Create aggregates only if they do not exist

src/pg/sql/04_py_agg.sql

@@ -10,10 +10,22 @@ CREATE OR REPLACE FUNCTION
     END
     $$ LANGUAGE plpgsql;
 
-
-CREATE AGGREGATE CDB_PyAgg(NUMERIC[])(
-    SFUNC = CDB_PyAggS,
-    STYPE = Numeric[],
-    INITCOND = "{}" 
-);
-
+-- Create aggregate if it did not exist
+DO $$
+BEGIN
+    IF NOT EXISTS (
+        SELECT *
+        FROM pg_catalog.pg_proc p
+            LEFT JOIN pg_catalog.pg_namespace n ON n.oid = p.pronamespace
+        WHERE n.nspname = 'cdb_crankshaft'
+            AND p.proname = 'cdb_pyagg'
+            AND p.proisagg)
+    THEN
+        CREATE AGGREGATE CDB_PyAgg(NUMERIC[]) (
+            SFUNC = CDB_PyAggS,
+            STYPE = Numeric[],
+            INITCOND = "{}"
+        );
+    END IF;
+END
+$$ LANGUAGE plpgsql;