What determines the cost of an index scan?

Hi list,
I have experienced the following situation: A join between two tables
(one with ~900k rows, the other with ~1600k rows) takes about 20 sec on
our productive database. I have created two tables in our test database
with the same data, but with fewer fields. (I have omitted several
fields that do not participate in the join.) If I try the join in our
test database it takes about 8 sec. Both queries have the same query plan:

prod=# explain analyze select 1 from dtng."Z_UL" inner join
dtng."Z_BARRIER" using ("ISIN", "ID_NOTATION");
                                                                  QUERY PLAN

----------------------------------------------------------------------------------------------------------------------------------------------
Merge Join  (cost=0.00..131201.15 rows=39376 width=0) (actual
time=0.198..16086.185 rows=1652076 loops=1)
  Merge Cond: ((("Z_UL"."ISIN")::bpchar = ("Z_BARRIER"."ISIN")::bpchar)
AND ("Z_UL"."ID_NOTATION" = "Z_BARRIER"."ID_NOTATION"))
  ->  Index Scan using "Z_UL_pkey" on "Z_UL"  (cost=0.00..34806.57
rows=897841 width=20) (actual time=0.075..1743.396 rows=897841 loops=1)
  ->  Index Scan using "Z_BARRIER_ISIN_ID_NOTATION_key" on "Z_BARRIER"
(cost=0.00..83255.17 rows=1652076 width=20) (actual time=0.076..3389.676
rows=1652076 loops=1)
Total runtime: 18123.042 ms

test=# explain analyze select 1 from table1 inner join table2 using
(key1, key2);

QUERY PLAN

-------------------------------------------------------------------------------------------------------------------------------------------------------
 Merge Join  (cost=0.00..82443.05 rows=36158 width=0) (actual
time=0.092..8036.490 rows=1652076 loops=1)
   Merge Cond: ((table1.key1 = table2.key1) AND (table1.key2 = table2.key2))
   ->  Index Scan using table1_pkey on table1  (cost=0.00..22719.56
rows=897841 width=20) (actual time=0.026..845.916 rows=897841 loops=1)
   ->  Index Scan using table2_key1_key2_key on table2
(cost=0.00..46638.20 rows=1652076 width=20) (actual time=0.049..1843.047
rows=1652076 loops=1)
 Total runtime: 8460.956 ms

No disk io occurs in either server, so I guess that the whole data is
already in memory. Both servers are idle. Both use the same PostgreSQL
version (8.2.9). Both servers are 64bit machines. However, the servers
have different CPUs and memory: The production server has 4 Dual-Core
AMD Opteron 8214 processors (2.2 GHz) and 16 GB memory, the test server
has 2 Dual-Core Intel Xeon 5130 processors (2.0 GHz) and 8 GB memory. I
have not yet done a CPU and memory benchmark, but this is my next step.

Where does this difference come from? Pure cpu performance? Do the
additional fields in the productive database have an impact on the
performance? Or do I miss something?

Regards,
    Christian

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