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    • Gleb Shchepa's avatar
      Bug #37799: SELECT with a BIT column in WHERE clause · a591f2cb
      Gleb Shchepa authored
                  returns unexpected result
      
      If:
        1. a table has a not nullable BIT column c1 with a length
           shorter than 8 bits and some additional not nullable
           columns c2 etc, and
        2. the WHERE clause is like: (c1 = constant) AND c2 ...,
      the SELECT query returns unexpected result set.
      
      
      The server stores BIT columns in a tricky way to save disk
      space: if column's bit length is not divisible by 8, the
      server places reminder bits among the null bits at the start
      of a record. The rest bytes are stored in the record itself,
      and Field::ptr points to these rest bytes.
      
      However if a bit length of the whole column is less than 8,
      there are no remaining bytes, and there is nothing to store in
      the record at its regular place. In this case Field::ptr points
      to bytes actually occupied by the next column in a record.
      If both columns (BIT and the next column) are NOT NULL,
      the Field::eq function incorrectly deduces that this is the
      same column, so query transformation/equal item elimination
      code (see build_equal_items_for_cond) may mix these columns
      and damage conditions containing references to them.
      
      
      mysql-test/r/type_bit.result:
        Added test case for bug #37799.
      mysql-test/t/type_bit.test:
        Added test case for bug #37799.
      sql/field.h:
        1. The Field::eq function has been modified to take types of
        comparing columns into account to distinguish between BIT and
        not BIT columns referencing the same bytes in a record.
        
        2. Unnecessary type comparison has been removed from the
        Field_bit::eq function (moved to Field::eq).
      a591f2cb