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

Appendix C
Code Implementation
C.1 Temporal Raster Functions
PostgreSQL allows also abilities for users to define new types and their own functions. Then
CREATE AGGREGATE can be used to provide the desired features that are not provided by PostgreSQL.
An aggregate function is identified by its name and input data type. An aggregate function is made
from one or two ordinary functions: a state transition function sfunc and an optional final calculation
function ffunc.
An temporary variable of type stype is used to hold the current value of the state transition
function. At each input data item, the state transition function is invoked to calculate a new state
value. After all the data has been processed, the final function is invoked once to calculate the
aggregate result. If there is no final function then the aggregate returns the current state value as
final result. sum is an example of this kind of aggregate, where for each input, sum adds its value to
current state value and returns the sum of all input value as final result.
A final function is specified in the case where the final result is different from the input data type.
Average aggregate function is an example of this case and will be examined thereafter. If the final
function is declared "strict", then it will not be called when the current state value is null. Instead a
null result will be returned automatically.
Example of creating the aggregate function ST_Avg:
CREATE AGGREGATE ST_Avg(st_rastertemp) (
SFUNC=array_append,
STYPE=st_rastertemp[],
FINALFUNC=ST_AvgTempRaster
);
CREATE OR REPLACE FUNCTION ST_AvgTempRaster(arrayRastTemp st_rastertemp[])
RETURNS raster AS
$$
DECLARE
newraster raster;
oldRast1 raster;
oldRast2 raster;
width int;
height int;
cx int;
cy int;
size int;
s int;
value double precision = 0;
intType boolean = true;
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