fix(postgres): Support WITHIN GROUP ( order_by_clause ) FILTER for Postgres (#4652)

* Support filter in aggregates

* Support only for Postgres

* lint

* Default on

* Feedback minus _parse_window

Author: gl3nnleblanc

sqlglot/expressions.py

@@ -1577,7 +1577,7 @@ def recursive(self) -> bool:
 
 
 class WithinGroup(Expression):
-    arg_types = {"this": True, "expression": False}
+    arg_types = {"this": True, "expression": False, "filter": False}
 
 
 # clickhouse supports scalar ctes

sqlglot/generator.py

@@ -2724,7 +2724,11 @@ def windowspec_sql(self, expression: exp.WindowSpec) -> str:
     def withingroup_sql(self, expression: exp.WithinGroup) -> str:
         this = self.sql(expression, "this")
         expression_sql = self.sql(expression, "expression")[1:]  # order has a leading space
-        return f"{this} WITHIN GROUP ({expression_sql})"
+        filter_sql = self.sql(expression, "filter")[1:]
+        result = f"{this} WITHIN GROUP ({expression_sql})"
+        if filter_sql:
+            result = f"{result} FILTER ({filter_sql})"
+        return result
 
     def between_sql(self, expression: exp.Between) -> str:
         this = self.sql(expression, "this")

sqlglot/parser.py

@@ -6614,7 +6614,13 @@ def _parse_window(
         # https://learn.microsoft.com/en-us/sql/t-sql/functions/percentile-disc-transact-sql?view=sql-server-ver16
         if self._match_text_seq("WITHIN", "GROUP"):
             order = self._parse_wrapped(self._parse_order)
-            this = self.expression(exp.WithinGroup, this=this, expression=order)
+            if self._match(TokenType.FILTER):
+                where_clause = self._parse_wrapped(self._parse_where)
+                this = self.expression(
+                    exp.WithinGroup, this=this, expression=order, filter=where_clause
+                )
+            else:
+                this = self.expression(exp.WithinGroup, this=this, expression=order)
 
         # SQL spec defines an optional [ { IGNORE | RESPECT } NULLS ] OVER
         # Some dialects choose to implement and some do not.

tests/dialects/test_postgres.py

@@ -833,6 +833,10 @@ def test_postgres(self):
             "/* + some comment */ SELECT b.foo, b.bar FROM baz AS b",
         )
 
+        self.validate_identity(
+            "SELECT PERCENTILE_CONT(0.5) WITHIN GROUP (ORDER BY a) FILTER (WHERE CAST(b AS BOOLEAN)) AS mean_value FROM (VALUES (0, 't')) AS fake_data(a, b)"
+        )
+
     def test_ddl(self):
         # Checks that user-defined types are parsed into DataType instead of Identifier
         self.parse_one("CREATE TABLE t (a udt)").this.expressions[0].args["kind"].assert_is(