executor: use before statement snapshot for UnionScan (pingcap#19276) (…

…pingcap#19841)

executor/mem_reader.go

@@ -272,7 +272,7 @@ func iterTxnMemBuffer(ctx sessionctx.Context, kvRanges []kv.KeyRange, fn process
 		return err
 	}
 	for _, rg := range kvRanges {
-		iter, err := txn.GetMemBuffer().Iter(rg.StartKey, rg.EndKey)
+		iter, err := txn.GetMemBufferSnapshot().Iter(rg.StartKey, rg.EndKey)
 		if err != nil {
 			return err
 		}

kv/kv.go

@@ -164,6 +164,8 @@ type Transaction interface {
 	Valid() bool
 	// GetMemBuffer return the MemBuffer binding to this transaction.
 	GetMemBuffer() MemBuffer
+	// GetMemBufferSnapshot is used to return a snapshot of MemBuffer without any statement modify.
+	GetMemBufferSnapshot() MemBuffer
 	// SetVars sets variables to the transaction.
 	SetVars(vars *Variables)
 	// SetAssertion sets an assertion for an operation on the key.

kv/mock.go

@@ -105,6 +105,10 @@ func (t *mockTxn) SetCap(cap int) {
 
 }
 
+func (t *mockTxn) GetMemBufferSnapshot() MemBuffer {
+	return nil
+}
+
 func (t *mockTxn) Reset() {
 	t.valid = false
 }

session/txn.go

@@ -233,6 +233,11 @@ func (st *TxnState) GetMemBuffer() kv.MemBuffer {
 	return kv.NewBufferStoreFrom(st.Transaction.GetMemBuffer(), st.buf)
 }
 
+// GetMemBufferSnapshot overrides the Transaction interface.
+func (st *TxnState) GetMemBufferSnapshot() kv.MemBuffer {
+	return st.Transaction.GetMemBuffer()
+}
+
 // BatchGet overrides the Transaction interface.
 func (st *TxnState) BatchGet(keys []kv.Key) (map[string][]byte, error) {
 	bufferValues := make([][]byte, len(keys))

store/tikv/txn.go

@@ -519,3 +519,7 @@ func (txn *tikvTxn) Size() int {
 func (txn *tikvTxn) GetMemBuffer() kv.MemBuffer {
 	return txn.us.GetMemBuffer()
 }
+
+func (txn *tikvTxn) GetMemBufferSnapshot() kv.MemBuffer {
+	panic("unsupported operation")
+}