diff --git a/examples/pessimistic.rs b/examples/pessimistic.rs
index f4e8c44..ce14394 100644
--- a/examples/pessimistic.rs
+++ b/examples/pessimistic.rs
@@ -71,4 +71,5 @@ async fn main() {
         .await
         .expect("Committing read-only transaction should not fail");
     println!("{:?}", (key1, result));
+    txn3.commit().await.unwrap();
 }
diff --git a/src/transaction/buffer.rs b/src/transaction/buffer.rs
index 265b2a9..390a6d4 100644
--- a/src/transaction/buffer.rs
+++ b/src/transaction/buffer.rs
@@ -8,13 +8,38 @@ use std::{
 use tikv_client_proto::kvrpcpb;
 use tokio::sync::Mutex;
 
+#[derive(Default)]
+struct Mutations {
+    primary_key: Option<Key>,
+    key_mutation_map: BTreeMap<Key, Mutation>,
+}
+
+impl Mutations {
+    fn insert(&mut self, key: impl Into<Key>, mutation: Mutation) {
+        let key = key.into();
+        if self.primary_key.is_none() {
+            self.primary_key = Some(key.clone());
+        }
+        self.key_mutation_map.insert(key, mutation);
+    }
+
+    pub fn get_primary_key_or(&mut self, key: Key) -> Key {
+        self.primary_key.get_or_insert(key).clone()
+    }
+}
+
 /// A caching layer which buffers reads and writes in a transaction.
 #[derive(Default)]
 pub struct Buffer {
-    mutations: Mutex<BTreeMap<Key, Mutation>>,
+    mutations: Mutex<Mutations>,
 }
 
 impl Buffer {
+    /// Get the primary key of the buffer, if not exists, use `key` as the primary key.
+    pub async fn get_primary_key_or(&self, key: Key) -> Key {
+        self.mutations.lock().await.get_primary_key_or(key)
+    }
+
     /// Get a value from the buffer. If the value is not present, run `f` to get
     /// the value.
     pub async fn get_or_else<F, Fut>(&self, key: Key, f: F) -> Result<Option<Value>>
@@ -54,6 +79,7 @@ impl Buffer {
             ) = keys
                 .map(|key| {
                     let value = mutations
+                        .key_mutation_map
                         .get(&key)
                         .map(Mutation::get_value)
                         .unwrap_or(MutationValue::Undetermined);
@@ -92,7 +118,7 @@ impl Buffer {
     {
         // read from local buffer
         let mut mutations = self.mutations.lock().await;
-        let mutation_range = mutations.range(range.clone());
+        let mutation_range = mutations.key_mutation_map.range(range.clone());
 
         // fetch from TiKV
         // fetch more entries because some of them may be deleted.
@@ -137,8 +163,10 @@ impl Buffer {
 
     /// Lock the given key if necessary.
     pub async fn lock(&self, key: Key) {
-        let mut mutations = self.mutations.lock().await;
+        let mutations = &mut self.mutations.lock().await;
+        mutations.primary_key.get_or_insert(key.clone());
         let value = mutations
+            .key_mutation_map
             .entry(key)
             // Mutated keys don't need a lock.
             .or_insert(Mutation::Lock);
@@ -163,10 +191,14 @@ impl Buffer {
 
     /// Converts the buffered mutations to the proto buffer version
     pub async fn to_proto_mutations(&self) -> Vec<kvrpcpb::Mutation> {
-        self.mutations
-            .lock()
-            .await
+        let mutations = self.mutations.lock().await;
+        let (primary, other) = mutations
+            .key_mutation_map
             .iter()
+            .partition::<Vec<_>, _>(|(key, _)| *key == mutations.primary_key.as_ref().unwrap());
+        primary
+            .into_iter()
+            .chain(other.into_iter())
             .filter_map(|(key, mutation)| mutation.to_proto_with_key(key))
             .collect()
     }
@@ -175,6 +207,7 @@ impl Buffer {
         self.mutations
             .lock()
             .await
+            .key_mutation_map
             .get(&key)
             .map(Mutation::get_value)
             .unwrap_or(MutationValue::Undetermined)
@@ -269,7 +302,10 @@ mod tests {
             ))
             .unwrap()
             .collect::<Vec<_>>(),
-            vec![KvPair(Key::from(b"key1".to_vec()), b"value".to_vec()),]
+            vec![KvPair(
+                Key::from(b"key1".to_vec()),
+                Value::from(b"value".to_vec()),
+            ),]
         );
     }
 
diff --git a/src/transaction/transaction.rs b/src/transaction/transaction.rs
index bd448db..75eb599 100644
--- a/src/transaction/transaction.rs
+++ b/src/transaction/transaction.rs
@@ -468,7 +468,7 @@ impl Transaction {
 
     /// Pessimistically lock the keys.
     ///
-    /// Once resovled it acquires a lock on the key in TiKV.
+    /// Once resolved it acquires a lock on the key in TiKV.
     /// The lock prevents other transactions from mutating the entry until it is released.
     ///
     /// Only valid for pessimistic transactions, panics if called on an optimistic transaction.
@@ -481,19 +481,19 @@ impl Transaction {
             "`pessimistic_lock` is only valid to use with pessimistic transactions"
         );
 
-        let mut keys: Vec<Vec<u8>> = keys
+        let keys: Vec<Vec<u8>> = keys
             .into_iter()
             .map(|it| it.into())
             .map(|it: Key| it.into())
             .collect();
-        keys.sort();
-        let primary_lock = keys[0].clone();
+        let first_key = keys[0].clone();
+        let primary_lock = self.buffer.get_primary_key_or(first_key.into()).await;
         let lock_ttl = DEFAULT_LOCK_TTL;
         let for_update_ts = self.rpc.clone().get_timestamp().await.unwrap().version();
         self.options.push_for_update_ts(for_update_ts);
         new_pessimistic_lock_request(
             keys,
-            primary_lock.into(),
+            primary_lock,
             self.timestamp.version(),
             lock_ttl,
             for_update_ts,