COALESCE As A Performance Enhancer??!?

I am using a clever product called SQL Expert Pro to optimize
problematic SQL statements. It works by generating syntactically
identical variations of the original SQL, finding all unique execution
plans, and batch testing them.

Anyway, it generally turns out that a very odd change dramatically
improves performance (for a PeopleSoft database on SQL Server 7). Run
time goes from 52 seconds to 8 seconds.

The change is to replace a join condition in a where clause with an
odd equivalent COALESCE construct, e.g. replace WHERE C.PAY_ID =
D.PAY_ID with WHERE C.PAY_ID = COALESCE(D.PAY_ID, D.PAY_ID).

Has anyone seen this sort of behavior before? Why would it be
advantageous to COALESCE on the same field twice?

The original and COALESCE'd versions are shown below. Does the fact
that this is a nine-table join over large tables have anything to do
with it?

Original Query (52 sec)

SELECT F.ACCT_ID,
I.ENTITY_NAME,
A.TNDR_SOURCE_CD,
C.PAY_EVENT_ID,
C.NON_CIS_NAME,
C.NON_CIS_REF_NBR,
C.NON_CIS_COMMENT,
C.PAY_AMT,
D.PAY_SEG_AMT,
E.ACCOUNTING_DT,
':1',
':2',
':3',
E.FREEZE_OPRID,
E.FREEZE_DTTM,
E.FT_TYPE_FLG
FROM PS_CI_TNDR_CTL A,
PS_CI_PAY_TNDR B,
PS_CI_PAY C,
PS_CI_PAY_SEG D,
PS_CI_SA F,
PS_CI_SA_TYPE G,
PS_CI_ACCT_PER H,
PS_CI_PER_NAME I,
PS_CW_FT E
WHERE A.TNDR_CTL_ID = B.TNDR_CTL_ID
AND A.TNDR_SOURCE_CD LIKE 'STK%'
AND B.PAY_EVENT_ID = C.PAY_EVENT_ID
AND C.PAY_ID = D.PAY_ID
AND D.PAY_SEG_ID = E.SIBLING_ID
AND E.ACCOUNTING_DT BETWEEN '2003-10-01' AND '2003-10-31'
AND E.FT_TYPE_FLG IN ('PS', 'PX')
AND NOT EXISTS (SELECT 'X'
FROM PS_CW_INTERFACE_ID J
WHERE J.PAYOR_ACCT_ID = F.ACCT_ID)
AND E.SA_ID = F.SA_ID
AND F.SA_TYPE_CD = G.SA_TYPE_CD
AND G.DEBT_CL_CD = 'NCIS'
AND F.ACCT_ID = H.ACCT_ID
AND H.PER_ID = I.PER_ID
ORDER BY 2

Optimized Query (8 sec)

SELECT F.ACCT_ID,
I.ENTITY_NAME,
A.TNDR_SOURCE_CD,
C.PAY_EVENT_ID,
C.NON_CIS_NAME,
C.NON_CIS_REF_NBR,
C.NON_CIS_COMMENT,
C.PAY_AMT,
D.PAY_SEG_AMT,
E.ACCOUNTING_DT,
':1',
':2',
':3',
E.FREEZE_OPRID,
E.FREEZE_DTTM,
E.FT_TYPE_FLG
FROM PS_CI_TNDR_CTL A,
PS_CI_PAY_TNDR B,
PS_CI_PAY C,
PS_CI_PAY_SEG D,
PS_CI_SA F,
PS_CI_SA_TYPE G,
PS_CI_ACCT_PER H,
PS_CI_PER_NAME I,
PS_CW_FT E
WHERE A.TNDR_CTL_ID = COALESCE(B.TNDR_CTL_ID,B.TNDR_CTL_ID)
AND A.TNDR_SOURCE_CD LIKE 'STK%'
AND B.PAY_EVENT_ID = COALESCE(C.PAY_EVENT_ID, C.PAY_EVENT_ID)
AND C.PAY_ID = COALESCE(D.PAY_ID, D.PAY_ID)
AND D.PAY_SEG_ID = COALESCE(E.SIBLING_ID, E.SIBLING_ID)
AND COALESCE(E.ACCOUNTING_DT, E.ACCOUNTING_DT) BETWEEN '2003-10-01'
AND '2003-10-31'
AND COALESCE(E.FT_TYPE_FLG, E.FT_TYPE_FLG) IN ('PS', 'PX')
AND NOT EXISTS (SELECT 'X'
FROM PS_CW_INTERFACE_ID J
WHERE COALESCE(J.PAYOR_ACCT_ID, J.PAYOR_ACCT_ID) =
F.ACCT_ID)
AND E.SA_ID = COALESCE(F.SA_ID,F.SA_ID)
AND F.SA_TYPE_CD = COALESCE(G.SA_TYPE_CD,G.SA_TYPE_CD)
AND G.DEBT_CL_CD = 'NCIS'
AND F.ACCT_ID = COALESCE(H.ACCT_ID ,H.ACCT_ID)
AND H.PER_ID = COALESCE(I.PER_ID ,I.PER_ID)
ORDER BY 2