count is ten times faster

Hi there,

I have a simple aggregate query: SELECT count("PK_ID") AS "b1" FROM "tbA"
WHERE "PK_ID" > "f1"( 'c1' ), which has the following execution plan:
"Aggregate  (cost=2156915.42..2156915.43 rows=1 width=4)"
"  ->  Seq Scan on "tbA"  (cost=0.00..2137634.36 rows=7712423 width=4)"
"        Filter: ("PK_ID" > "f1"('c1'::character varying))"

I tried to get the same result with the following query:
SELECT (
     SELECT count("PK_ID") AS "b1" FROM "tbA" ) -
     (
          SELECT count("PK_ID") AS "b1"
          FROM "tbA"
          WHERE "PK_ID" <= "f1"( 'c1' )
     )
with the execution plan:
"Result  (cost=248952.95..248952.96 rows=1 width=0)"
"  InitPlan"
"    ->  Aggregate  (cost=184772.11..184772.12 rows=1 width=4)"
"          ->  Seq Scan on "tbA"  (cost=0.00..165243.49 rows=7811449
width=4)"
"    ->  Aggregate  (cost=64180.81..64180.82 rows=1 width=4)"
"          ->  Index Scan using "tbA_pkey" on "tbA"  (cost=0.25..63933.24
rows=99026 width=4)"
"                Index Cond: ("PK_ID" <= "f1"('c1'::character varying))"

How do you explain the cost is about ten times lower in the 2nd query than
the first ?

TIA,
Sabin