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  1. MariaDB Server
  2. MDEV-5174

Filesort excessively uses disk space for TINYBLOB and TINYTEXT columns

        Details

        • Type: Bug
        • Status: Open
        • Priority: Minor
        • Resolution: Unresolved
        • Affects Version/s: 10.0.4, 5.5.33, 5.1.67, 5.2.14, 5.3.12
        • Fix Version/s: None
        • Component/s: None
        • Labels:
          None

          Description

          Filesort uses Field::sort_length() to calculate space needed to sort the field.

          In case of BLOB and TEXT fields and all their
          TINY, MEDIUM, LONG variations the length is calculated as follows:

          uint32 Field_blob::sort_length() const

          { return (uint32) (current_thd->variables.max_sort_length + (field_charset == &my_charset_bin ? 0 : packlength)); }

          The default value of max_sort_length is 1024.

          This is bad for TINYBLOB and TINYTEXT.

          • It should be enough to use 256 bytes to sort TINYBLOB,
          • It should be enough to use 256*strxfrm_multiply bytes to sort TINYTEXT.
            (where strxfrm_multiply is 1 for many collations, which makes 256 bytes again)

          So TINYBLOB (and TINYTEXT in most cases) use 4 times more space
          for sorting than it's actually needed. That should affect performance
          very negatively.

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                  • Assignee:
                    bar Alexander Barkov
                    Reporter:
                    bar Alexander Barkov
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                    Dates

                    • Created:
                      Updated: