Reduce number of AST walks in while_changing

Author: georgesittas

sqlglot/helper.py

@@ -211,25 +211,31 @@ def while_changing(expression: Expression, func: t.Callable[[Expression], E]) ->
     Returns:
         The transformed expression.
     """
+    end_hash: t.Optional[int] = None
+
     while True:
-        for n in reversed(tuple(expression.walk())):
-            n._hash = hash(n)
+        # No need to walk the AST– we've already cached the hashes in the previous iteration
+        if end_hash is None:
+            for n in reversed(tuple(expression.walk())):
+                n._hash = hash(n)
 
         start_hash = hash(expression)
         expression = func(expression)
 
+        expression_nodes = tuple(expression.walk())
+
         # Uncache previous caches so we can recompute them
-        for n in reversed(tuple(expression.walk())):
+        for n in reversed(expression_nodes):
             n._hash = None
             n._hash = hash(n)
 
         end_hash = hash(expression)
 
-        # ... and reset the hash so we don't risk it becoming out of date if a mutation happens
-        for n in expression.walk():
-            n._hash = None
-
         if start_hash == end_hash:
+            # ... and reset the hash so we don't risk it becoming out of date if a mutation happens
+            for n in expression_nodes:
+                n._hash = None
+
             break
 
     return expression