Implement aggregation and grouping pushdown

Makefile

@@ -1,6 +1,6 @@
 srcdir       = .
 MODULE_big   = multicorn
-OBJS         =  src/errors.o src/python.o src/query.o src/multicorn.o
+OBJS         = src/deparse.o src/errors.o src/python.o src/query.o src/multicorn.o
 
 
 DATA         = $(filter-out $(wildcard sql/*--*.sql),$(wildcard sql/*.sql))
@@ -19,7 +19,7 @@ directories.stamp:
 
 $(OBJS): directories.stamp
 
-install: python_code 
+install: python_code
 
 sql/$(EXTENSION)--$(EXTVERSION).sql: sql/$(EXTENSION).sql directories.stamp
 	cp $< $@

python/multicorn/__init__.py

@@ -212,6 +212,31 @@ def can_sort(self, sortkeys):
         """
         return []
 
+    def can_pushdown_upperrel(self):
+        """
+        Method called from the planner to ask the FDW whether it supports upper
+        relation pushdown (i.e. aggregation, grouping, etc.), and if so return
+        a data structure with appropriate details.
+
+        Return:
+            None if pushdown not supported, otherwise a dictionary containing
+            more granular details for the planning phase, in the form:
+
+            {
+                "groupby_supported": <true_or_false>, # can be ommited if false
+                "agg_functions": {
+                    <PG_agg_func_name>: <foreign_agg_func_name>,
+                    ...
+                },
+            }
+
+            Each entry in `agg_functions` dict corresponds to a maping between
+            the name of a aggregation function in PostgreSQL, and the equivalent
+            foreign function. If no mapping exists for an aggregate function any
+            queries containing it won't be pushed down.
+        """
+        return None
+
     def get_path_keys(self):
         u"""
         Method called from the planner to add additional Path to the planner.
@@ -269,7 +294,7 @@ def get_path_keys(self):
         """
         return []
 
-    def explain(self, quals, columns, sortkeys=None, verbose=False):
+    def explain(self, quals, columns, sortkeys=None, aggs=None, group_clauses=None, verbose=False):
         """Hook called on explain.
 
         The arguments are the same as the :meth:`execute`, with the addition of
@@ -280,7 +305,7 @@ def explain(self, quals, columns, sortkeys=None, verbose=False):
         """
         return []
 
-    def execute(self, quals, columns, sortkeys=None):
+    def execute(self, quals, columns, sortkeys=None, aggs=None, group_clauses=None):
         """Execute a query in the foreign data wrapper.
 
         This method is called at the first iteration.
@@ -311,8 +336,16 @@ def execute(self, quals, columns, sortkeys=None):
                 You should return AT LEAST those columns when returning a
                 dict. If returning a sequence, every column from the table
                 should be in the sequence.
+
+        Kwargs:
             sortkeys (list): A list of :class:`SortKey`
                 that the FDW said it can enforce.
+            aggs (dict): A dictionary mapping aggregation key with function and
+                column to be used in the aggregation operation. Result should be
+                returned under the provided aggregation key.
+            group_clauses (list): A list of columns used in GROUP BY statements.
+                For each column provided the returned response should have a
+                corresponding value in each row using that column name as the key.
 
         Returns:
             An iterable of python objects which can be converted back to PostgreSQL.