Go to the first, previous, next, last section, table of contents.
MySQL 4.1 introduces spatial extensions to allow the
generation, storage, and analysis of geographic features.
Currently, these features are available for MyISAM tables only.
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
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/docs/99-049.pdf.
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
analyze 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 the term geospatial feature to
refer to geographic features.
Geometry is another word that denotes a geographic feature.
Originally the word geometry means measurement of the earth.
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.
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.
The geometry classes define a hierarchy as follows:
Geometry (non-instantiable)
Point (instantiable)
Curve (non-instantiable)
LineString (instantiable)
Surface (non-instantiable)
GeometryCollection (instantiable)
MultiPoint (instantiable)
MultiCurve (non-instantiable)
MultiLineString (instantiable)
MultiSurface (non-instantiable)
MultiPolygon (instantiable)
It is not possible to create objects in non-instantiable classes.
It is possible to create objects in instantiable classes.
All classes have properties, and instantiable classes may also
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 specialized 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 generalize 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 extensibility.
Point, LineString, Polygon, GeometryCollection,
MultiPoint, MultiLineString, and
MultiPolygon are instantiable classes.
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.)
18.2.3 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.
Every geometry occupies 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 the 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 types (
LineString, MultiPoint,
MultiLineString)
are either simple or 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 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 for empty geometry.
- 0 for geometry with no length and no area.
- 1 for geometry with non-zero length and zero area.
- 2 for geometry 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.
A Point is a geometry that represents a single
location in coordinate space.
18.2.5 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.
18.2.6 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.
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.
18.2.8 Curve Properties
-
A
Curve has the coordinates of its points.
-
A
Curve is defined as a 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.
A LineString is a Curve with linear interpolation between points.
18.2.10 LineString Examples
-
On a world map,
LineString objects could represent rivers.
-
In a city map,
LineString objects could represent streets.
18.2.11 LineString Properties
-
A
LineString has coordinates of 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.
A Surface is a two-dimensional geometry. It is a non-instantiable
class. Its only instantiable subclass is Polygon.
18.2.13 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.
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.
18.2.15 Polygon Examples
-
On a region map,
Polygon objects could represent forests, districts, etc.
18.2.16 Polygon Assertions
-
The boundary of a
Polygon consists of a set of LinearRing objects
(that is, LineString objects that are both simple and closed) that make up its
exterior and interior boundaries.
-
A
Polygon has no rings which cross. The rings in the boundary of a
Polygon may intersect at a Point, but only as a tangent.
-
A
Polygon has no lines, spikes, or punctures.
-
A
Polygon has an interior which is a connected point set.
-
A
Polygon may have holes.
The exterior of a Polygon with holes is not connected.
Each hole defines a connected component of the exterior.
The preceding assertions make a Polygon a simple geometry.
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).
There are no other constraints on the elements of a GeometryCollection,
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
A MultiPoint is a geometry collection composed of
Point elements. The points are not connected or ordered
in any way.
18.2.19 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.
18.2.20 MultiPoint Properties
-
A
MultiPoint is a zero-dimensional geometry.
-
A
MultiPoint is simple if no two of its Point values are
equal (have identical coordinate values).
-
A
MultiPoint's boundary is the empty set.
A MultiCurve is a geometry collection composed of
Curve elements. MultiCurve is a non-instantiable class.
18.2.22 MultiCurve Properties
-
A
MultiCurve is 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.
-
A
MultiCurve boundary 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.
-
A closed
MultiCurve's boundary is always empty.
A MultiLineString is a MultiCurve geometry collection composed
of LineString elements.
18.2.24 MultiLineString Examples
-
On a region map, a
MultiLineString could represent a river system or
a highway system.
A MultiSurface is a geometry collection composed of surface elements.
MultiSurface is a non-instantiable class. Its only instantiable
subclass is MultiPolygon.
18.2.26 MultiSurface Assertions
-
Two
MultiSurface surfaces have no interiors which intersect.
-
Two
MultiSurface elements have boundaries which
intersect at most at a finite number of points.
A MultiPolygon is a MultiSurface object composed of
Polygon elements.
18.2.28 MultiPolygon Examples
-
On a region map, a
MultiPolygon could represent a system of lakes.
18.2.29 MultiPolygon Assertions
-
A
MultiPolygon has no two Polygon elements with interiors
that intersect.
-
A
MultiPolygon has no two Polygon elements that cross
(crossing is also forbidden by the previous assertion), or that
touch at an infinite number of points.
-
A
MultiPolygon may not have cut lines, spikes or punctures. A
MultiPolygon is a regular, closed point set.
-
A
MultiPolygon which has more than one Polygon has an
interior which 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.
18.2.30 MultiPolygon Properties
-
A
MultiPolygon is a two-dimensional geometry.
-
A
MultiPolygon boundary 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 Polygon element.
-
Every
Curve in the boundary of an Polygon element is
in the boundary of the MultiPolygon.
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.
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 grammar 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.
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.
This section describes the datatypes you can use for representing
spatial data in MySQL, and the functions available for creating and retrieving
spatial values.
MySQL has datatypes that correspond to OpenGIS classes.
Some of these types hold single geometry values:
GEOMETRY
POINT
LINESTRING
POLYGON
GEOMETRY can store geometry values of any type.
The other single-value types,
POINT and LINESTRING and POLYGON,
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,
MULTIPOINT and MULTILINESTRING and MULTIPOLYGON and GEOMETRYCOLLECTION,
restrict collection members to those having a particular geometry type.
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.
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.
GeomCollFromText(wkt[,srid])
-
GeometryCollectionFromText(wkt[,srid])
-
Constructs a
GEOMETRYCOLLECTION value using its WKT representation and SRID.
GeomFromText(wkt[,srid])
-
GeometryFromText(wkt[,srid])
-
Constructs a geometry value of any type using its WKT representation and SRID.
LineFromText(wkt[,srid])
-
LineStringFromText(wkt[,srid])
-
Constructs a
LINESTRING value using its WKT representation and SRID.
MLineFromText(wkt[,srid])
-
MultiLineStringFromText(wkt[,srid])
-
Constructs a
MULTILINESTRING value using its WKT representation and SRID.
MPointFromText(wkt[,srid])
-
MultiPointFromText(wkt[,srid])
-
Constructs a
MULTIPOINT value using its WKT representation and SRID.
MPolyFromText(wkt[,srid])
-
MultiPolygonFromText(wkt[,srid])
-
Constructs a
MULTIPOLYGON value using its WKT representation and SRID.
PointFromText(wkt[,srid])
-
Constructs a
POINT value using its WKT representation and SRID.
PolyFromText(wkt[,srid])
-
PolygonFromText(wkt[,srid])
-
Constructs a
POLYGON 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 implement these functions:
BdMPolyFromText(wkt,srid)
-
Constructs a
MultiPolygon value from a
MultiLineString value in WKT format containing
an arbitrary collection of closed LineString values.
BdPolyFromText(wkt,srid)
-
Constructs a
Polygon value from a
MultiLineString value in WKT format containing
an arbitrary collection of closed LineString values.
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.
GeomCollFromWKB(wkb[,srid])
-
GeometryCollectionFromWKB(wkt[,srid])
-
Constructs a
GEOMETRYCOLLECTION value using its WKB representation and SRID.
GeomFromWKB(wkb[,srid])
-
GeometryFromWKB(wkt[,srid])
-
Constructs a geometry value of any type using its WKB representation and SRID.
LineFromWKB(wkb[,srid])
-
LineStringFromWKB(wkb[,srid])
-
Constructs a
LINESTRING value using its WKB representation and SRID.
MLineFromWKB(wkb[,srid])
-
MultiLineStringFromWKB(wkb[,srid])
-
Constructs a
MULTILINESTRING value using its WKB representation and SRID.
MPointFromWKB(wkb[,srid])
-
MultiPointFromWKB(wkb[,srid])
-
Constructs a
MULTIPOINT value using its WKB representation and SRID.
MPolyFromWKB(wkb[,srid])
-
MultiPolygonFromWKB(wkb[,srid])
-
Constructs a
MULTIPOLYGON value using its WKB representation and SRID.
PointFromWKB(wkb[,srid])
-
Constructs a
POINT value using its WKB representation and SRID.
PolyFromWKB(wkb[,srid])
-
PolygonFromWKB(wkb[,srid])
-
Constructs a
POLYGON 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 implement these functions:
BdMPolyFromWKB(wkb,srid)
-
Constructs a
MultiPolygon value from a
MultiLineString value in WKB format containing
an arbitrary collection of closed LineString values.
BdPolyFromWKB(wkb,srid)
-
Constructs a
Polygon value from a
MultiLineString value in WKB format containing
an arbitrary collection of closed LineString values.
Note: MySQL does not 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.
GeometryCollection(g1,g2,...)
-
Constructs a WKB
GeometryCollection. If any argument is not a
well-formed WKB representation of a geometry, the return value is
NULL.
LineString(pt1,pt2,...)
-
Constructs a WKB
LineString value 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.
MultiPoint(pt1,pt2,...)
-
Constructs a WKB
MultiPoint value using WKB Point arguments.
If any argument is not a WKBPoint, 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.
Point(x,y)
-
Constructs a WKB
Point using its coordinates.
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.
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
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 you can perform the conversion 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:
Geometry values stored in a table can be fetched in internal
format. You can also convert them into WKT or WKB format.
Fetching geometry values using internal format can be useful in
table-to-table transfers:
CREATE TABLE geom2 (g GEOMETRY) SELECT g FROM geom;
The AsText() function 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) |
+-------------------------+
The AsBinary() function converts a geometry from internal format into a BLOB containing
the WKB value.
SELECT AsBinary(g) FROM geom;
After populating spatial columns with values, you are ready to
query and analyze 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
MySQL supports the following functions for converting geometry values between
internal format and either WKT or WKB format:
AsBinary(g)
-
Converts a value in internal geometry format to its WKB representation
and returns the binary result.
AsText(g)
-
Converts a value in internal geometry format to its WKT representation
and returns the string result.
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) |
+--------------------------+
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
section 18.4.2.1 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
section 18.4.2.2 Creating Geometry Values Using WKB 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.
The functions listed in this section do not restrict their argument and
accept a geometry value of any type.
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
section 18.2.2 Class Geometry.)
mysql> SELECT Dimension(GeomFromText('LineString(1 1,2 2)'));
+------------------------------------------------+
| Dimension(GeomFromText('LineString(1 1,2 2)')) |
+------------------------------------------------+
| 1 |
+------------------------------------------------+
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))
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 |
+------------------------------------------+
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 |
+-----------------------------------------------+
The OpenGIS specification also defines the following functions, which MySQL
does not 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 geometry 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)
-
Currently, this function is a placeholder and should not be used.
If implemented, its behavior will be as described in the next paragraph.
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.
A Point consists of 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 |
+--------------------------------------+
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, its StartPoint() 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 implement:
IsRing(ls)
-
Returns 1 if the
LineString value ls is closed
(that is, 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.
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 |
+--------------------------------------------------------------------+
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 |
+----------------------------------------------------------------------------+
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) |
+-----------------------------------------------------------------------------------------------+
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 |
+----------------------------------------------------------------------------------------+
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 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.
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) |
+------------------------------------------------------------------------------------------+
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 |
+------------------------------------------------------------------------------------+
In the section section 18.5.2 Geometry 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)
OpenGIS proposes a number of other functions that can produce
geometries. They are designed to implement Spatial Operators.
These functions are not implemented in MySQL.
They may appear in future releases.
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.
Difference(g1,g2)
-
Returns a geometry that represents the point set difference of the geometry
value
g1 with g2.
Intersection(g1,g2)
-
Returns a geometry that represents the point set intersection of the geometry
values
g1 with g2.
SymDifference(g1,g2)
-
Returns a geometry that represents the point set symmetric difference of the
geometry value
g1 with g2.
Union(g1,g2)
-
Returns a geometry that represents the point set union of the geometry values
g1 and g2.
The functions described in these sections take two geometries as input
parameters and return a qualitive or quantitive relation between them.
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 |
+----------------------+----------------------+
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).
MBREqual(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.
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 |
+--------------------+--------------------+
The OpenGIS specification defines the following functions. Currently,
MySQL does not implement them according to the specification. Those that
are implemented return the same result as the corresponding
MBR-based functions. This includes functions in the following list
other than Distance() and Related().
These functions may be implemented in future releases with 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.
Distance(g1,g2)
-
Returns as a double-precision number
the shortest distance between any two points in the two geometries.
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.
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 if this
function is implemented.
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.
Search operations in non-spatial databases can be optimized
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 optimize
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 uses 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.
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:
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
The optimizer 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 SELECT statement, 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 may also be used for optimizing
other functions.
See section 18.5.4 Functions for Testing Spatial Relations Between Geometric Objects.
- 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.
- The OpenGIS function
Length() on LineString and MultiLineString currently should be called in MySQL as GLength()
-
The problem is that there is an existing SQL function
Length() which 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.
Go to the first, previous, next, last section, table of contents.
|
| Who's Online |
| Guest Users: 6 |
 |
Deliver First Class Web Sites: 101 Essential Checklists
Want to learn how to make your web sites usable and accessible? Want to ensure that your sites meet current best practice, without spending hours trawling through incomprehensible specifications and recommendations from dozens of different books, research papers, and web sites? Want to make sure that the sites you build are "right the first time," requiring no costly redevelopments? |
More Sample Chapters |
|
Free Sample Chapters