Adds a warning to the stores len function

lib/src/store/memory.rs

@@ -195,6 +195,8 @@ impl MemoryStore {
     }
 
     /// Returns the number of quads in the store
+    ///
+    /// Warning: this function executes a full scan
     pub fn len(&self) -> usize {
         self.indexes()
             .spog

lib/src/store/rocksdb.rs

@@ -150,6 +150,8 @@ impl RocksDbStore {
     }
 
     /// Returns the number of quads in the store
+    ///
+    /// Warning: this function executes a full scan
     pub fn len(&self) -> usize {
         self.db
             .full_iterator_cf(self.spog_cf(), IteratorMode::Start)

lib/src/store/sled.rs

@@ -151,6 +151,8 @@ impl SledStore {
     }
 
     /// Returns the number of quads in the store
+    ///
+    /// Warning: this function executes a full scan
     pub fn len(&self) -> usize {
         self.quads.len() / 6
     }