hum, should i use this function in sql_select.cc? remove_eq_conds()
/**
  Remove const and eq items. Return new item, or NULL if no condition
  cond_value is set to according:
  COND_OK    query is possible (field = constant)
  COND_TRUE  always true ( 1 = 1 )
  COND_FALSE always false ( 1 = 2 )

  SYNPOSIS
    remove_eq_conds()
    thd THD environment
    cond                        the condition to handle
    cond_value                  the resulting value of the condition

  NOTES
    calls the inner_remove_eq_conds to check all the tree reqursively

  RETURN
    *COND with the simplified condition
*/