diff --git a/rust-toolchain b/rust-toolchain
new file mode 100644
index 00000000..3e2f5f2c
--- /dev/null
+++ b/rust-toolchain
@@ -0,0 +1 @@
+nightly-2021-03-15
diff --git a/src/transaction/buffer.rs b/src/transaction/buffer.rs
index 1d7f61b9..2ec1d86c 100644
--- a/src/transaction/buffer.rs
+++ b/src/transaction/buffer.rs
@@ -1,17 +1,21 @@
 // Copyright 2019 TiKV Project Authors. Licensed under Apache-2.0.
 
 use crate::{BoundRange, Key, KvPair, Result, Value};
+use derive_new::new;
 use std::{
-    collections::{BTreeMap, HashMap},
+    collections::{btree_map::Entry, BTreeMap, HashMap},
     future::Future,
 };
 use tikv_client_proto::kvrpcpb;
 use tokio::sync::{Mutex, MutexGuard};
 
-#[derive(Default)]
+#[derive(new)]
 struct InnerBuffer {
+    #[new(default)]
     primary_key: Option<Key>,
+    #[new(default)]
     entry_map: BTreeMap<Key, BufferEntry>,
+    is_pessimistic: bool,
 }
 
 impl InnerBuffer {
@@ -29,16 +33,22 @@ impl InnerBuffer {
 }
 
 /// A caching layer which buffers reads and writes in a transaction.
-#[derive(Default)]
 pub struct Buffer {
     mutations: Mutex<InnerBuffer>,
 }
 
 impl Buffer {
+    pub fn new(is_pessimistic: bool) -> Buffer {
+        Buffer {
+            mutations: Mutex::new(InnerBuffer::new(is_pessimistic)),
+        }
+    }
+
     /// Get the primary key of the buffer.
     pub async fn get_primary_key(&self) -> Option<Key> {
         self.mutations.lock().await.primary_key.clone()
     }
+
     /// 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).clone()
@@ -203,28 +213,30 @@ impl Buffer {
 
     /// Mark a value as Insert mutation into the buffer (does not write through).
     pub async fn insert(&self, key: Key, value: Value) {
-        self.mutations
-            .lock()
-            .await
-            .insert(key, BufferEntry::Insert(value));
+        let mut mutations = self.mutations.lock().await;
+        let mut entry = mutations.entry_map.entry(key.clone());
+        match entry {
+            Entry::Occupied(ref mut o) if matches!(o.get(), BufferEntry::Del) => {
+                o.insert(BufferEntry::Put(value));
+            }
+            _ => mutations.insert(key, BufferEntry::Insert(value)),
+        }
     }
 
     /// Mark a value as deleted.
     pub async fn delete(&self, key: Key) {
         let mut mutations = self.mutations.lock().await;
-        let value = mutations
-            .entry_map
-            .entry(key.clone())
-            .or_insert(BufferEntry::Del);
-
-        let new_value: BufferEntry;
-        if let BufferEntry::Insert(_) = value {
-            new_value = BufferEntry::CheckNotExist
-        } else {
-            new_value = BufferEntry::Del
+        let is_pessimistic = mutations.is_pessimistic;
+        let mut entry = mutations.entry_map.entry(key.clone());
+
+        match entry {
+            Entry::Occupied(ref mut o)
+                if matches!(o.get(), BufferEntry::Insert(_)) && !is_pessimistic =>
+            {
+                o.insert(BufferEntry::CheckNotExist);
+            }
+            _ => mutations.insert(key, BufferEntry::Del),
         }
-
-        mutations.insert(key, new_value);
     }
 
     /// Converts the buffered mutations to the proto buffer version
@@ -378,7 +390,7 @@ mod tests {
     #[tokio::test]
     #[allow(unreachable_code)]
     async fn set_and_get_from_buffer() {
-        let buffer = Buffer::default();
+        let buffer = Buffer::new(false);
         buffer
             .put(b"key1".to_vec().into(), b"value1".to_vec())
             .await;
@@ -411,7 +423,7 @@ mod tests {
     #[tokio::test]
     #[allow(unreachable_code)]
     async fn insert_and_get_from_buffer() {
-        let buffer = Buffer::default();
+        let buffer = Buffer::new(false);
         buffer
             .insert(b"key1".to_vec().into(), b"value1".to_vec())
             .await;
@@ -453,13 +465,13 @@ mod tests {
         let v2: Value = b"value2".to_vec();
         let v2_ = v2.clone();
 
-        let buffer = Buffer::default();
+        let buffer = Buffer::new(false);
         let r1 = block_on(buffer.get_or_else(k1.clone(), move |_| ready(Ok(Some(v1_)))));
         let r2 = block_on(buffer.get_or_else(k1.clone(), move |_| ready(panic!())));
         assert_eq!(r1.unwrap().unwrap(), v1);
         assert_eq!(r2.unwrap().unwrap(), v1);
 
-        let buffer = Buffer::default();
+        let buffer = Buffer::new(false);
         let r1 = block_on(
             buffer.batch_get_or_else(vec![k1.clone(), k2.clone()].into_iter(), move |_| {
                 ready(Ok(vec![(k1_, v1__).into(), (k2_, v2_).into()]))
diff --git a/src/transaction/lowering.rs b/src/transaction/lowering.rs
index ce1243d0..413c6bf8 100644
--- a/src/transaction/lowering.rs
+++ b/src/transaction/lowering.rs
@@ -106,8 +106,34 @@ pub fn new_pessimistic_rollback_request(
     )
 }
 
+pub trait PessimisticLock: Clone {
+    fn key(self) -> Key;
+
+    fn assertion(&self) -> kvrpcpb::Assertion;
+}
+
+impl PessimisticLock for Key {
+    fn key(self) -> Key {
+        self
+    }
+
+    fn assertion(&self) -> kvrpcpb::Assertion {
+        kvrpcpb::Assertion::None
+    }
+}
+
+impl PessimisticLock for (Key, kvrpcpb::Assertion) {
+    fn key(self) -> Key {
+        self.0
+    }
+
+    fn assertion(&self) -> kvrpcpb::Assertion {
+        self.1
+    }
+}
+
 pub fn new_pessimistic_lock_request(
-    keys: impl Iterator<Item = Key>,
+    locks: impl Iterator<Item = impl PessimisticLock>,
     primary_lock: Key,
     start_version: Timestamp,
     lock_ttl: u64,
@@ -115,13 +141,15 @@ pub fn new_pessimistic_lock_request(
     need_value: bool,
 ) -> kvrpcpb::PessimisticLockRequest {
     requests::new_pessimistic_lock_request(
-        keys.map(|key| {
-            let mut mutation = kvrpcpb::Mutation::default();
-            mutation.set_op(kvrpcpb::Op::PessimisticLock);
-            mutation.set_key(key.into());
-            mutation
-        })
-        .collect(),
+        locks
+            .map(|pl| {
+                let mut mutation = kvrpcpb::Mutation::default();
+                mutation.set_op(kvrpcpb::Op::PessimisticLock);
+                mutation.set_assertion(pl.assertion());
+                mutation.set_key(pl.key().into());
+                mutation
+            })
+            .collect(),
         primary_lock.into(),
         start_version.version(),
         lock_ttl,
diff --git a/src/transaction/transaction.rs b/src/transaction/transaction.rs
index 1fe8be4c..47374a90 100644
--- a/src/transaction/transaction.rs
+++ b/src/transaction/transaction.rs
@@ -66,7 +66,7 @@ impl<PdC: PdClient> Transaction<PdC> {
         Transaction {
             status: Arc::new(RwLock::new(status)),
             timestamp,
-            buffer: Default::default(),
+            buffer: Buffer::new(options.is_pessimistic()),
             rpc,
             options,
             is_heartbeat_started: false,
@@ -401,8 +401,11 @@ impl<PdC: PdClient> Transaction<PdC> {
             return Err(Error::DuplicateKeyInsertion);
         }
         if self.is_pessimistic() {
-            self.pessimistic_lock(iter::once(key.clone()), false)
-                .await?;
+            self.pessimistic_lock(
+                iter::once((key.clone(), kvrpcpb::Assertion::NotExist)),
+                false,
+            )
+            .await?;
         }
         self.buffer.insert(key, value.into()).await;
         Ok(())
@@ -630,7 +633,7 @@ impl<PdC: PdClient> Transaction<PdC> {
     /// Only valid for pessimistic transactions, panics if called on an optimistic transaction.
     async fn pessimistic_lock(
         &mut self,
-        keys: impl IntoIterator<Item = Key>,
+        keys: impl IntoIterator<Item = impl PessimisticLock>,
         need_value: bool,
     ) -> Result<Vec<KvPair>> {
         assert!(
@@ -638,12 +641,12 @@ impl<PdC: PdClient> Transaction<PdC> {
             "`pessimistic_lock` is only valid to use with pessimistic transactions"
         );
 
-        let keys: Vec<Key> = keys.into_iter().collect();
+        let keys: Vec<_> = keys.into_iter().collect();
         if keys.is_empty() {
             return Ok(vec![]);
         }
 
-        let first_key = keys[0].clone();
+        let first_key = keys[0].clone().key();
         let primary_lock = self.buffer.get_primary_key_or(&first_key).await;
         let for_update_ts = self.rpc.clone().get_timestamp().await?;
         self.options.push_for_update_ts(for_update_ts.clone());
@@ -667,7 +670,7 @@ impl<PdC: PdClient> Transaction<PdC> {
         self.start_auto_heartbeat().await;
 
         for key in keys {
-            self.buffer.lock(key).await;
+            self.buffer.lock(key.key()).await;
         }
 
         pairs
@@ -891,6 +894,13 @@ impl TransactionOptions {
         self.auto_heartbeat = false;
         self
     }
+
+    pub fn is_pessimistic(&self) -> bool {
+        match self.kind {
+            TransactionKind::Pessimistic(_) => true,
+            TransactionKind::Optimistic => false,
+        }
+    }
 }
 
 /// The default TTL of a lock in milliseconds.
diff --git a/tests/integration_tests.rs b/tests/integration_tests.rs
index 74e7c9b4..61fd3010 100644
--- a/tests/integration_tests.rs
+++ b/tests/integration_tests.rs
@@ -596,6 +596,49 @@ async fn pessimistic_rollback() -> Result<()> {
     Ok(())
 }
 
+#[tokio::test]
+#[serial]
+async fn pessimistic_delete() -> Result<()> {
+    clear_tikv().await;
+    let client =
+        TransactionClient::new_with_config(vec!["127.0.0.1:2379"], Default::default()).await?;
+
+    // The transaction will lock the keys and must release the locks on commit, even when values are
+    // not written to the DB.
+    let mut txn = client.begin_pessimistic().await?;
+    txn.put(vec![1], vec![42]).await?;
+    txn.delete(vec![1]).await?;
+    txn.insert(vec![2], vec![42]).await?;
+    txn.delete(vec![2]).await?;
+    txn.put(vec![3], vec![42]).await?;
+    txn.commit().await?;
+
+    // Check that the keys are not locked.
+    let mut txn2 = client.begin_optimistic().await?;
+    txn2.put(vec![1], vec![42]).await?;
+    txn2.put(vec![2], vec![42]).await?;
+    txn2.put(vec![3], vec![42]).await?;
+    txn2.commit().await?;
+
+    // As before, but rollback instead of commit.
+    let mut txn = client.begin_pessimistic().await?;
+    txn.put(vec![1], vec![42]).await?;
+    txn.delete(vec![1]).await?;
+    txn.delete(vec![2]).await?;
+    txn.insert(vec![2], vec![42]).await?;
+    txn.delete(vec![2]).await?;
+    txn.put(vec![3], vec![42]).await?;
+    txn.rollback().await?;
+
+    let mut txn2 = client.begin_optimistic().await?;
+    txn2.put(vec![1], vec![42]).await?;
+    txn2.put(vec![2], vec![42]).await?;
+    txn2.put(vec![3], vec![42]).await?;
+    txn2.commit().await?;
+
+    Ok(())
+}
+
 #[tokio::test]
 #[serial]
 async fn lock_keys() -> Result<()> {