is it a known issue or just a bug?

Folks,

Last week one of my students confronted me with a nice little SQL 
statement which made me call gdb ...

Consider the following scenario:

[hs@fedora bug]$ cat q1.sql
create temporary sequence seq_ab;

select * from (Select nextval('seq_ab') as nv,                      * from    ( select 
t_product.id,t_text.value,t_price.price                                from    t_product,t_price,t_text
      where   t_product.id = t_price.product_id                                and t_product.name = t_text.id
                    and t_text.lang='de'                                and t_price.typ = 'default'
  order by price desc ) as t ) as u                --      WHERE nv <= 1                ;
 
[hs@fedora bug]$ psql test < q1.sql
CREATE SEQUENCE nv | id |  value  | price
----+----+---------+-------  1 |  3 | Banane  |    12  2 |  1 | T-Shirt |    10  3 |  2 | Apfel   |     7
(3 rows)

this query returns the right result.
however, when uncommenting the WHERE clause things look different:

[hs@fedora bug]$ cat q2.sql
create temporary sequence seq_ab;

select * from (Select nextval('seq_ab') as nv,                      * from    ( select 
t_product.id,t_text.value,t_price.price                                from    t_product,t_price,t_text
      where   t_product.id = t_price.product_id                                and t_product.name = t_text.id
                    and t_text.lang='de'                                and t_price.typ = 'default'
  order by price desc ) as t ) as u                        WHERE nv <= 1                ;
 
[hs@fedora bug]$ psql test < q2.sql
CREATE SEQUENCE nv | id |  value  | price
----+----+---------+-------  4 |  1 | T-Shirt |    10
(1 row)

Obviously nv = 4 is wrong ...
Looking at the execution plan of the second query the problem seems 
quite obvious:
                                               QUERY PLAN
-------------------------------------------------------------------------------------------------------- Subquery Scan
t (cost=69.24..69.26 rows=1 width=68)   ->  Sort  (cost=69.24..69.25 rows=1 width=68)         Sort Key: t_price.price
     ->  Hash Join  (cost=22.51..69.23 rows=1 width=68)               Hash Cond: ("outer".name = "inner".id)
  Join Filter: (nextval('seq_ab'::text) <= 1)               ->  Nested Loop  (cost=0.00..46.68 rows=5 width=40)
           ->  Seq Scan on t_price  (cost=0.00..22.50 rows=5 
 
width=36)                           Filter: (typ = 'default'::text)                     ->  Index Scan using
t_product_pkeyon t_product 
 
(cost=0.00..4.82 rows=1 width=8)                           Index Cond: (t_product.id = "outer".product_id)
->  Hash  (cost=22.50..22.50 rows=5 width=36)                     ->  Seq Scan on t_text  (cost=0.00..22.50 rows=5 
 
width=36)                           Filter: (lang = 'de'::text)
(14 rows)


nextval() is called again when processing the WHERE clause.
this was fine if nextval() would return the same thing again and again 
(which is not the job of nextval).
if the planner materialized the subquery things would materialize the 
subquery in case of unstable functions things would work in this case.

I know I temp table would easily fix this query and it is certainly not 
the best query I have ever seen but still it seems like a bug and I just  wanted to know whether it is a know issue or
not.
Looking at the code I did not quite know whether this is something which 
should / can be fixed or not.

here is the data:
------------------------------------------------------
CREATE TABLE t_text (        id      int4,        lang    text,        value   text
);

CREATE TABLE t_group (        id      int4,        name    int4,           -- mehrsprachig in t_text        valid
boolean
);

INSERT INTO t_group VALUES (1, 1, 't');
INSERT INTO t_text  VALUES (1, 'de', 'Obst');
INSERT INTO t_text  VALUES (1, 'en', 'Fruits');

INSERT INTO t_group VALUES (2, 2, 't');
INSERT INTO t_text  VALUES (2, 'de', 'Kleidung');
INSERT INTO t_text  VALUES (2, 'en', 'Clothes');


CREATE UNIQUE INDEX idx_group_id ON t_group (id);

CREATE TABLE t_product (        id              int4,        name            int4,   -- mehrsprachig in t_text
active         boolean,        PRIMARY KEY (id)
 
);

INSERT INTO t_product VALUES (1, 3, 't');
INSERT INTO t_text  VALUES (3, 'de', 'T-Shirt');
INSERT INTO t_text  VALUES (3, 'en', 'T-Shirt');

INSERT INTO t_product VALUES (2, 4, 't');
INSERT INTO t_text  VALUES (4, 'de', 'Apfel');
INSERT INTO t_text  VALUES (4, 'en', 'Apple');

INSERT INTO t_product VALUES (3, 5, 't');
INSERT INTO t_text  VALUES (5, 'de', 'Banane');
INSERT INTO t_text  VALUES (5, 'en', 'Banana');


CREATE TABLE t_product_group (        product_id      int4    REFERENCES t_product(id)
     ON UPDATE CASCADE                                        ON DELETE CASCADE,        group_id        int4
REFERENCESt_group(id)                                        ON UPDATE CASCADE
ONDELETE CASCADE
 
);

INSERT INTO t_product_group VALUES (2, 1);
INSERT INTO t_product_group VALUES (3, 1);
INSERT INTO t_product_group VALUES (1, 2);

CREATE TABLE t_price (        id              int4,        typ             text,        price           numeric,
product_id     int4    REFERENCES t_product(id)                                        ON UPDATE CASCADE
                       ON DELETE CASCADE,        PRIMARY KEY (id)
 
);

INSERT INTO t_price VALUES (1, 'default', '10', 1);
INSERT INTO t_price VALUES (2, 'sonder', '20', 1);
INSERT INTO t_price VALUES (3, 'spezial', '30', 1);
INSERT INTO t_price VALUES (4, 'default', '7', 2);
INSERT INTO t_price VALUES (5, 'default', '12', 3);

Best regards,
    Hans

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