diff --git a/src/transaction/client.rs b/src/transaction/client.rs
index 50ddd76..c4e8557 100644
--- a/src/transaction/client.rs
+++ b/src/transaction/client.rs
@@ -183,7 +183,9 @@ impl Client {
 
     /// Request garbage collection (GC) of the TiKV cluster.
     ///
-    /// GC deletes MVCC records whose timestamp is lower than the given `safepoint`.
+    /// GC deletes MVCC records whose timestamp is lower than the given `safepoint`. We must guarantee
+    ///  that all transactions started before this timestamp had committed. We can keep an active
+    /// transaction list in application to decide which is the minimal start timestamp of them.
     ///
     /// For each key, the last mutation record (unless it's a deletion) before `safepoint` is retained.
     ///
diff --git a/src/transaction/transaction.rs b/src/transaction/transaction.rs
index 8b6f962..c089244 100644
--- a/src/transaction/transaction.rs
+++ b/src/transaction/transaction.rs
@@ -635,6 +635,11 @@ impl<PdC: PdClient> Transaction<PdC> {
         res
     }
 
+    /// Get the start timestamp of this transaction.
+    pub fn start_timestamp(&self) -> Timestamp {
+        self.timestamp.clone()
+    }
+
     /// Send a heart beat message to keep the transaction alive on the server and update its TTL.
     ///
     /// Returns the TTL set on the transaction's locks by TiKV.