compiler: Improve validation of ORDER BY clause with column number

Only targets in the SELECT target list can be referenced by column
number in the ORDER BY clause, not auxiliary targets added to compute
the GROUP BY clause.

beanquery/compiler.py

@@ -219,10 +219,6 @@ def _compile_order_by(self, order_by, c_targets):
         if not order_by:
             return [], None
 
-        new_targets = c_targets[:]
-        c_target_expressions = [c_target.c_expr for c_target in c_targets]
-        order_spec = []
-
         # Compile order-by expressions and resolve them to their targets if
         # possible. A ORDER-BY column may be one of the following:
         #
@@ -232,7 +228,16 @@ def _compile_order_by(self, order_by, c_targets):
         #
         # References by name are converted to indexes. New expressions are
         # inserted into the list of targets as invisible targets.
-        targets_name_map = {target.name: index for index, target in enumerate(c_targets)}
+
+        new_targets = c_targets[:]
+        c_target_expressions = [c_target.c_expr for c_target in c_targets]
+        targets_name_map = {target.name: idx for idx, target in enumerate(c_targets) if target.name is not None}
+        # Only targets appearing in the SELECT targets list can be
+        # referenced by index. These are guaranteed to have a valid name.
+        n_targets = len(targets_name_map)
+
+        order_spec = []
+
         for spec in order_by:
             column = spec.column
             descending = spec.ordering
@@ -241,7 +246,7 @@ def _compile_order_by(self, order_by, c_targets):
             # Process target references by index.
             if isinstance(column, int):
                 index = column - 1
-                if not 0 <= index < len(c_targets):
+                if not 0 <= index < n_targets:
                     raise CompilationError(f'invalid ORDER-BY column index {column}')
 
             else: