IMMUTABLE function's flag do not work: 7.3.4, plpgsql

Hi folks. I notice that immutable flag does nothing when i invoke
my plpgsql function within one session with same args.

tele=# SELECT version();
version
-------------------------------------------------------------
PostgreSQL 7.3.4 on i686-pc-linux-gnu, compiled by GCC 2.96

At first EXPLAIN ANALYZE shown strange runtime :)

[15:41]/0:ant(at)monstr:~>time psql -c 'EXPLAIN ANALYZE SELECT calc_total(1466476, 1062363600, 1064955599)' tele
QUERY PLAN
----------------------------------------------------------------------------------
Result (cost=0.00..0.01 rows=1 width=0) (actual time=0.00..0.00 rows=1 loops=1)
Total runtime: 0.02 msec
^^^^^^^^^
(2 rows)

real 0m19.282s
^^^^^^^^^

At second. calc_total() is immutable function:

tele=# SELECT provolatile from pg_proc where proname = 'calc_total' and pronargs =3;
provolatile
-------------
i

but it seems that it's not cached in one session:

[15:38]/0:ant(at)monstr:~>psql tele
Welcome to psql 7.3.4, the PostgreSQL interactive terminal.

Type: \copyright for distribution terms
\h for help with SQL commands
\? for help on internal slash commands
\g or terminate with semicolon to execute query
\q to quit

tele=# EXPLAIN ANALYZE SELECT calc_total(1466476, 1062363600, 1064955599);
QUERY PLAN
----------------------------------------------------------------------------------
Result (cost=0.00..0.01 rows=1 width=0) (actual time=0.00..0.00 rows=1 loops=1)
Total runtime: 0.02 msec
(2 rows)

tele=# EXPLAIN ANALYZE SELECT calc_total(1466476, 1062363600, 1064955599);
QUERY PLAN
----------------------------------------------------------------------------------
Result (cost=0.00..0.01 rows=1 width=0) (actual time=0.00..0.00 rows=1 loops=1)
Total runtime: 0.02 msec
(2 rows)

What i miss?

Thanks,
Andriy Tkachuk

http://www.imt.com.ua