COMMIT takes long time for read only transaction

Hi, all.

Postgres 8.4.4, quite often I have situation like this:

011-05-26 18:25:38.842 CEST [16163] <ample@knjiz> [192.168.1.5(32431)]
SET LOG:  duration: 0.029 ms  statement: BEGIN; SET TRANSACTION
ISOLATION LEVEL READ COMMITTED
2011-05-26 18:25:38.843 CEST [16163] <ample@knjiz> [192.168.1.5(32431)]
SELECT LOG:  duration: 0.259 ms  statement: SELECT
2011-05-26 18:25:38.847 CEST [16163] <ample@knjiz> [192.168.1.5(32431)]
SELECT LOG:  duration: 2.205 ms  statement: SELECT
2011-05-26 18:25:38.850 CEST [16163] <ample@knjiz> [192.168.1.5(32431)]
SELECT LOG:  duration: 0.185 ms  statement: SELECT
2011-05-26 18:25:39.250 CEST [16163] <ample@knjiz> [192.168.1.5(32431)]
SELECT LOG:  duration: 0.232 ms  statement: SELECT
2011-05-26 18:25:39.252 CEST [16163] <ample@knjiz> [192.168.1.5(32431)]
SELECT LOG:  duration: 0.289 ms  statement: SELECT
2011-05-26 18:25:39.255 CEST [16163] <ample@knjiz> [192.168.1.5(32431)]
SELECT LOG:  duration: 0.639 ms  statement: SELECT
2011-05-26 18:25:39.257 CEST [16163] <ample@knjiz> [192.168.1.5(32431)]
SELECT LOG:  duration: 0.135 ms  statement: SELECT
2011-05-26 18:25:39.258 CEST [16163] <ample@knjiz> [192.168.1.5(32431)]
SELECT LOG:  duration: 0.156 ms  statement: SELECT
2011-05-26 18:25:51.624 CEST [16163] <ample@knjiz> [192.168.1.5(32431)]
COMMIT LOG:  duration: 11159.202 ms  statement: COMMIT
2011-05-26 18:26:05.788 CEST [16163] <ample@knjiz> [192.168.1.5(32431)]
COMMIT LOG:  duration: 326.856 ms  statement: COMMIT

This is a grep from a postgres log when I had logging of all statements
enabled. I did grep by the local pid (16163), and what I don't
understand is:
- why is first COMMIT taking 11 seconds, when there are only SELECTs in
the transaction? SELECTS don't call any user defined functions, just
plain reads from the tables.
- what is that second COMMIT? If I issue COMMIT without 'BEGIN'
shouldn't I get: "COMMIT WARNING:  there is no transaction in progress"

    Mario