Problem: after introduction of "WL#2649 Number-to-string conversions"

This query:
  SET NAMES cp850; -- Or any other non-latin1 ASCII-based character set
  SELECT * FROM t1
  WHERE datetime_column='2010-01-01 00:00:00'
started to add extra character set conversion: 
  SELECT * FROM t1
  WHERE CONVERT(datetime_column USING cp850)='2010-01-01 00:00:00';

so index on DATETIME column was not used anymore.
Fix:
  avoid convertion of NUMERIC/DATETIME items
  (i.e. those with derivation DERIVATION_NUMERIC).