Perf: memoize collectSubfields

Collecting subfields occurs after resolving a field's value and before resolving subfield values. This step collects fragment spreads and checks inline fragment conditions. When fetching a list of things, this step is computed with the same inputs and expecting the same outputs for each item in the list. Memoizing ensures the work is done at most once per type returned from a list.

I tested this against the introspection query (which is both synchronous and complex) against a large schema and saw a ~15%-25% reduction in runtime. In practice I don't expect most queries to see this level of speedup as most queries are limited by backend communication and not execution overhead.

Author: leebyron

src/execution/execute.js

@@ -1236,6 +1236,34 @@ function collectAndExecuteSubfields(
   result: mixed,
 ): mixed {
   // Collect sub-fields to execute to complete this value.
+  const subFieldNodes = collectSubfields(exeContext, returnType, fieldNodes);
+  return executeFields(exeContext, returnType, result, path, subFieldNodes);
+}
+
+/**
+ * A memoized collection of relevant subfields in the context of the return
+ * type. Memoizing ensures the subfields are not repeatedly calculated, which
+ * saves overhead when resolving lists of values.
+ */
+const subfieldCache: WeakMap<
+  $ReadOnlyArray<FieldNode>,
+  WeakMap<GraphQLObjectType, ObjMap<Array<FieldNode>>>,
+> = new WeakMap();
+function collectSubfields(
+  exeContext: ExecutionContext,
+  returnType: GraphQLObjectType,
+  fieldNodes: $ReadOnlyArray<FieldNode>,
+): ObjMap<Array<FieldNode>> {
+  let cacheByReturnType = subfieldCache.get(fieldNodes);
+  if (cacheByReturnType) {
+    const cachedSubFieldNodes = cacheByReturnType.get(returnType);
+    if (cachedSubFieldNodes) {
+      return cachedSubFieldNodes;
+    }
+  } else {
+    cacheByReturnType = new WeakMap();
+    subfieldCache.set(fieldNodes, cacheByReturnType);
+  }
   let subFieldNodes = Object.create(null);
   const visitedFragmentNames = Object.create(null);
   for (let i = 0; i < fieldNodes.length; i++) {
@@ -1250,8 +1278,8 @@ function collectAndExecuteSubfields(
       );
     }
   }
-
-  return executeFields(exeContext, returnType, result, path, subFieldNodes);
+  cacheByReturnType.set(returnType, subFieldNodes);
+  return subFieldNodes;
 }
 
 /**