Improper processing of random values in sub-queries

PostgreSQL version: 7.4.3 (RPMs from ftp.au.postgresql.org)

Operating Sysem: Fedora Core 1

GIVEN

CREATE TABLE data (i integer);

AND

SELECT count(*), min(i), max(i) FROM data;
count | min | max
-------+-----+-----
1340 | 3 | 20
(1 row)

(Actual data attached as data.dmp)

NOW CONSIDER the following query, executed using psql:

SELECT i, j from (
SELECT i, ((i + 18 * random())::integer % 20 + 1) AS j FROM data
) foo WHERE j = 15;

Column j should be constant (15) for all rows. In fact I get the
following output:

i | j
----+----
8 | 2
8 | 1
8 | 13
8 | 5
8 | 5
9 | 19
9 | 1
10 | 8
10 | 4
11 | 5
12 | 9
19 | 10
3 | 15
3 | 16
3 | 5
3 | 18
3 | 11
3 | 10
3 | 7
3 | 13
3 | 10
3 | 18
3 | 2
.
.
.
(74 rows)

The same problem occurs for any constraint of j in the range 1..20