SELECT *
FROM (
    SELECT q.*
        ,ROWNUM RN
    FROM (
        SELECT T1.X,T2.Y
    FROM TABLE1 T1 LEFT OUTER JOIN
            TABLE2 T2 ON (T1.X=T2.X)
            LEFT OUTER JOIN
            TABLE2 T3 ON (T1.X=T3.X AND T1.Y = T3.Y AND T2.Z = T3.Z)
            WHERE
            <CONDITION 1>
            <CONDITION 2>
        )
        ORDER BY T2.TIMESTAMP1 DESC
    ) Q
)
WHERE RN >= 1 AND RN <= 20;