Uploaded image for project: 'MariaDB Server'
  1. MariaDB Server
  2. MDEV-25449

Add MY_COLLATION_HANDLER::strnncollsp_nchars()

    XMLWordPrintable

    Details

      Description

      Field_string::cmp() seems to do unnecessary work trimming trailing spaces:

      int Field_string::cmp(const uchar *a_ptr, const uchar *b_ptr) const
      {
        size_t a_len, b_len;
       
        if (mbmaxlen() != 1)
        {
          size_t char_len= Field_string::char_length();
          a_len= field_charset()->charpos(a_ptr, a_ptr + field_length, char_len);
          b_len= field_charset()->charpos(b_ptr, b_ptr + field_length, char_len);
        }
        else
          a_len= b_len= field_length;
        /*
          We have to remove end space to be able to compare multi-byte-characters
          like in latin_de 'ae' and 0xe4
        */
        return field_charset()->strnncollsp(a_ptr, a_len,
                                            b_ptr, b_len);
      }
      

      In absolute majority cases, the difference between strings is found in the very beginning of the compared strings. So doing charpos() on the two arguments, before passing them to the actual comparison function, looks like an inefficient waste of CPU.

      A better approach would be to implement a new comparison function with this tentative API:

      int strnncollsp_nchars(CHARSET_INFO *cs,
                             const char *s1, size_t len1,
                             const char *s2, size_t len2,
                             size_t nchars);
      

      Internally, the exact virtial implementations of strnncollsp_nchars() would do the same with what strnncollsp() do in the same collation, but with an extra limit on "nchars".

      This new function should also help to fix a bug in the similar code in InnoDB: see MDEV-25440 for details.

        Attachments

          Issue Links

            Activity

              People

              Assignee:
              serg Sergei Golubchik
              Reporter:
              bar Alexander Barkov
              Votes:
              0 Vote for this issue
              Watchers:
              4 Start watching this issue

                Dates

                Created:
                Updated: