Don't reset StringBuffers in loops when not needed

- Moved out creating StringBuffers in loops and instead create them
  outside and just reset the buffer if it was not allocated (to avoid
  a possible malloc/free for every entry)

Other things related to set_buffer_if_not_allocated()
- Changed Valuebuffer to not call set_buffer_if_not_allocated() when
  it is created.
- Fixed geometry functions to reset string length before calling
  String::reserve().  This is because one should not access length()
  of an undefined.
- Added Item_func_conv_charset::save_in_field() as the item is using
  str_value to store cached values, which conflicts with
  Item::save_str_in_field().
- Changed Item_proc_string to not store the string value in sql_string
  as this clashes with Item::save_str_in_field().
- Locally store value of full_name_cstring() in analyse::end_of_records()
  as Item::save_str_in_field() may overwrite it.
- Marked some strings as set_thread_specific()
- Added String::free_buffer() to be used internally in String functions
  to just free the buffer but not reset other String values.
- Fixed uses_buffer_owned_by() to check for allocated length instead of
  strlength, which could be marked MEM_UNDEFINED().

1 files changed, 6 insertions, 2 deletions

diff --git a/sql/sql_show.cc b/sql/sql_show.cc
index bd73220310f..8e7e042264b 100644
--- a/sql/sql_show.cc
+++ b/sql/sql_show.cc
@@ -2410,6 +2410,7 @@ int show_create_table_ex(THD *thd, TABLE_LIST *table_list,
   /* Add table level check constraints */
   if (share->table_check_constraints)
   {
+    StringBuffer<MAX_FIELD_WIDTH> str(&my_charset_utf8mb4_general_ci);
     for (uint i= share->field_check_constraints;
          i < share->table_check_constraints ; i++)
     {
@@ -2418,7 +2419,8 @@ int show_create_table_ex(THD *thd, TABLE_LIST *table_list,
       if (share->period.constr_name.streq(check->name))
         continue;
 
-      StringBuffer<MAX_FIELD_WIDTH> str(&my_charset_utf8mb4_general_ci);
+      str.set_buffer_if_not_allocated(&my_charset_utf8mb4_general_ci);
+      str.length(0);                            // Print appends to str
       check->print(&str);
 
       packet->append(STRING_WITH_LEN(",\n  "));
@@ -3036,14 +3038,16 @@ int select_result_text_buffer::append_row(List<Item> &items, bool send_names)
       rows.push_back(row, thd->mem_root))
     return true;
 
+  StringBuffer<32> buf;
+
   while ((item= it++))
   {
     DBUG_ASSERT(column < n_columns);
-    StringBuffer<32> buf;
     const char *data_ptr; 
     char *ptr;
     size_t data_len;
 
+    buf.set_buffer_if_not_allocated(&my_charset_bin);
     if (send_names)
     {
       DBUG_ASSERT(strlen(item->name.str) == item->name.length);