EXPLAIN SELECT logic_id, since, till FROM COND_dummy               
           WHERE logic_id = 9999
           AND ((since >= '2005-01-01 00:00:00' 
                AND since < '2005-01-01 00:00:01')
             OR (till  >  '2005-01-01 00:00:00' 
                AND till  < '2005-01-01 00:00:01')
             OR ('2005-01-01 00:00:00' >= since 
                 AND '2005-01-01 00:00:01' < till))                      
         LIMIT 1;