Oracle analytical functions to the rescue

 Below is a typical example of how using analytical functions speeds up a query, which is having a subquery inside.

Original query:

SELECT
DISTINCT CUSTOMER_ID, SUBSCRIBER_NO, TEST3.TEST3, MODEL_DESC,
                        ALT_MODEL_DESC
                        FROM TEST1, TEST2, TEST3
                        WHERE TEST1.CUSTOMER_ID = 517091995
                        AND TEST1.UNIT_ESN = ESN
                        AND TEST1.ESN_SEQ_NO =
                        (SELECT MAX (ESN_SEQ_NO)
                        FROM TEST1 PD1
                        WHERE PD1.CUSTOMER_ID = TEST1.CUSTOMER_ID
                        AND PD1.SUBSCRIBER_NO = TEST1.SUBSCRIBER_NO
                        AND LENGTH(UNIT_ESN) <> 20 )
                        AND TEST2.TEST3 = TEST3.TEST3
                        AND ( TEST3.EXPIRATION_DATE  IS NULL OR TEST3.EXPIRATION_DATE >= SYSDATE          )

;


The new query, using analytical functions (much faster and accesing the TEST1 table only once):

SELECT
DISTINCT CUSTOMER_ID, SUBSCRIBER_NO, TEST3.TEST3, MODEL_DESC,
                        ALT_MODEL_DESC
                        FROM
(select UNIT_ESN,ESN_SEQ_NO,CUSTOMER_ID, SUBSCRIBER_NO, max(ESN_SEQ_NO) over (partition by CUSTOMER_ID,SUBSCRIBER_NO)  as max_ESN_SEQ_NO
from TEST1 where LENGTH(UNIT_ESN) <> 20 ) phd, TEST2, TEST3
                        WHERE phd.CUSTOMER_ID = 517091995
                        and ESN_SEQ_NO=phd.max_ESN_SEQ_NO
                        AND UNIT_ESN = ESN
                        AND TEST2.TEST3 = TEST3.TEST3
                        AND ( TEST3.EXPIRATION_DATE  IS NULL OR TEST3.EXPIRATION_DATE >= SYSDATE)
;