feat: fix ifthen and switch visitor

src/gateway/converter/conversion_options.py

@@ -39,7 +39,9 @@ def arrow():
 
 def datafusion():
     """Return standard options to connect to a Datafusion backend."""
-    return ConversionOptions(backend=BackendOptions(BackendEngine.DATAFUSION))
+    options = ConversionOptions(backend=BackendOptions(BackendEngine.DATAFUSION))
+    options.use_switch_expressions_where_possible = False
+    return options
 
 
 def duck_db():

src/gateway/converter/substrait_plan_visitor.py

@@ -178,7 +178,7 @@ def visit_record(self, record: algebra_pb2.Expression.MultiOrList.Record) -> Any
     def visit_if_value(self, if_clause: algebra_pb2.Expression.SwitchExpression.IfValue) -> Any:
         """Visits an if value."""
         if if_clause.HasField('if'):
-            self.visit_expression(getattr(if_clause, 'if'))
+            self.visit_literal(getattr(if_clause, 'if'))
         if if_clause.HasField('then'):
             self.visit_expression(if_clause.then)
 
@@ -239,10 +239,17 @@ def visit_window_rel_function(
         if function.HasField('output_type'):
             self.visit_type(function.output_type)
 
+    def visit_if_clause(self, if_clause: algebra_pb2.Expression.IfThen.IfClause) -> Any:
+        """Visits an if value."""
+        if if_clause.HasField('if'):
+            self.visit_expression(getattr(if_clause, 'if'))
+        if if_clause.HasField('then'):
+            self.visit_expression(if_clause.then)
+
     def visit_if_then(self, if_then: algebra_pb2.Expression.IfThen) -> Any:
         """Visits an if then."""
         for if_then_if in if_then.ifs:
-            self.visit_if_value(if_then_if)
+            self.visit_if_clause(if_then_if)
         if if_then.HasField('else'):
             self.visit_expression(getattr(if_then, 'else'))