===============================================================================================================
Case 1
===============================================================================================================
select count(*) from (
        SELECT *
        from (SELECT pbdedup_party_1.party_id
              from pbdedup_party_1
              where pbdedup_party_1.batch_no = 93
              UNION
              SELECT pbdedup_party_2.party_id
              from pbdedup_party_2
              where pbdedup_party_2.batch_no = 93
              ) as a
           , pb_person
                left outer join pbdedup_person_parts ON pb_person.party_id = pbdedup_person_parts.party_id
        where a.party_id = pb_person.party_id
) as b
---------------------------------------------------------------------------------------------------------------
Aggregate  (cost=4251.07..4251.07 rows=1 width=12)
  ->  Hash Join  (cost=1291.73..4251.07 rows=1 width=12)
        ->  Hash Join  (cost=1284.45..4072.80 rows=34196 width=8)
              ->  Seq Scan on pb_person  (cost=0.00..1496.96 rows=34196 width=4)
              ->  Hash  (cost=1081.96..1081.96 rows=34196 width=4)
                    ->  Seq Scan on pbdedup_person_parts  (cost=0.00..1081.96 rows=34196 width=4)
        ->  Hash  (cost=7.27..7.27 rows=1 width=4)
              ->  Subquery Scan a  (cost=7.27..7.27 rows=1 width=4)
                    ->  Unique  (cost=7.27..7.27 rows=1 width=4)
                          ->  Sort  (cost=7.27..7.27 rows=2 width=4)
                                ->  Append  (cost=0.00..7.26 rows=2 width=4)
                                      ->  Subquery Scan *SELECT* 1  (cost=0.00..2.25 rows=1 width=4)
                                            ->  Seq Scan on pbdedup_party_1  (cost=0.00..2.25 rows=1 width=4)
                                      ->  Subquery Scan *SELECT* 2  (cost=0.00..5.01 rows=1 width=4)
                                            ->  Index Scan using IDX_pbdedup_party_2__batch_no_g on pbdedup_party_2  (cost=0.00..5.01 rows=1 width=4)




===============================================================================================================
Case 2
===============================================================================================================
select count(*) from (
        SELECT *
        from (SELECT pbdedup_party_1.party_id
              from pbdedup_party_1
              where pbdedup_party_1.batch_no = 93
              UNION
              SELECT pbdedup_party_2.party_id
              from pbdedup_party_2
              where pbdedup_party_2.batch_no = 93
              ) as a
                left outer join pbdedup_person_parts ON a.party_id = pbdedup_person_parts.party_id
           , pb_person
        where a.party_id = pb_person.party_id
) as b
---------------------------------------------------------------------------------------------------------------
Aggregate  (cost=18.82..18.82 rows=1 width=12)
  ->  Nested Loop  (cost=7.27..18.81 rows=1 width=12)
        ->  Nested Loop  (cost=7.27..13.06 rows=1 width=8)
              ->  Subquery Scan a  (cost=7.27..7.27 rows=1 width=4)
                    ->  Unique  (cost=7.27..7.27 rows=1 width=4)
                          ->  Sort  (cost=7.27..7.27 rows=2 width=4)
                                ->  Append  (cost=0.00..7.26 rows=2 width=4)
                                      ->  Subquery Scan *SELECT* 1  (cost=0.00..2.25 rows=1 width=4)
                                            ->  Seq Scan on pbdedup_party_1  (cost=0.00..2.25 rows=1 width=4)
                                      ->  Subquery Scan *SELECT* 2  (cost=0.00..5.01 rows=1 width=4)
                                            ->  Index Scan using IDX_pbdedup_party_2__batch_no_g on pbdedup_party_2  (cost=0.00..5.01 rows=1 width=4)
              ->  Index Scan using pbdedup_person_parts_pkey on pbdedup_person_parts  (cost=0.00..5.77 rows=1 width=4)
        ->  Index Scan using pb_person_pkey on pb_person  (cost=0.00..5.74 rows=1 width=4)