add ST_EXTENT aggregate function

site_cubicweb.py

@@ -7,7 +7,7 @@
 # psql <database> -f /usr/share/postgresql/9.1/contrib/postgis-1.5/spatial_ref_sys.sql
 # cubicweb-ctl db-init <instance>
 
-from logilab.database import FunctionDescr
+from logilab.database import FunctionDescr, AggrFunctionDescr
 from logilab.database import get_db_helper
 from logilab.database.sqlgen import SQLExpression
 
@@ -302,6 +302,11 @@ class ST_M(FunctionDescr):
     rtype = 'Float'
 
 
+class ST_EXTENT(AggrFunctionDescr):
+    supported_backends = ('postgres',)
+    rtype = 'Geometry'
+
+
 register_function(ST_EQUALS)
 register_function(ST_INTERSECTS)
 register_function(ST_INTERSECTION)
@@ -337,3 +342,4 @@ register_function(ST_X)
 register_function(ST_Y)
 register_function(ST_Z)
 register_function(ST_M)
+register_function(ST_EXTENT)