cache simplified

Author: fjetter

Diff for: dask_expr/_core.py

@@ -48,6 +48,7 @@ def __new__(cls, *args, **kwargs):
             return Expr._instances[_name]
 
         Expr._instances[_name] = inst
+        inst.fully_simplified = False
         if inst._required_attribute:
             dep = next(iter(_dependencies))._meta
             if not hasattr(dep, inst._required_attribute):
@@ -299,9 +300,12 @@ def simplify_once(self, dependents: defaultdict):
         expr:
             output expression
         """
+        if self.fully_simplified:
+            return self
         expr = self
-
+        n = 0
         while True:
+            n += 1
             out = expr._simplify_down()
             if out is None:
                 out = expr
@@ -338,7 +342,8 @@ def simplify_once(self, dependents: defaultdict):
                 expr = type(expr)(*new_operands)
 
             break
-
+        if expr is self:
+            expr.fully_simplified = True
         return expr
 
     def simplify(self) -> Expr: