java - Return rows from a PL/pgSQL function

Question

I have a procedure in PostgreSQL:

CREATE OR REPLACE FUNCTION get_geom_difference()
RETURNS void AS
$$
BEGIN
SELECT filedata.num,st_area(ST_Difference(ST_TRANSFORM(filedata.the_geom,70066),filedata_temp.the_geom))
FROM filedata, filedata_temp
Where filedata.num=filedata_temp.num

end;
$$
LANGUAGE 'plpgsql'

I call it in Java and want to get result of this procedure. How to change this procedure to make it possible get a result? And how to work with it in JDBC?

Now I use this:

Integer fileId;
Class.forName("org.postgresql.Driver");
Connection connect= null;
connect = DriverManager.getConnection("jdbc:postgresql://localhost:5432/postgis","postgres","123456");
java.sql.CallableStatement proc =  connect.prepareCall("{?=call get_geom_difference()}");
proc.registerOutParameter(1, java.sql.Types.Integer);
proc.execute();
ResultSet results = (ResultSet) proc.getObject(1);
while (results.next()) {
fileId=r.getInt("num");
}
proc.close();
connect.close();
out.println(fileId);

But When I try to call the procedure in JDBC I get

error org.apache.jasper.JasperException: An exception occurred processing JSP page /commit_changes.jsp at line 25

Line 25 is: proc.execute();

Answer 1

Proper function definition

CREATE OR REPLACE FUNCTION get_geom_difference()
  RETURNS TABLE (num mumeric, my_area geometry) AS
$BODY$
   SELECT f.num
         ,st_area(ST_Difference(ST_TRANSFORM(f.the_geom, 70066), t.the_geom))
   FROM   filedata f
   JOIN   filedata_temp t USING (num);
$BODY$
LANGUAGE sql;

You are returning a SET of a composite type (two columns), you have to declare the the function accordingly. RETURNS TABLE is the most convenient way to do this.

Be sure to table-qualify the column names in the query so they do not conflict with OUT columns of the same name.

You can use a language SQL function for this basic query (or you could just execute the raw SQL), no need for plpgsql.

Call the function in SQL

SELECT * FROM get_geom_difference();

Do it via JDBC

I quote the manual here

Functions that return data as a set should not be called via the CallableStatement interface, but instead should use the normal Statement or PreparedStatement interfaces.

I also took this example from the site and adapted it:

Statement stmt = conn.createStatement();
stmt.execute(" <function definition from above goes here> ");
ResultSet rs = stmt.executeQuery("SELECT * FROM get_geom_difference();");
while (rs.next()) {
    // do something
}
rs.close();
stmt.close();

You could also use a refcursor. Read more in the manual.