why?

I've two queries -

1. emrxdbs=# explain select * from patient A  where exists (select NULL from
patient B where B.mrn=A.mrn and B.dob=A.dob and B.sex=A.sex and
B.lastname=A.lastname and B.firstname=A.firstname group by B.mrn, B.dob,
B.sex, B.lastname, B.firstname having A.patseq < max(B.patseq)) limit 10;
NOTICE:  QUERY PLAN:

Limit  (cost=0.00..121.50 rows=10 width=141) ->  Seq Scan on patient a  (cost=0.00..6955296.53 rows=572430 width=141)
   SubPlan         ->  Aggregate  (cost=6.03..6.05 rows=1 width=42)               ->  Group  (cost=6.03..6.05 rows=1
width=42)                    ->  Sort  (cost=6.03..6.03 rows=1 width=42)                           ->  Index Scan using
patient_name_idxon patient
 
b  (cost=0.00..6.02 rows=1 width=42)

2. emrxdbs=# explain select * from patient A  where exists (select NULL from
patient B where B.mrn=A.mrn and B.dob=A.dob and B.sex=A.sex and
B.lastname=A.lastname and B.firstname=A.firstname and B.mrn='3471585'  group
by B.mrn, B.dob, B.sex, B.lastname, B.firstname having A.patseq <
max(B.patseq)) limit 10;
NOTICE:  QUERY PLAN:

Limit  (cost=0.00..121.45 rows=10 width=141) ->  Seq Scan on patient a  (cost=0.00..6951997.59 rows=572430 width=141)
   SubPlan         ->  Aggregate  (cost=6.03..6.05 rows=1 width=42)               ->  Group  (cost=6.03..6.04 rows=1
width=42)                    ->  Sort  (cost=6.03..6.03 rows=1 width=42)                           ->  Index Scan using
patient_mrnfac_idxon
 
patient b  (cost=0.00..6.02 rows=1 width=42)

The first query results come back fairly quick, the 2nd one just sits there
forever.
It looks similar in the two query plans.

Let me know.

thanks.
johnl