fix: aggregate keys to read ahead of time

src/uproot/_dask.py

@@ -897,16 +897,23 @@ def read_tree(self, tree: HasBranches, start: int, stop: int) -> AwkArray:
         # Populate container with placeholders if keys aren't required
         # Otherwise, read from disk
         container = {}
-        for buffer_key, dtype in form.expected_from_buffers(
+
+        expected_from_buffers = form.expected_from_buffers(
             buffer_key=form_info.buffer_key
-        ).items():
-            # Which key(s) does this buffer require. This code permits the caller
-            # to require multiple keys to compute a single buffer.
-            keys_for_buffer = form_info.keys_for_buffer_keys({buffer_key})
+        )
+
+        # At this point, we've dispensed with the information pertaining to
+        # which buffer keys were read. Instead, we'll populate the mapping for
+        # each buffer key whose source TTree keys were loaded.
+        keys_to_read = set.intersection(
+            *(form_info.keys_for_buffer_keys({k}) for k in expected_from_buffers)
+        )
+
+        for buffer_key, dtype in expected_from_buffers.items():
             # If reading this buffer loads a permitted key, read from the tree
             # We might not have _all_ keys if e.g. buffer A requires one
             # but not two of the keys required for buffer B
-            if all(k in self.common_keys for k in keys_for_buffer):
+            if all(k in self.common_keys for k in keys_to_read):
                 container[buffer_key] = mapping[buffer_key]
             # Otherwise, introduce a placeholder
             else: