The job run with error: SQL0437W Performance of this complex query may be sub-optimal. Reason code: "3". SQLSTATE=01602

update CVG set cvg_amt = select cvg_amt from 
    (select CASE WHEN CVG.CVG_AMT IS NULL THEN COM.CVG_AMT ELSE CVG.CVG_AMT END AS CVG_AMT
	from cvg
	left join 
	(select A.id, A.cvg_amt, CASE WHEN A.PROCESS_DT > B.PROCESS_DT THEN A.CVG_AMT ELSE B.CVG_AMT END AS CVG_AMT
	from 
	( select A_first.id, A_first.cvg_amt, A_first.process_dt, A_first.gen_id from 
	  (select id, cvg_amt, process_dt, gen_id, eff_dt from A where eff_dt < '2015-10-28') as A_first,
		(select id, process_dt, max(gen_id ) as max_gen 
			from (select id, process_dt, gen_id  from A as A1 where eff_dt < '2015-10-28') , 
	       (select A2.id, max(A2.process_dt) as max_dt from A as A2 where eff_dt < '2015-10-28' group by A2.id)
        where eff_dt < '2015-10-28' and A2.id = A1.id and A2.max_dt=A1.process_dt group by A1.id, A1.process_dt) as A_second
	  where A_first.eff_dt<'2015-10-28'
			and A_first.id=A_second.id 
			and A_first.process_dt = A_second.process_dt
			and A_first.gen_id = A_second.gen_id
	 ) A
	 left join
	 
	 ( select B_first.id, B_first.cvg_amt, B_first.process_dt, B_first.gen_id from 
	  (select id, cvg_amt, process_dt, gen_id, eff_dt from B where eff_dt < '2015-10-28') as B_first,
		(select id, process_dt, max(gen_id ) as max_gen 
			from (select id, process_dt, gen_id  from B as B1 where eff_dt < '2015-10-28') , 
	       (select B2.id, max(B2.process_dt) as max_dt from B as B2 where eff_dt < '2015-10-28' group by B2.id)
        where eff_dt < '2015-10-28' and B2.id = B1.id and B2.max_dt=B1.process_dt group by B1.id, B1.process_dt) as B_second
	  where B_first.eff_dt<'2015-10-28'
			and B_first.id=B_second.id 
			and B_first.process_dt = B_second.process_dt
			and B_first.gen_id = B_second.gen_id
	 ) B on B.id = A.id
	 ) COM ON COM.ID = CVG.ID)

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