Question about cost-calculation

Hi,

according the doc
(http://www.postgresql.org/docs/8.3/interactive/runtime-config-query.html#GUC-CPU-OPERATOR-COST)

Quote:
Sets the planner's estimate of the cost of processing each operator or function executed during a query.

i expected for a 1000 row test-table a cost per function of 2.5
(cpu_operator_cost = 0.0025), but i got 5.

First, a full table scan:

test=*# explain analyse select i from s_test;
QUERY PLAN
-------------------------------------------------------------------------------------------------------
Seq Scan on s_test (cost=0.00..15.00 rows=1000 width=4) (actual time=0.013..2.315 rows=1000 loops=1)
Total runtime: 4.232 ms
(2 rows)

cost=15.

Now, the same query but with one or two functions on a particular column:

test=*# explain analyse select i, cos(i) from s_test;
QUERY PLAN
-------------------------------------------------------------------------------------------------------
Seq Scan on s_test (cost=0.00..20.00 rows=1000 width=4) (actual time=0.026..3.043 rows=1000 loops=1)
Total runtime: 5.017 ms
(2 rows)

test=*# explain analyse select i, cos(i), md5(i) from s_test;
QUERY PLAN
-------------------------------------------------------------------------------------------------------
Seq Scan on s_test (cost=0.00..25.00 rows=1000 width=4) (actual time=0.040..5.414 rows=1000 loops=1)
Total runtime: 7.444 ms
(2 rows)

cost increased by 5 per function-call, why, why not 2.5?

Version: 8.1

Andreas
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Andreas Kretschmer
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