Under terms of MDEV-27490 we'll swith to Unicode-14.0.0. In this Unicode version a string can grow during a casedb conversion. We need to remove all my_casedn_str() calls done for identifiers.

Under tems of this task, let's remove my_casedn_str() whose results are further passed to append_identifier().

Let's add this helper function:

bool append_identifier_opt_casedn(THD *thd, String *to,

                                  const LEX_CSTRING &ident, bool casedn)

{

  if (!casedn)

    return append_identifier(thd, to, &ident);

  CharBuffer<MAX_ALIAS_NAME> buff;

  LEX_CSTRING ls= buff.copy_casedn(system_charset_info, ident).to_lex_cstring();

  return append_identifier(thd, to, &ls);

}

and reuse it in:

• Item_ident::print()
• Item_func_nextval::print()
• Item_func_setval::print()

This change is also expected to reduce the code size.