Refs #35102 -- Optimized replace_expressions()/relabelling aliases by adding early return.

This avoids costly hashing.

Thanks Anthony Shaw for the report.

Co-Authored-By: Simon Charette <charette.s@gmail.com>

django/db/models/expressions.py

@@ -402,10 +402,13 @@ class BaseExpression:
         return clone
 
     def replace_expressions(self, replacements):
+        if not replacements:
+            return self
         if replacement := replacements.get(self):
             return replacement
+        if not (source_expressions := self.get_source_expressions()):
+            return self
         clone = self.copy()
-        source_expressions = clone.get_source_expressions()
         clone.set_source_expressions(
             [
                 expr.replace_expressions(replacements) if expr else None

django/db/models/sql/query.py

@@ -972,6 +972,8 @@ class Query(BaseExpression):
         relabelling any references to them in select columns and the where
         clause.
         """
+        if not change_map:
+            return self
         # If keys and values of change_map were to intersect, an alias might be
         # updated twice (e.g. T4 -> T5, T5 -> T6, so also T4 -> T6) depending
         # on their order in change_map.

django/db/models/sql/where.py

@@ -204,6 +204,8 @@ class WhereNode(tree.Node):
         Relabel the alias values of any children. 'change_map' is a dictionary
         mapping old (current) alias values to the new values.
         """
+        if not change_map:
+            return self
         for pos, child in enumerate(self.children):
             if hasattr(child, "relabel_aliases"):
                 # For example another WhereNode
@@ -225,6 +227,8 @@ class WhereNode(tree.Node):
         return clone
 
     def replace_expressions(self, replacements):
+        if not replacements:
+            return self
         if replacement := replacements.get(self):
             return replacement
         clone = self.create(connector=self.connector, negated=self.negated)