Add caching to recursive simplify_once calls (#797)

dask_expr/_core.py

@@ -267,7 +267,7 @@ def rewrite(self, kind: str):
 
         return expr
 
-    def simplify_once(self, dependents: defaultdict):
+    def simplify_once(self, dependents: defaultdict, simplified: dict):
         """Simplify an expression
 
         This leverages the ``._simplify_down`` and ``._simplify_up``
@@ -278,12 +278,18 @@ def simplify_once(self, dependents: defaultdict):
 
         dependents: defaultdict[list]
             The dependents for every node.
+        simplified: dict
+            Cache of simplified expressions for these dependents.
 
         Returns
         -------
         expr:
             output expression
         """
+        # Check if we've already simplified for these dependents
+        if self._name in simplified:
+            return simplified[self._name]
+
         expr = self
 
         while True:
@@ -314,7 +320,10 @@ def simplify_once(self, dependents: defaultdict):
                 if isinstance(operand, Expr):
                     # Bandaid for now, waiting for Singleton
                     dependents[operand._name].append(weakref.ref(expr))
-                    new = operand.simplify_once(dependents=dependents)
+                    new = operand.simplify_once(
+                        dependents=dependents, simplified=simplified
+                    )
+                    simplified[operand._name] = new
                     if new._name != operand._name:
                         changed = True
                 else:
@@ -332,7 +341,7 @@ def simplify(self) -> Expr:
         expr = self
         while True:
             dependents = collect_dependents(expr)
-            new = expr.simplify_once(dependents=dependents)
+            new = expr.simplify_once(dependents=dependents, simplified={})
             if new._name == expr._name:
                 break
             expr = new

dask_expr/_expr.py

@@ -1742,9 +1742,7 @@ def vals(self):
 
     @functools.cached_property
     def _meta(self):
-        args = [
-            meta_nonempty(op._meta) if isinstance(op, Expr) else op for op in self._args
-        ]
+        args = [op._meta if isinstance(op, Expr) else op for op in self._args]
         return make_meta(self.operation(*args, **self._kwargs))
 
     def _tree_repr_argument_construction(self, i, op, header):