diff --git a/src/share/system_variable/ob_system_variable_factory.h b/src/share/system_variable/ob_system_variable_factory.h
index da2de06dc8..0435bffa62 100644
--- a/src/share/system_variable/ob_system_variable_factory.h
+++ b/src/share/system_variable/ob_system_variable_factory.h
@@ -1470,4 +1470,4 @@ private:
 
 }
 }
-#endif //OCEANBASE_SHARE_SYSTEM_VARIABLE_OB_SYSTEM_VARIABLE_FACTORY_
\ No newline at end of file
+#endif //OCEANBASE_SHARE_SYSTEM_VARIABLE_OB_SYSTEM_VARIABLE_FACTORY_
diff --git a/src/sql/engine/subquery/ob_subplan_scan_op.cpp b/src/sql/engine/subquery/ob_subplan_scan_op.cpp
index 7abb4c639a..c6bb2f026e 100644
--- a/src/sql/engine/subquery/ob_subplan_scan_op.cpp
+++ b/src/sql/engine/subquery/ob_subplan_scan_op.cpp
@@ -55,6 +55,18 @@ int ObSubPlanScanOp::inner_get_next_row()
       LOG_WARN("get row from child failed", K(ret));
     }
   } else {
+    // eval child's output expr
+    // For some expression in the subquery, we must eval, even if it not output.
+    // e.g.
+    //      select 1 from (select @a=3);
+    for (int64_t i = 0; OB_SUCC(ret) && i < child_->get_spec().output_.count(); i++) {
+      ObExpr *expr = child_->get_spec().output_[i];
+      ObDatum *datum = NULL;
+      if (OB_FAIL(expr->eval(eval_ctx_, datum))) {
+        LOG_WARN("expr evaluate failed", K(ret), K(*expr));
+      }
+    }
+
     for (int64_t i = 0; OB_SUCC(ret) && i < MY_SPEC.projector_.count(); i += 2) {
       ObExpr* from = MY_SPEC.projector_[i];
       ObExpr* to = MY_SPEC.projector_[i + 1];
diff --git a/src/sql/resolver/expr/ob_expr_info_flag.h b/src/sql/resolver/expr/ob_expr_info_flag.h
index 6bc9627e5b..fd8c8b1375 100644
--- a/src/sql/resolver/expr/ob_expr_info_flag.h
+++ b/src/sql/resolver/expr/ob_expr_info_flag.h
@@ -56,6 +56,7 @@ enum ObExprInfoFlag {
   IS_VOLATILE_CONST,  // the const expr may be altered by overwrite, non-const in execution.
   IS_ORA_ROWSCN_EXPR,
   IS_VAR_EXPR,
+  IS_ASSIGN_EXPR,
   IS_CONST_EXPR,   // expression contains calculable expression
   CNT_CONST_EXPR,  // IS_CONST_EXPR and CNT_CONST_EXPR at most one is true
 
@@ -98,6 +99,7 @@ enum ObExprInfoFlag {
   CNT_VOLATILE_CONST,
   CNT_ORA_ROWSCN_EXPR,
   CNT_VAR_EXPR,
+  CNT_ASSIGN_EXPR,
 
   BE_USED,         // expression has been applied
   IS_SIMPLE_COND,  // column = const
@@ -117,11 +119,11 @@ enum ObExprInfoFlag {
 };
 
 #define IS_INFO_MASK_BEGIN IS_CONST
-#define IS_INFO_MASK_END IS_VAR_EXPR
+#define IS_INFO_MASK_END IS_ASSIGN_EXPR
 #define CNT_INFO_MASK_BEGIN CNT_CONST
-#define CNT_INFO_MASK_END CNT_VAR_EXPR
+#define CNT_INFO_MASK_END CNT_ASSIGN_EXPR
 #define INHERIT_MASK_BEGIN CNT_CONST_EXPR
-#define INHERIT_MASK_END CNT_VAR_EXPR
+#define INHERIT_MASK_END CNT_ASSIGN_EXPR
 
 inline const char* get_expr_info_flag_str(const ObExprInfoFlag flag)
 {
diff --git a/src/sql/resolver/expr/ob_raw_expr_info_extractor.cpp b/src/sql/resolver/expr/ob_raw_expr_info_extractor.cpp
index 85f08cc83f..bf29c8056d 100644
--- a/src/sql/resolver/expr/ob_raw_expr_info_extractor.cpp
+++ b/src/sql/resolver/expr/ob_raw_expr_info_extractor.cpp
@@ -161,7 +161,8 @@ int ObRawExprInfoExtractor::add_const(ObRawExpr& expr)
           T_FUN_SYS_ROWKEY_TO_ROWID == expr.get_expr_type() || T_OP_CONNECT_BY_ROOT == expr.get_expr_type() ||
           T_FUN_SYS_CONNECT_BY_PATH == expr.get_expr_type() || T_FUN_SYS_GUID == expr.get_expr_type() ||
           T_FUN_SYS_STMT_ID == expr.get_expr_type() || T_FUN_SYS_SLEEP == expr.get_expr_type() ||
-          T_OP_PRIOR == expr.get_expr_type() || T_OP_GET_USER_VAR == expr.get_expr_type())) {
+          T_OP_ASSIGN == expr.get_expr_type() || T_OP_PRIOR == expr.get_expr_type() || 
+          T_OP_GET_USER_VAR == expr.get_expr_type())) {
     is_const_expr = false;
   }
   if (is_const_expr) {
@@ -233,7 +234,7 @@ bool ObRawExprInfoExtractor::not_calculable_expr(const ObRawExpr& expr)
          expr.has_flag(CNT_ALIAS) || expr.has_flag(CNT_ENUM_OR_SET) || expr.has_flag(CNT_VALUES) ||
          expr.has_flag(CNT_SEQ_EXPR) || expr.has_flag(CNT_SYS_CONNECT_BY_PATH) || expr.has_flag(CNT_RAND_FUNC) ||
          expr.has_flag(CNT_SO_UDF) || expr.has_flag(CNT_PRIOR) || expr.has_flag(CNT_EXEC_PARAM) ||
-         expr.has_flag(CNT_VOLATILE_CONST) || expr.has_flag(CNT_VAR_EXPR);
+         expr.has_flag(CNT_VOLATILE_CONST) || expr.has_flag(CNT_VAR_EXPR) || expr.has_flag(CNT_ASSIGN_EXPR);
 }
 
 int ObRawExprInfoExtractor::visit(ObOpRawExpr& expr)
@@ -314,6 +315,10 @@ int ObRawExprInfoExtractor::visit(ObOpRawExpr& expr)
           if (OB_FAIL(expr.add_flag(IS_OR))) {
             LOG_WARN("failed to add flag IS_OR", K(ret));
           }
+        } else if (expr.get_expr_type() == T_OP_ASSIGN) {
+          if (OB_FAIL(expr.add_flag(IS_ASSIGN_EXPR))) {
+            LOG_WARN("failed to add flag IS_ASSIGN_EXPR", K(ret));
+          }
         }
       } else if (3 == expr.get_param_count()) {
         // triple operator
diff --git a/unittest/sql/optimizer/test_optimizer_select.result b/unittest/sql/optimizer/test_optimizer_select.result
index 579701d01c..e3c3fd6619 100644
--- a/unittest/sql/optimizer/test_optimizer_select.result
+++ b/unittest/sql/optimizer/test_optimizer_select.result
@@ -19310,17 +19310,19 @@ Outputs & filters:
 
 SQL: Select distinct 1, 1 + (@var:=1) from t7; 
 
-===================================
-|ID|OPERATOR  |NAME|EST. ROWS|COST|
------------------------------------
-|0 |TABLE SCAN|t7  |1        |36  |
-===================================
+======================================
+|ID|OPERATOR     |NAME|EST. ROWS|COST|
+--------------------------------------
+|0 |HASH DISTINCT|    |1        |122 |
+|1 | TABLE SCAN  |t7  |100      |88  |
+======================================
 
 Outputs & filters: 
 -------------------------------------
-  0 - output([?], [?]), filter(nil), 
+  0 - output([?], [? + (T_OP_ASSIGN, 'var', ?)]), filter(nil), 
+      distinct([? + (T_OP_ASSIGN, 'var', ?)])
+  1 - output([? + (T_OP_ASSIGN, 'var', ?)]), filter(nil), 
       access([t7.c1]), partitions(p0), 
-      limit(1), offset(nil), 
       is_index_back=false, 
       range_key([t7.c1]), range(MIN ; MAX)always true