improved sql parser.

src/main/java/com/alibaba/druid/sql/ast/expr/SQLAggregateExpr.java

@@ -19,6 +19,7 @@ import java.io.Serializable;
 import java.util.ArrayList;
 import java.util.List;
 
+import com.alibaba.druid.sql.SQLUtils;
 import com.alibaba.druid.sql.ast.SQLExpr;
 import com.alibaba.druid.sql.ast.SQLExprImpl;
 import com.alibaba.druid.sql.ast.SQLKeep;
@@ -115,6 +116,10 @@ public class SQLAggregateExpr extends SQLExprImpl implements Serializable {
         this.ignoreNulls = ignoreNulls;
     }
 
+    public String toString() {
+        return SQLUtils.toSQLString(this);
+    }
+
 
     @Override
     protected void accept0(SQLASTVisitor visitor) {

src/main/java/com/alibaba/druid/sql/ast/expr/SQLFlashbackExpr.java

@@ -1,3 +1,18 @@
+/*
+ * Copyright 1999-2017 Alibaba Group Holding Ltd.
+ *
+ * Licensed under the Apache License, Version 2.0 (the "License");
+ * you may not use this file except in compliance with the License.
+ * You may obtain a copy of the License at
+ *
+ *      http://www.apache.org/licenses/LICENSE-2.0
+ *
+ * Unless required by applicable law or agreed to in writing, software
+ * distributed under the License is distributed on an "AS IS" BASIS,
+ * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+ * See the License for the specific language governing permissions and
+ * limitations under the License.
+ */
 package com.alibaba.druid.sql.ast.expr;
 
 import com.alibaba.druid.sql.ast.SQLExpr;

src/main/java/com/alibaba/druid/sql/ast/statement/SQLTableSource.java

@@ -17,6 +17,7 @@ package com.alibaba.druid.sql.ast.statement;
 
 import java.util.List;
 
+import com.alibaba.druid.sql.ast.SQLExpr;
 import com.alibaba.druid.sql.ast.SQLHint;
 import com.alibaba.druid.sql.ast.SQLObject;
 
@@ -31,4 +32,7 @@ public interface SQLTableSource extends SQLObject {
     SQLTableSource clone();
 
     String computeAlias();
+
+    SQLExpr getFlashback();
+    void setFlashback(SQLExpr flashback);
 }