Disk read is quite high

Question

All,
I wrote query as below which is having quite high disk usage. I believe this to be reason for slow performance of my code (Although bad on performance tuning, but based on what I read on blogs that disk reads should be less). Any suggestion/help of how can this be performed fast.
SELECT DECODE (MSSO.SOURCE_TYPE, 1, 'T', 2, 'M', 3, 'B') SOURCE_TYPE ,
MSSO.RANK ,MSSO.ALLOCATION_PERCENT ,MSSO.VENDOR_ID ,MSSO.VENDOR_SITE_ID ,
MSSO.SOURCE_ORGANIZATION_ID ,TO_CHAR (MSRO.EFFECTIVE_DATE, 'RRRR/MM/DD')
ACTIVE_DATE ,TO_CHAR (MSRO.DISABLE_DATE, 'RRRR/MM/DD') END_DATE
FROM
MRP_SOURCING_RULES MSR ,MRP_SR_RECEIPT_ORG MSRO ,MRP_SR_SOURCE_ORG MSSO
WHERE MSR.SOURCING_RULE_ID = MSRO.SOURCING_RULE_ID AND MSSO.SR_RECEIPT_ID =
MSRO.SR_RECEIPT_ID AND MSR.SOURCING_RULE_ID = :B1 AND MSSO.RANK = (SELECT
MIN (RANK) FROM MRP_SR_SOURCE_ORG MSSO1 WHERE MSSO.SR_RECEIPT_ID =
MSSO1.SR_RECEIPT_ID) AND TRUNC (NVL (MSRO.EFFECTIVE_DATE, SYSDATE)) <=
TRUNC (SYSDATE) AND TRUNC (NVL (MSRO.DISABLE_DATE, SYSDATE)) >= TRUNC
(SYSDATE) AND ( EXISTS ( SELECT 1 FROM HR_ORGANIZATION_INFORMATION OOD ,
PO_VENDOR_SITES_ALL S WHERE OOD.ORGANIZATION_ID = :B2 AND
OOD.ORG_INFORMATION3 = S.ORG_ID AND OOD.ORG_INFORMATION_CONTEXT =
'Accounting Information' AND S.VENDOR_SITE_ID = MSSO.VENDOR_SITE_ID) OR
MSSO.VENDOR_SITE_ID IS NULL OR MSSO.SOURCE_TYPE <>3)
call count cpu elapsed disk query current rows
Parse 1 0.00 0.00 0 0 0 0
Execute 89400 23.16 23.35 0 0 0 0
Fetch 89400 69.28 86.57 1338 1740766 0 88732
total 178801 92.44 109.93 1338 1740766 0 88732
Thanks
Sachin

Answer 1

And below is the Explain Plan it generated
Misses in library cache during parse: 1
Misses in library cache during execute: 1
Optimizer mode: ALL_ROWS
Parsing user id: 173     (recursive depth: 1)
Number of plan statistics captured: 1
Rows (1st) Rows (avg) Rows (max)  Row Source Operation
         1          1          1  FILTER  (cr=15 pr=11 pw=0 time=0 us)
         1          1          1   HASH GROUP BY (cr=15 pr=11 pw=0 time=0 us cost=12 size=131 card=1)
         1          1          1    FILTER  (cr=15 pr=11 pw=0 time=0 us)
         1          1          1     NESTED LOOPS  (cr=15 pr=11 pw=0 time=0 us)
         1          1          1      NESTED LOOPS  (cr=14 pr=11 pw=0 time=0 us cost=11 size=131 card=1)
         1          1          1       NESTED LOOPS  (cr=11 pr=11 pw=0 time=0 us cost=8 size=116 card=1)
         1          1          1        NESTED LOOPS  (cr=7 pr=7 pw=0 time=0 us cost=5 size=62 card=1)
         1          1          1         INDEX UNIQUE SCAN MRP_SOURCING_RULES_U1 (cr=3 pr=3 pw=0 time=0 us cost=2 size=18 card=1)(object id 39415)
         1          1          1         TABLE ACCESS BY INDEX ROWID MRP_SR_RECEIPT_ORG (cr=4 pr=4 pw=0 time=0 us cost=3 size=44 card=1)
         1          1          1          INDEX RANGE SCAN MRP_SR_RECEIPT_ORG_U2 (cr=3 pr=3 pw=0 time=0 us cost=2 size=0 card=1)(object id 39599)
         1          1          1        TABLE ACCESS BY INDEX ROWID MRP_SR_SOURCE_ORG (cr=4 pr=4 pw=0 time=0 us cost=3 size=54 card=1)
         1          1          1         INDEX RANGE SCAN MRP_SR_SOURCE_ORG_U2 (cr=3 pr=3 pw=0 time=0 us cost=2 size=0 card=1)(object id 39656)
         1          1          1       INDEX RANGE SCAN MRP_SR_SOURCE_ORG_U2 (cr=3 pr=0 pw=0 time=0 us cost=2 size=0 card=1)(object id 39656)
         1          1          1      TABLE ACCESS BY INDEX ROWID MRP_SR_SOURCE_ORG (cr=1 pr=0 pw=0 time=0 us cost=3 size=15 card=1)
         0          0          0     NESTED LOOPS  (cr=0 pr=0 pw=0 time=0 us cost=4 size=30 card=1)
         0          0          0      TABLE ACCESS BY INDEX ROWID PO_VENDOR_SITES_ALL (cr=0 pr=0 pw=0 time=0 us cost=2 size=9 card=1)
         0          0          0       INDEX UNIQUE SCAN PO_VENDOR_SITES_U1 (cr=0 pr=0 pw=0 time=0 us cost=1 size=0 card=1)(object id 3521718)
         0          0          0      TABLE ACCESS BY INDEX ROWID HR_ORGANIZATION_INFORMATION (cr=0 pr=0 pw=0 time=0 us cost=2 size=21 card=1)
         0          0          0       INDEX RANGE SCAN HR_ORGANIZATION_INFORMATIO_FK2 (cr=0 pr=0 pw=0 time=0 us cost=1 size=0 card=1)(object id 3521010)