Question about postmaster's CPU usage

Hi all,

   When I use postgres and issue a simple sequential scan for a table inventory using query "select * from inventory;", I can see from "top" that postmaster is using 100% CPU, which limits the query execution time. My question is that, why CPU is the bottleneck here and what is postmaster doing? Is there any way to improve the performance? Thanks!

PID USER      PR  NI  VIRT  RES  SHR S %CPU %MEM    TIME+  COMMAND
15401 postgres  20   0 4371m 517m 515m R 99.8  3.2   0:30.14 postmaster

Query: select * from inventory;

explain analyze select * from inventory;
                                                        QUERY PLAN

--------------------------------------------------------------------------------------------------------------
------------
 Seq Scan on inventory  (cost=0.00..180937.00 rows=11745000 width=16) (actual time=0.005..1030.403 rows=117450
00 loops=1)
 Total runtime: 1750.889 ms
(2 rows)

Table "public.inventory"
        Column        |  Type   | Modifiers
----------------------+---------+-----------
 inv_date_sk          | integer | not null
 inv_item_sk          | integer | not null
 inv_warehouse_sk     | integer | not null
 inv_quantity_on_hand | integer |
Indexes:
    "inventory_pkey" PRIMARY KEY, btree (inv_date_sk, inv_item_sk, inv_warehouse_sk)