BUG #6644: Query give wrong results when 'is not null' is applied in where clause

The following bug has been logged on the website:

Bug reference: 6644
Logged by: Karl Walbrecht
Email address: kwalbrecht(at)cghtech(dot)com
PostgreSQL version: 9.0.3
Operating system: SunOS 5.10
Description:

geom.ArcToStreight(integer) is a function which returns the following type:

Composite type "geom.t_line_ref"
Column | Type
-----------+-------------------
line_id | integer
start_id | integer
mid_id | integer
end_id | integer
line | geometry
line_type | character varying
chart_id | integer

The following queries show 1) the total number of rows. 2) the number of
null rows. 3) the number of not null rows.

The problem is that results of query 2 + the results of query 3 should equal
the result of query 1 which they do not.

SELECT count(*)
FROM (
select geom.ArcToStreight(line_id) as a
from sdat_legacy.lines
where mid_node is not null
) foo
;

count
-------
14177
(1 row)

SELECT count(*)
FROM (
select geom.ArcToStreight(line_id) as a
from sdat_legacy.lines
where mid_node is not null
) foo
WHERE foo.a is null
;

count
-------
13796
(1 row)

SELECT count(*)
FROM (
select geom.ArcToStreight(line_id) as a
from sdat_legacy.lines
where mid_node is not null
) foo
WHERE foo.a is not null
;

count
-------
0
(1 row)

Clearly something is amiss.

14197 Total number of rows
- 13796 Number of null row
--------
381 Expected number of not null rows

0 Number of not null rows returned