Query performance - Mailing list pgsql-performance

In the below query both table has less than 1 million data. Can u tell me the reason of this plan?
why its takin extensive cost , seq scan and sorting?? wat is Materialize?

select 1 from  service_detail
left join non_service_detail on non_service_detail_service_id = service_detail.service_detail_id



Merge Left Join  (cost=62451.86..67379.08 rows=286789 width=0)
  Merge Cond: (service_detail.service_detail_id = non_service_detail.non_service_detail_service_id)
  ->  Sort  (cost=18610.57..18923.27 rows=125077 width=8)
        Sort Key: service_detail.service_detail_id
        ->  Seq Scan on service_detail  (cost=0.00..6309.77 rows=125077 width=8)
  ->  Materialize  (cost=43841.28..47426.15 rows=286789 width=8)
        ->  Sort  (cost=43841.28..44558.26 rows=286789 width=8)
              Sort Key: non_service_detail.non_service_detail_service_id
              ->  Seq Scan on non_service_detail  (cost=0.00..13920.89 rows=286789 width=8)

Thanks,
Arvind S


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