[CP][BUGFIX] fix text substr len 0 error

src/sql/engine/expr/ob_expr_lob_utils.cpp

@@ -77,6 +77,10 @@ void ObTextStringDatumResult::set_result()
   res_datum_->set_string(buffer_, pos_);
 }
 
+void ObTextStringDatumResult::set_result_null()
+{
+  res_datum_->set_null();
+}
 
 int ObTextStringObObjResult::init(int64_t res_len, ObIAllocator *allocator)
 {

src/sql/engine/expr/ob_expr_lob_utils.h

@@ -41,7 +41,7 @@ public:
   int init(int64_t res_len, ObIAllocator *allocator = NULL);
   int init_with_batch_idx(int64_t res_len, int64_t batch_idx);
   void set_result();
-
+  void set_result_null();
 private:
   char * buff_alloc (const int64_t size);
 

src/sql/engine/expr/ob_expr_substr.cpp

@@ -557,7 +557,9 @@ static int eval_substr_text(const ObCollationType &cs_type,
   } else if (OB_FAIL(input_iter.get_char_len(total_char_len))) {
     LOG_WARN("get input char len failed", K(ret));
   } else if (FALSE_IT(result_byte_len = MIN((pos >= 0 ? total_byte_len - pos + 1 : -pos * mbmaxlen), (MIN((len), (total_char_len)) * mbmaxlen)))) {
-  } else if (len < 0 || pos > total_char_len) {
+  } else if (len <= 0 && lib::is_oracle_mode()) {
+    output_result.set_result_null();
+  } else if (pos > total_char_len || len <= 0) {
     if (!is_batch) {
       ret = output_result.init(0); // fill empty lob result
     } else {
@@ -574,8 +576,6 @@ static int eval_substr_text(const ObCollationType &cs_type,
     } else {
       ret = output_result.init_with_batch_idx(result_byte_len, batch_idx);
     }
-  }
-
     if (OB_FAIL(ret)) {
       LOG_WARN("init stringtext result failed", K(ret));
     } else {
@@ -616,6 +616,7 @@ static int eval_substr_text(const ObCollationType &cs_type,
         }
       }
     }
+  }
   return ret;
 }