PostGIS Raster : Extending PostgreSQL for The Support of ... - CoDE

• Well Known Binary Well Known Binary (WKB) refers to the binary form of the Well Known
Text described above. It is used when inserting a raster using ST_RasterFromWKB() (not yet
implemented) and retrieving a raster using ST_AsBinary().
Example:
SELECT ST_AsBinary(rast) As rastbin
FROM dummy_rast WHERE rid=1;
rastbin
-------------------------------------------------------------------------------
0010000000000000000000000000000000000000000000000000000100000000000000000003400
0000000000000000034000000000000000000000000000000000000000000000000001200000000
0012000024000
• Hexadecimal WKB Hexadecimal WKB (HEXWKB) is a hexadecimal representation of the WKB
form. This form is used by the Python loader raster2pgsql.py and when outputting the value
of a raster field like SELECT rast FROM raster_table.
Example:
SELECT rast from dummy_rast WHERE rid = 2;
---------------------------------------------------------------------------------
01000003009A9999999999A93F9A9999999999A9BF000000E02B274A4100000000771956410000000
0000000000000000000000000FFFFFFFF050005000400FDFEFDFEFEFDFEFEFDF9FAFEFEFCF9FBFDFE
FEFDFCFAFEFEFE04004E627AADD16076B4F9FE6370A9F5FE59637AB0E54F58617087040046566487A
1506CA2E3FA5A6CAFFBFE4D566DA4CB3E454C5665
4.15 Physical Storage
PostGIS Raster enables user to store raster images in the database (storing in-database raster)
or merely register them as external files (registering out-database raster). When registering a raster,
only the raster metadata (width, height, number of band, georeference and path to the actual raster
file) are stored in the database and not the pixel values.
4.15.1 Storing In-database Raster Versus Storing Out-database Raster
The choice made between out-database raster and in-database raster depends on user applications.
When user applications are read-only applications, that means that they only retrieve data and do
not prefer to modify data. Out-database raster storage facilitates these processes by providing:
• Fast application access: storing out-database raster stores the path of the raster image file
conserved in the filesystem. It can speed up the reading of rasters files (JPEG, GIF, PNG) for
web applications in the way that only the path is needed to read when retrieving raster data
and there is no need to convert it from the PostGIS Raster type to an image file.
• Fast loading database: in the first time of loading a raster coverage (a table that contains
a colon of type out-database raster or in-database raster), it is executed faster than normally
since only the raster metadata has to be loaded or copied from the database.
• Applicable SQL operators and functions: all operators and functions, that are available for
in-database raster data (except those involving write operations to the raster data like setting
pixel value), work also with out-database rasters.
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