Replace elide_data_returned with count timeout

optimade/server/config.py

@@ -157,9 +157,10 @@ class ServerConfig(BaseSettings):
         None, description="Host settings to pass through to the `Elasticsearch` class."
     )
 
-    elide_data_returned: bool = Field(
-        False,
-        description="Whether to skip counting all the results for every query (to set the `data_returned` field), as this may be too strenuous for large databases. Currently only supports MongoDB.",
+    mongo_count_timeout: int = Field(
+        5,
+        description="""Number of seconds to allow MongoDB to perform a full database count before falling back to `null`.
+This operation can require a full COLLSCAN for empty queries which can be prohibitively slow if the database does not fit into the active set, hence a timeout can drastically speed-up response times.""",
     )
 
     mongo_database: str = Field(

optimade/server/entry_collections/mongo.py

@@ -80,10 +80,11 @@ def count(self, **kwargs: Any) -> int:
         for k in list(kwargs.keys()):
             if k not in ("filter", "skip", "limit", "hint", "maxTimeMS"):
                 del kwargs[k]
-        if "filter" not in kwargs:  # "filter" is needed for count_documents()
-            kwargs["filter"] = {}
+        if "filter" not in kwargs:
             return self.collection.estimated_document_count()
         else:
+            if "maxTimeMS" not in kwargs:
+                kwargs["maxTimeMS"] = 1000 * CONFIG.mongo_count_timeout
             return self.collection.count_documents(**kwargs)
 
     def insert(self, data: List[EntryResource]) -> None: