diff --git a/src/sql/engine/expr/ob_datum_cast.cpp b/src/sql/engine/expr/ob_datum_cast.cpp
index 631df07d5d..2ca98d66d9 100644
--- a/src/sql/engine/expr/ob_datum_cast.cpp
+++ b/src/sql/engine/expr/ob_datum_cast.cpp
@@ -515,16 +515,10 @@ static int common_copy_string(const ObExpr &expr,
   int ret = OB_SUCCESS;
   char *out_ptr = NULL;
   int64_t len = align_offset + src.length();
-  if (expr.res_buf_len_ < len) {
-    if (OB_ISNULL(out_ptr = expr.get_str_res_mem(ctx, len))) {
-      ret = OB_ALLOCATE_MEMORY_FAILED;
-      LOG_WARN("allocate memory failed", K(ret));
-    }
+  if (OB_ISNULL(out_ptr = expr.get_str_res_mem(ctx, len))) {
+    ret = OB_ALLOCATE_MEMORY_FAILED;
+    LOG_WARN("allocate memory failed", K(ret));
   } else {
-    out_ptr = const_cast<char*>(res_datum.ptr_);
-  }
-
-  if (OB_SUCC(ret)) {
     MEMMOVE(out_ptr + align_offset, src.ptr(), len - align_offset);
     MEMSET(out_ptr, 0, align_offset);
     res_datum.set_string(out_ptr, len);
@@ -552,16 +546,10 @@ static int common_copy_string_zf(const ObExpr &expr,
   } else if (CM_IS_ZERO_FILL(expr.extra_) && out_len > src.length()) {
     char *out_ptr = NULL;
     // out_ptr may overlap with src, so memmove is used.
-    if (expr.res_buf_len_ < out_len) {
-      if (OB_ISNULL(out_ptr = expr.get_str_res_mem(ctx, out_len))) {
-        ret = OB_ALLOCATE_MEMORY_FAILED;
-        LOG_WARN("allocate memory failed", K(ret));
-      }
+    if (OB_ISNULL(out_ptr = expr.get_str_res_mem(ctx, out_len))) {
+      ret = OB_ALLOCATE_MEMORY_FAILED;
+      LOG_WARN("allocate memory failed", K(ret));
     } else {
-      out_ptr = const_cast<char*>(res_datum.ptr_);
-    }
-
-    if (OB_SUCC(ret)) {
       int64_t zf_len = out_len - src.length();
       if (0 < zf_len) {
         MEMMOVE(out_ptr + zf_len, src.ptr(), src.length());
diff --git a/src/sql/engine/expr/ob_expr.cpp b/src/sql/engine/expr/ob_expr.cpp
index e6f6d07560..cc02245651 100644
--- a/src/sql/engine/expr/ob_expr.cpp
+++ b/src/sql/engine/expr/ob_expr.cpp
@@ -574,6 +574,16 @@ void ObExpr::reset_datums_ptr(char *frame, const int64_t size) const
   }
 }
 
+void ObExpr::reset_datum_ptr(char *frame, const int64_t size, const int64_t idx) const
+{
+  ObDatum *datum = reinterpret_cast<ObDatum *>(frame + datum_off_);
+  datum += idx;
+  char *ptr = frame + res_buf_off_ + (batch_idx_mask_ & idx) * res_buf_len_;
+  if (datum->ptr_ != ptr && idx < size) {
+    datum->ptr_ = ptr;
+  }
+}
+
 int ObExpr::eval_one_datum_of_batch(ObEvalCtx &ctx, common::ObDatum *&datum) const
 {
   int ret = OB_SUCCESS;
@@ -599,6 +609,7 @@ int ObExpr::eval_one_datum_of_batch(ObEvalCtx &ctx, common::ObDatum *&datum) con
   }
   datum = reinterpret_cast<ObDatum *>(frame + datum_off_) + ctx.get_batch_idx();
   if (need_evaluate) {
+    reset_datum_ptr(frame, ctx.get_batch_size(), ctx.get_batch_idx());
     ret = eval_func_(*this, ctx, *datum);
     if (OB_SUCC(ret)) {
       ObBitVector *evaluated_flags = to_bit_vector(frame + eval_flags_off_);
diff --git a/src/sql/engine/expr/ob_expr.h b/src/sql/engine/expr/ob_expr.h
index cf0458d352..23b4815e76 100644
--- a/src/sql/engine/expr/ob_expr.h
+++ b/src/sql/engine/expr/ob_expr.h
@@ -571,6 +571,7 @@ private:
   char *alloc_str_res_mem(ObEvalCtx &ctx, const int64_t size, const int64_t idx) const;
   // reset datums pointer to reserved buffer.
   void reset_datums_ptr(char *frame, const int64_t size) const;
+  void reset_datum_ptr(char *frame, const int64_t size, const int64_t idx) const;
   int eval_one_datum_of_batch(ObEvalCtx &ctx, common::ObDatum *&datum) const;
   int do_eval_batch(ObEvalCtx &ctx, const ObBitVector &skip, const int64_t size) const;