.

11.6 Optimising Spatial Analysis

It is known that search operations in non-spatial databases can be optimised using indexes. This is true for spatial databases as well. With the help of a great variety of multi-dimensional indexing methods that have already been designed, it's possible to optimise spatial searches. The most typical of these are:

• Point queries that search for all objects that contain a given point
• Region queries that search for all objects that overlap a given region

MySQL utilises R-Trees with quadratic splitting to index spatial columns. A spatial index is built using the MBR of a geometry. For most geometries, the MBR is a minimum rectangle that surrounds the geometries. For a horizontal or a vertical linestring, the MBR is a rectangle degenerated into the linestring. For a point, the MBR is a rectangle degenerated into the point.

11.6.1 Creating Spatial Indexes

MySQL can create spatial indexes using syntax similar to that for creating regular indexes, but extended with the SPATIAL keyword. Spatial columns that are indexed currently must be declared NOT NULL. The following examples demonstrate how to create spatial indexes.

• With CREATE TABLE:

  mysql> CREATE TABLE geom (g GEOMETRY NOT NULL, SPATIAL INDEX(g));

• With ALTER TABLE:

  mysql> ALTER TABLE geom ADD SPATIAL INDEX(g);

• With CREATE INDEX:

  mysql> CREATE SPATIAL INDEX sp_index ON geom (g);

To drop spatial indexes, use ALTER TABLE or DROP INDEX:

• With ALTER TABLE:

  mysql> ALTER TABLE geom DROP INDEX g;

• With DROP INDEX:

  mysql> DROP INDEX sp_index ON geom;

Example: Suppose that a table geom contains more than 32000 geometries, which are stored in the column g of type GEOMETRY. The table also has an AUTO_INCREMENT column fid for storing object ID values.

mysql> SHOW FIELDS FROM geom;
+-------+----------+------+-----+---------+----------------+
| Field | Type     | Null | Key | Default | Extra          |
+-------+----------+------+-----+---------+----------------+
| fid   | int(11)  |      | PRI | NULL    | auto_increment |
| g     | geometry |      |     |         |                |
+-------+----------+------+-----+---------+----------------+
2 rows in set (0.00 sec)

mysql> SELECT COUNT(*) FROM geom;
+----------+
| count(*) |
+----------+
|    32376 |
+----------+
1 row in set (0.00 sec)

To add a spatial index on the column g, use this statement:

mysql> ALTER TABLE geom ADD SPATIAL INDEX(g);
Query OK, 32376 rows affected (4.05 sec)
Records: 32376  Duplicates: 0  Warnings: 0

11.6.2 Using a Spatial Index

The optimiser investigates whether available spatial indexes can be involved in the search for queries that use a function such as MBRContains() or MBRWithin() in the WHERE clause. For example, let's say we want to find all objects that are in the given rectangle:

mysql> SELECT fid,AsText(g) FROM geom WHERE
mysql> MBRContains(GeomFromText('Polygon((30000 15000,31000 15000,31000 16000,30000 16000,30000 15000))'),g);
+-----+-----------------------------------------------------------------------------+
| fid | AsText(g)                                                                   |
+-----+-----------------------------------------------------------------------------+
|  21 | LINESTRING(30350.4 15828.8,30350.6 15845,30333.8 15845,30333.8 15828.8)     |
|  22 | LINESTRING(30350.6 15871.4,30350.6 15887.8,30334 15887.8,30334 15871.4)     |
|  23 | LINESTRING(30350.6 15914.2,30350.6 15930.4,30334 15930.4,30334 15914.2)     |
|  24 | LINESTRING(30290.2 15823,30290.2 15839.4,30273.4 15839.4,30273.4 15823)     |
|  25 | LINESTRING(30291.4 15866.2,30291.6 15882.4,30274.8 15882.4,30274.8 15866.2) |
|  26 | LINESTRING(30291.6 15918.2,30291.6 15934.4,30275 15934.4,30275 15918.2)     |
| 249 | LINESTRING(30337.8 15938.6,30337.8 15946.8,30320.4 15946.8,30320.4 15938.4) |
|   1 | LINESTRING(30250.4 15129.2,30248.8 15138.4,30238.2 15136.4,30240 15127.2)   |
|   2 | LINESTRING(30220.2 15122.8,30217.2 15137.8,30207.6 15136,30210.4 15121)     |
|   3 | LINESTRING(30179 15114.4,30176.6 15129.4,30167 15128,30169 15113)           |
|   4 | LINESTRING(30155.2 15121.4,30140.4 15118.6,30142 15109,30157 15111.6)       |
|   5 | LINESTRING(30192.4 15085,30177.6 15082.2,30179.2 15072.4,30194.2 15075.2)   |
|   6 | LINESTRING(30244 15087,30229 15086.2,30229.4 15076.4,30244.6 15077)         |
|   7 | LINESTRING(30200.6 15059.4,30185.6 15058.6,30186 15048.8,30201.2 15049.4)   |
|  10 | LINESTRING(30179.6 15017.8,30181 15002.8,30190.8 15003.6,30189.6 15019)     |
|  11 | LINESTRING(30154.2 15000.4,30168.6 15004.8,30166 15014.2,30151.2 15009.8)   |
|  13 | LINESTRING(30105 15065.8,30108.4 15050.8,30118 15053,30114.6 15067.8)       |
| 154 | LINESTRING(30276.2 15143.8,30261.4 15141,30263 15131.4,30278 15134)         |
| 155 | LINESTRING(30269.8 15084,30269.4 15093.4,30258.6 15093,30259 15083.4)       |
| 157 | LINESTRING(30128.2 15011,30113.2 15010.2,30113.6 15000.4,30128.8 15001)     |
+-----+-----------------------------------------------------------------------------+
20 rows in set (0.00 sec)

Now let's check the way this query is executed, using EXPLAIN:

mysql> EXPLAIN SELECT fid,AsText(g) FROM geom WHERE
mysql> MBRContains(GeomFromText('Polygon((30000 15000,31000 15000,31000 16000,30000 16000,30000 15000))'),g);
+----+-------------+-------+-------+---------------+------+---------+------+------+-------------+
| id | select_type | table | type  | possible_keys | key  | key_len | ref  | rows | Extra       |
+----+-------------+-------+-------+---------------+------+---------+------+------+-------------+
|  1 | SIMPLE      | geom  | range | g             | g    |      32 | NULL |   50 | Using where |
+----+-------------+-------+-------+---------------+------+---------+------+------+-------------+
1 row in set (0.00 sec)

Now let's check what would happen if we didn't have a spatial index:

mysql> EXPLAIN SELECT fid,AsText(g) FROM g IGNORE INDEX (g) WHERE
mysql> MBRContains(GeomFromText('Polygon((30000 15000,31000 15000,31000 16000,30000 16000,30000 15000))'),g);
+----+-------------+-------+------+---------------+------+---------+------+-------+-------------+
| id | select_type | table | type | possible_keys | key  | key_len | ref  | rows  | Extra       |
+----+-------------+-------+------+---------------+------+---------+------+-------+-------------+
|  1 | SIMPLE      | geom  | ALL  | NULL          | NULL |    NULL | NULL | 32376 | Using where |
+----+-------------+-------+------+---------------+------+---------+------+-------+-------------+
1 row in set (0.00 sec)

Let's execute the above query, ignoring the spatial key we have:

mysql> SELECT fid,AsText(g) FROM geom IGNORE INDEX (g) WHERE
mysql> MBRContains(GeomFromText('Polygon((30000 15000,31000 15000,31000 16000,30000 16000,30000 15000))'),g);
+-----+-----------------------------------------------------------------------------+
| fid | AsText(g)                                                                   |
+-----+-----------------------------------------------------------------------------+
|   1 | LINESTRING(30250.4 15129.2,30248.8 15138.4,30238.2 15136.4,30240 15127.2)   |
|   2 | LINESTRING(30220.2 15122.8,30217.2 15137.8,30207.6 15136,30210.4 15121)     |
|   3 | LINESTRING(30179 15114.4,30176.6 15129.4,30167 15128,30169 15113)           |
|   4 | LINESTRING(30155.2 15121.4,30140.4 15118.6,30142 15109,30157 15111.6)       |
|   5 | LINESTRING(30192.4 15085,30177.6 15082.2,30179.2 15072.4,30194.2 15075.2)   |
|   6 | LINESTRING(30244 15087,30229 15086.2,30229.4 15076.4,30244.6 15077)         |
|   7 | LINESTRING(30200.6 15059.4,30185.6 15058.6,30186 15048.8,30201.2 15049.4)   |
|  10 | LINESTRING(30179.6 15017.8,30181 15002.8,30190.8 15003.6,30189.6 15019)     |
|  11 | LINESTRING(30154.2 15000.4,30168.6 15004.8,30166 15014.2,30151.2 15009.8)   |
|  13 | LINESTRING(30105 15065.8,30108.4 15050.8,30118 15053,30114.6 15067.8)       |
|  21 | LINESTRING(30350.4 15828.8,30350.6 15845,30333.8 15845,30333.8 15828.8)     |
|  22 | LINESTRING(30350.6 15871.4,30350.6 15887.8,30334 15887.8,30334 15871.4)     |
|  23 | LINESTRING(30350.6 15914.2,30350.6 15930.4,30334 15930.4,30334 15914.2)     |
|  24 | LINESTRING(30290.2 15823,30290.2 15839.4,30273.4 15839.4,30273.4 15823)     |
|  25 | LINESTRING(30291.4 15866.2,30291.6 15882.4,30274.8 15882.4,30274.8 15866.2) |
|  26 | LINESTRING(30291.6 15918.2,30291.6 15934.4,30275 15934.4,30275 15918.2)     |
| 154 | LINESTRING(30276.2 15143.8,30261.4 15141,30263 15131.4,30278 15134)         |
| 155 | LINESTRING(30269.8 15084,30269.4 15093.4,30258.6 15093,30259 15083.4)       |
| 157 | LINESTRING(30128.2 15011,30113.2 15010.2,30113.6 15000.4,30128.8 15001)     |
| 249 | LINESTRING(30337.8 15938.6,30337.8 15946.8,30320.4 15946.8,30320.4 15938.4) |
+-----+-----------------------------------------------------------------------------+
20 rows in set (0.46 sec)

When the index is not used, the execution time for this query rises from 0.00 seconds to 0.46 seconds.

In future releases, spatial indexes will also be used for optimising other functions. See section Functions For Testing Spatial Relations Between Geometric Objects.

11.7 MySQL Conformance And Compatibility

11.7.1 GIS Features That Are Not Yet Implemented

Additional Metadata Views

OpenGIS specifications propose several additional metadata views. For example, a system view named GEOMETRY_COLUMNS contains a description of geometry columns, one row for each geometry column in the database.

Functions to add/drop spatial columns

OpenGIS assumes that columns can be added or dropped using special AddGeometryColumn() and DropGeometryColumn() functions. In MySQL, this is done using the ALTER TABLE, CREATE INDEX, and DROP INDEX statements instead.

Factors related to Spatial Reference Systems and their IDs (SRIDs):

• Functions like Length() and Area() assume a planar coordinate system.
• All objects currently are considered to be in the same planar coordinate system.

The OpenGIS function Length() on LineString and MultiLineString currently should be called in MySQL as GLength()

The problem is that it conflicts with the existing SQL function Length() that calculates the length of string values, and sometimes it's not possible to distinguish whether the function is called in a textual or spatial context. We need either to solve this somehow, or decide on another function name.

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Generated: 2007-01-26 17:58:44