fix problem of reallocated string

Author: sanja-byelkin

sql/filesort.cc

@@ -634,6 +634,9 @@ static uchar *read_buffpek_from_file(IO_CACHE *buffpek_pointers, uint count,
 }
 
 #ifndef DBUG_OFF
+
+static char dbug_row_print_buf[4096];
+
 /*
   Print table's current row into a buffer and return a pointer to it.
 
@@ -649,11 +652,9 @@ static uchar *read_buffpek_from_file(IO_CACHE *buffpek_pointers, uint count,
 const char* dbug_print_row(TABLE *table, const uchar *rec, bool print_names)
 {
   Field **pfield;
-  const size_t alloc_size= 512;
-  char *row_buff= (char *) alloc_root(&table->mem_root, alloc_size);
-  char *row_buff_tmp= (char *) alloc_root(&table->mem_root, alloc_size);
-  String tmp(row_buff_tmp, alloc_size, &my_charset_bin);
-  String output(row_buff, alloc_size, &my_charset_bin);
+  char row_buff_tmp[512];
+  String tmp(row_buff_tmp, sizeof(row_buff_tmp), &my_charset_bin);
+  String output(dbug_row_print_buf, sizeof(dbug_row_print_buf), &my_charset_bin);
 
   auto move_back_lambda= [table, rec]() mutable {
     table->move_fields(table->field, table->record[0], rec);
@@ -717,8 +718,10 @@ const char* dbug_print_row(TABLE *table, const uchar *rec, bool print_names)
     }
   }
   output.append(")");
-  
-  return output.c_ptr_safe();
+  if (output.c_ptr() == dbug_row_print_buf)
+    return dbug_row_print_buf;
+  else
+    return "Couldn't fit into buffer";
 }