A few more tweaks

docs/API/V1/list.md

@@ -30,7 +30,9 @@ private datasets, while specifying `public` will show only `public` datasets
 `daterange` boolean \
 Instead of returning a filtered set of datasets, return only the upload
 timestamps of the oldest and most recent datasets in the filtered set. This
-can be useful for initializing a date picker, for example.
+may be useful for initializing a date picker. If no datasets are selected by
+the specified filters, the `from` and `to` keys (see
+[results](#dataset-date-range)) will not be returned.
 
 `end` date/time \
 Select only datasets created on or before the specified time. Time should be
@@ -77,7 +79,9 @@ for the set of datasets. See [metadata](../metadata.md) for deails on the
 Pbench Server metadata namespaces. Because the `global` and `user` namespaces
 are completely dynamic, and the `dataset.metalog` sub-namespace varies greatly
 across Pbench Agent benchmark scripts, this mode provides a mechanism for a
-metadata visualizer to understand what's available for a set of datasets.
+metadata visualizer to understand what's available for a set of datasets. If no
+datasets are selected by the specified filters, the `keys` key (see
+[results](#key-namespace-summary) will be set to an empty object.
 
 `limit` integer \
 "Paginate" the selected datasets by returning at most `limit` datasets. This

lib/pbench/server/api/resources/datasets_list.py

@@ -332,14 +332,15 @@ def keyspace(self, query: Query) -> JSONOBJECT:
         Returns:
             The aggregated keyspace JSON object
         """
-        aggregate: JSONOBJECT = {
-            "dataset": {c.name: None for c in Dataset.__table__._columns}
-        }
-
         Database.dump_query(query, current_app.logger)
+        aggregate: JSONOBJECT = {}
 
         datasets = query.all()
         for d in datasets:
+            if not aggregate:
+                aggregate.update(
+                    {"dataset": {c.name: None for c in Dataset.__table__._columns}}
+                )
             for m in d.metadatas:
                 # "metalog" is a top-level key in the Metadata schema, but we
                 # report it as a sub-key of "dataset".