Optimization DB2 query with mass join - sql

Question

I have complex query:
select rma.RELATION_MANAGER_ID,
rm.ORG_STRUCTURE_ID,
rm.RELATIONSHIP_MANAGER_NM,
count(distinct ppa.PARTY_ID) as count_party
from RELATIONSHIP_MANAGER rm --15808 row
join RELATIONSHIP_MANAGER_MARKET rmm --1560 row
on rm.RELATIONSHIP_MANAGER_ID = rmm.RELATIONSHIP_MANAGER_ID
and rmm.INCLUDE_IN_REPORT = 'Y'
join MARKET_SEGMENT rm_ms --4 row
on rmm.MARKET_SEGMENT_ID = rm_ms.MARKET_SEGMENT_ID
and rm_ms.MARKET_SEGMENT = '01'
join RELATIONSHIP_MANAGER_ALLOCATION rma --61349 row
on rm.RELATIONSHIP_MANAGER_ID = rma.RELATIONSHIP_MANAGER_ID
join CMD_PARTY_PORTFOLIO_ALLOCATION ppa --3114096 row
on ppa.PORTFOLIO_ID = rma.PORTFOLIO_ID
join person ps --3112575 row
on ps.IS_DELETED != 1 and ppa.party_id = ps.party_id
join PARTY p --3114146 row
on ppa.party_id=p.party_id
join MARKET_SEGMENT ms --4 row
on p.MARKET_SEGMENT_ID = ms.MARKET_SEGMENT_ID and ms.MARKET_SEGMENT = '01'
where rm.IS_CM = 1 and rm.IS_DELETED != 1
group by rm.RELATIONSHIP_MANAGER_NM, rma.RELATIONSHIP_MANAGER_ID, rm.ORG_STRUCTURE_ID
Table columns have indexes:
rm.RELATIONSHIP_MANAGER_ID,
rmm.RELATIONSHIP_MANAGER_ID,
rmm.MARKET_SEGMENT_ID,
rm_ms.MARKET_SEGMENT_ID,
rma.RELATIONSHIP_MANAGER_ID,
ppa.PORTFOLIO_ID,
rma.PORTFOLIO_ID,
ppa.party_id,
ps.party_id,
p.party_id,
p.MARKET_SEGMENT_ID,
ms.MARKET_SEGMENT_ID
tables PARTY, PERSON have ~1-3 million row,
runtime of query ~20second. I am comment
join MARKET_SEGMENT ms
on p.MARKET_SEGMENT_ID = ms.MARKET_SEGMENT_ID --and ms.MARKET_SEGMENT = '01'
runtime of query became ~3 second.
Explain why this is happening, please ?
Explain plan dont help me.. How i can optimization the query?
EDIT:
platform is DB2 for z/OS V9.7,
added size of table
EDIT2: explain plan shows that the first is always join the small size of the table

Answer 1

Just for grins, see if this makes any difference:
WITH
MktSeg( MARKET_SEGMENT_ID ) AS
( SELECT MARKET_SEGMENT_ID
FROM MARKET_SEGMENT
WHERE MARKET_SEGMENT = '01' )
select rma.RELATION_MANAGER_ID,
rm.ORG_STRUCTURE_ID,
rm.RELATIONSHIP_MANAGER_NM,
count(distinct ppa.PARTY_ID) as count_party
from RELATIONSHIP_MANAGER rm --15808 row
join RELATIONSHIP_MANAGER_MARKET rmm --1560 row
on rm.RELATIONSHIP_MANAGER_ID = rmm.RELATIONSHIP_MANAGER_ID
and rmm.INCLUDE_IN_REPORT = 'Y'
join MktSeg rm_ms --4 row
on rmm.MARKET_SEGMENT_ID = rm_ms.MARKET_SEGMENT_ID
join RELATIONSHIP_MANAGER_ALLOCATION rma --61349 row
on rm.RELATIONSHIP_MANAGER_ID = rma.RELATIONSHIP_MANAGER_ID
join CMD_PARTY_PORTFOLIO_ALLOCATION ppa --3114096 row
on ppa.PORTFOLIO_ID = rma.PORTFOLIO_ID
join person ps --3112575 row
on ps.IS_DELETED != 1 and ppa.party_id = ps.party_id
join PARTY p --3114146 row
on ppa.party_id=p.party_id
join MktSeg ms --4 row
on p.MARKET_SEGMENT_ID = ms.MARKET_SEGMENT_ID
WHERE rm.IS_CM = 1 AND rm.IS_DELETED != 1
group by rm.RELATIONSHIP_MANAGER_NM, rma.RELATIONSHIP_MANAGER_ID,
rm.ORG_STRUCTURE_ID, rma.RELATION_MANAGER_ID
Note also that I've added an item to the Group By clause.