perf: subset unmixing prior to listing

docs/getting_started.md

@@ -112,6 +112,15 @@ optimal performance, this method should be called with the layouts enabled, with
 files organized with folders (see the [advanced section](#disable-layouts)), and
 on filters whose information is encoded in the folders.
 
+In case you are working on a small set of data, it is safe to ignore this
+warning
+
+```{code-cell}
+from fcollections.core import PerformanceWarning
+import warnings
+warnings.simplefilter("ignore", PerformanceWarning)
+```
+
 ## Access metadata
 
 The database can display information about the variables and attributes

src/fcollections/core/__init__.py

@@ -29,6 +29,7 @@
     FilesDatabase,
     IFilterBuilder,
     NotExistingPathError,
+    PerformanceWarning,
     SubsetsUnmixer,
 )
 from ._listing import (
@@ -114,4 +115,5 @@
     "walk",
     "VisitResult",
     "FileSystemMetadataCollector",
+    "PerformanceWarning",
 ]