Allow dialects to customize group by clause compilation

Refactored :class:`.SQLCompiler` to expose a
:meth:`.SQLCompiler.group_by_clause` method similar to the
:meth:`.SQLCompiler.order_by_clause` and :meth:`.SQLCompiler.limit_clause`
methods, which can be overridden by dialects to customize how GROUP BY
renders.  Pull request courtesy Samuel Chou.

Change-Id: I0a7238e55032558c27a0c56a72907c7b883456f1
Pull-request: https://github.com/zzzeek/sqlalchemy/pull/474
(cherry picked from commit 33fccc486111fc6b41eab651cc7325c83099ad45)

3 files changed, 59 insertions, 9 deletions

diff --git a/doc/build/changelog/unreleased_12/pr474.rst b/doc/build/changelog/unreleased_12/pr474.rst
new file mode 100644
index 000000000..571fe5e97
--- /dev/null
+++ b/doc/build/changelog/unreleased_12/pr474.rst
@@ -0,0 +1,9 @@
+.. change::
+    :tags: feature, sql
+    :versions: 1.3.0b1
+
+    Refactored :class:`.SQLCompiler` to expose a
+    :meth:`.SQLCompiler.group_by_clause` method similar to the
+    :meth:`.SQLCompiler.order_by_clause` and :meth:`.SQLCompiler.limit_clause`
+    methods, which can be overridden by dialects to customize how GROUP BY
+    renders.  Pull request courtesy Samuel Chou.
diff --git a/lib/sqlalchemy/sql/compiler.py b/lib/sqlalchemy/sql/compiler.py
index d93e08ece..e27a736ef 100644
--- a/lib/sqlalchemy/sql/compiler.py
+++ b/lib/sqlalchemy/sql/compiler.py
@@ -968,11 +968,7 @@ class SQLCompiler(Compiled):
              for i, c in enumerate(cs.selects))
         )
 
-        group_by = cs._group_by_clause._compiler_dispatch(
-            self, asfrom=asfrom, **kwargs)
-        if group_by:
-            text += " GROUP BY " + group_by
-
+        text += self.group_by_clause(cs, **dict(asfrom=asfrom, **kwargs))
         text += self.order_by_clause(cs, **kwargs)
         text += (cs._limit_clause is not None
                  or cs._offset_clause is not None) and \
@@ -1911,10 +1907,7 @@ class SQLCompiler(Compiled):
                 text += " \nWHERE " + t
 
         if select._group_by_clause.clauses:
-            group_by = select._group_by_clause._compiler_dispatch(
-                self, **kwargs)
-            if group_by:
-                text += " GROUP BY " + group_by
+            text += self.group_by_clause(select, **kwargs)
 
         if select._having is not None:
             t = select._having._compiler_dispatch(self, **kwargs)
@@ -1970,7 +1963,18 @@ class SQLCompiler(Compiled):
         """
         return select._distinct and "DISTINCT " or ""
 
+    def group_by_clause(self, select, **kw):
+        """allow dialects to customize how GROUP BY is rendered."""
+
+        group_by = select._group_by_clause._compiler_dispatch(self, **kw)
+        if group_by:
+            return " GROUP BY " + group_by
+        else:
+            return ""
+
     def order_by_clause(self, select, **kw):
+        """allow dialects to customize how ORDER BY is rendered."""
+
         order_by = select._order_by_clause._compiler_dispatch(self, **kw)
         if order_by:
             return " ORDER BY " + order_by
diff --git a/test/sql/test_compiler.py b/test/sql/test_compiler.py
index 7f2c44bf1..993008c07 100644
--- a/test/sql/test_compiler.py
+++ b/test/sql/test_compiler.py
@@ -28,6 +28,7 @@ from sqlalchemy.sql.expression import ClauseList, _literal_as_text, HasPrefixes
 from sqlalchemy.engine import default
 from sqlalchemy.dialects import mysql, mssql, postgresql, oracle, \
     sqlite, sybase
+from sqlalchemy.dialects.postgresql.base import PGCompiler, PGDialect
 from sqlalchemy.ext.compiler import compiles
 from sqlalchemy.sql import compiler
 
@@ -1297,6 +1298,42 @@ class SelectTest(fixtures.TestBase, AssertsCompiledSQL):
             "GROUP BY myothertable.othername ORDER BY myothertable.othername"
         )
 
+    def test_custom_order_by_clause(self):
+        class CustomCompiler(PGCompiler):
+            def order_by_clause(self, select, **kw):
+                return super(CustomCompiler, self).\
+                    order_by_clause(select, **kw) + " CUSTOMIZED"
+
+        class CustomDialect(PGDialect):
+            name = 'custom'
+            statement_compiler = CustomCompiler
+
+        stmt = select([table1.c.myid]).order_by(table1.c.myid)
+        self.assert_compile(
+            stmt,
+            "SELECT mytable.myid FROM mytable ORDER BY "
+            "mytable.myid CUSTOMIZED",
+            dialect=CustomDialect()
+        )
+
+    def test_custom_group_by_clause(self):
+        class CustomCompiler(PGCompiler):
+            def group_by_clause(self, select, **kw):
+                return super(CustomCompiler, self).\
+                    group_by_clause(select, **kw) + " CUSTOMIZED"
+
+        class CustomDialect(PGDialect):
+            name = 'custom'
+            statement_compiler = CustomCompiler
+
+        stmt = select([table1.c.myid]).group_by(table1.c.myid)
+        self.assert_compile(
+            stmt,
+            "SELECT mytable.myid FROM mytable GROUP BY "
+            "mytable.myid CUSTOMIZED",
+            dialect=CustomDialect()
+        )
+
     def test_for_update(self):
         self.assert_compile(
             table1.select(table1.c.myid == 7).with_for_update(),