BUG#8804: Better comment + TODO section with suggestion how to speedup

      full_scan 

  DESCRIPTION

    Resolve subquery using index lookup(s).

    First look for specified constant, 

    If not found and we need to check for NULLs, do that too.

    The engine is used to resolve subqueries in form

      oe IN (SELECT key FROM tbl WHERE subq_where) 

    The value of the predicate is calculated as follows: 

    1. If oe IS NULL, this is a special case, do a full table scan on

       table tbl and search for row that satisfies subq_where. If such 

       row is found, return NULL, otherwise return FALSE.

    2. Make an index lookup via key=oe, search for a row that satisfies

       subq_where. If found, return TRUE.

    3. If check_null==TRUE, make another lookup via key=NULL, search for a 

       row that satisfies subq_where. If found, return NULL, otherwise

       return FALSE.

  TODO

    The step #1 can be optimized further when the index has several key

    parts. Consider a subquery:

      (oe1, oe2) IN (SELECT keypart1, keypart2 FROM tbl WHERE subq_where)

    and suppose we need to evaluate it for {oe1, oe2}=={const1, NULL}.

    Current code will do a full table scan and obtain correct result. There

    is a better option: instead of evaluating

      SELECT keypart1, keypart2 FROM tbl WHERE subq_where            (1)

    and checking if it has produced any matching rows, evaluate

      SELECT keypart2 FROM tbl WHERE subq_where AND keypart1=const1  (2)

    If this query produces a row, the result is NULL (as we're evaluating 

    "(const1, NULL) IN { (const1, X), ... }", which has a value of UNKNOWN,

    i.e. NULL).  If the query produces no rows, the result is FALSE.

    NULL IN (SELECT ...) is a special case.

    We currently evaluate (1) by doing a full table scan. (2) can be

    evaluated by doing a "ref" scan on "keypart1=const1", which can be much

    cheaper. We can use index statistics to quickly check whether "ref" scan

    will be cheaper than full table scan.

  RETURN

    0