diff --git a/indexer/database/sql/lazy_tx.go b/indexer/database/sql/lazy_tx.go
index d34d8ae..701b7cb 100644
--- a/indexer/database/sql/lazy_tx.go
+++ b/indexer/database/sql/lazy_tx.go
@@ -3,6 +3,7 @@ package sql
 import (
 	"context"
 	"reflect"
+	"sync"
 	"time"
 
 	"github.com/cerc-io/plugeth-statediff/indexer/database/metrics"
@@ -15,6 +16,7 @@ const copyFromCheckLimit = 100
 type DelayedTx struct {
 	cache []interface{}
 	db    Database
+	sync.RWMutex
 }
 type cachedStmt struct {
 	sql  string
@@ -27,6 +29,8 @@ type copyFrom struct {
 	rows        [][]interface{}
 }
 
+type result int64
+
 func (cf *copyFrom) appendRows(rows [][]interface{}) {
 	cf.rows = append(cf.rows, rows...)
 }
@@ -44,6 +48,8 @@ func (tx *DelayedTx) QueryRow(ctx context.Context, sql string, args ...interface
 }
 
 func (tx *DelayedTx) findPrevCopyFrom(tableName []string, columnNames []string, limit int) (*copyFrom, int) {
+	tx.RLock()
+	defer tx.RUnlock()
 	for pos, count := len(tx.cache)-1, 0; pos >= 0 && count < limit; pos, count = pos-1, count+1 {
 		prevCopy, ok := tx.cache[pos].(*copyFrom)
 		if ok && prevCopy.matches(tableName, columnNames) {
@@ -59,6 +65,8 @@ func (tx *DelayedTx) CopyFrom(ctx context.Context, tableName []string, columnNam
 			"current", len(prevCopy.rows), "new", len(rows), "distance", distance)
 		prevCopy.appendRows(rows)
 	} else {
+		tx.Lock()
+		defer tx.Unlock()
 		tx.cache = append(tx.cache, &copyFrom{tableName, columnNames, rows})
 	}
 
@@ -66,8 +74,10 @@ func (tx *DelayedTx) CopyFrom(ctx context.Context, tableName []string, columnNam
 }
 
 func (tx *DelayedTx) Exec(ctx context.Context, sql string, args ...interface{}) (Result, error) {
+	tx.Lock()
+	defer tx.Unlock()
 	tx.cache = append(tx.cache, cachedStmt{sql, args})
-	return nil, nil
+	return result(0), nil
 }
 
 func (tx *DelayedTx) Commit(ctx context.Context) error {
@@ -85,6 +95,8 @@ func (tx *DelayedTx) Commit(ctx context.Context) error {
 			rollback(ctx, base)
 		}
 	}()
+	tx.Lock()
+	defer tx.Unlock()
 	for _, item := range tx.cache {
 		switch item := item.(type) {
 		case *copyFrom:
@@ -105,6 +117,13 @@ func (tx *DelayedTx) Commit(ctx context.Context) error {
 }
 
 func (tx *DelayedTx) Rollback(ctx context.Context) error {
+	tx.Lock()
+	defer tx.Unlock()
 	tx.cache = nil
 	return nil
 }
+
+// RowsAffected satisfies sql.Result
+func (r result) RowsAffected() (int64, error) {
+	return int64(r), nil
+}