"=" or ":=" ?

    Hello,

    I'm trying to optimize assign_vertex_id() function provided by
pgRouting/PostGIS. In this function, I can see :

DECLARE
       points record;
       i record;
       source_id int;
       target_id int;
       pre varchar;
       post varchar;

       srid integer;

       BEGIN

        BEGIN
            DROP TABLE vertices_tmp;
             EXCEPTION
            WHEN UNDEFINED_TABLE THEN
                 END;

        EXECUTE 'CREATE TABLE vertices_tmp (id serial)';

        FOR i IN EXECUTE 'SELECT srid FROM geometry_columns WHERE
f_table_name='''|| quote_ident(geom_table)||'''' LOOP
        END LOOP;

        srid := i.srid;

        EXECUTE 'SELECT addGeometryColumn(''vertices_tmp'', ''the_geom'',
'||srid||', ''POINT'', 2)';


                 CREATE INDEX vertices_tmp_idx ON vertices_tmp USING
GIST (the_geom);

                 pre = '';
                 post = '';

    I don't understand last assignations. In pgsql documentation, ther is
written that all assignations have to be written with ":=", not with
"=". What is the difference between "=" and ":=" ? I don't find any
information about "=".

    Regards,

    JKB