BUG #2487: Immutable functions results

The following bug has been logged online:

Bug reference: 2487
Logged by: Pedro J. Romero
Email address: multiacademia(at)eresmas(dot)com
PostgreSQL version: 8.0.1
Operating system: Linux
Description: Immutable functions results
Details:

Sorry if this not a bug, but I think so:

Look at this, I think this must be self explanatory, after two or three
readings:

select * from tablea;
fielda
------
16
(1 row)

select * from childtablea;
fielda|data_a
------|----------
16| 101
16| 127
16| 4315
(3 rows)

select fielda, otherfunction('childtablea', 'fielda', fielda) from tablea;

--otherfunction is not marked as immutable, it cannot
--So I cannot use it for index

fielda|otherfunction
------|-----------------
16|101-127-4315
(1 row)

select fielda, immutablefunction_a(fielda) from tablea;

fielda|immutablefunction_a
------|-------------------
16|101-127-4315
(1 row)

select otherfunction('childtablea', 'fielda', fielda),
immutablefunction_a(fielda), otherfunction('childtablea', 'fielda',
fielda)=immutablefunction_a(fielda) as are_the_same from tablea;

otherfunction|immutablefunction_a|are_the_same
-------------|-------------------|------------
101-127-4315 |101-127-4315 |t

--===========================================
--===========================================

--Same schema, same data, same database, other tables

select * from tableb;
fieldb
------
16
(1 row)

select * from childtableb;
fieldb|data_b
------|----------
16| 101
16| 127
16| 4315
(3 rows)

select fieldb,otherfunction('childtableb', 'fieldb', fieldb) from tableb;

fieldb|otherfunction
------|-----------------
16|101-127-4315
(1 row)

select fieldb, immutablefunction_b(fieldb) from tableb;

fieldb|immutablefunction
------|-----------------
16|101-127-4315
(1 row)

select otherfunction('childtableb', 'fieldb', fieldb),
immutablefunction_b(fieldb), otherfunction('childtableb', 'detailb',
fieldb)=immutablefunction_b(fieldb) as are_the_same from tableb;

otherfunction|immutablefunction_b|are_the_same
-------------|-------------------|------------
101-127-4315 |101-127-4315 |t

--===========================================
--===========================================

Ok, until this....

But....

vacuum full tablea;

vacuum full tableb;

select fielda, fieldb, otherfunction('childtablea', 'fielda', fielda) from
tablea left join tableb on otherfunction('childtablea', 'fielda',
fielda)=otherfunction('childtableb', 'fieldb', fieldb);

fielda|fieldb|otherfunction
------|------|-------------
16| 16|101-127-4315
(0 rows)

select fielda, fieldb from tablea left join tableb on
immutablefunction_a(fielda)=immutablefunction_b(fieldb);

fielda|fieldb
------|------
(0 rows)

vacuum full tablea;

vacuum full tableb;

select fielda, fieldb from tablea left join tableb on
immutablefunction_a(fielda)=immutablefunction_b(fieldb);

fielda|fieldb
------|------
(0 rows)

Why? If I cannot use the immutable function, performance is really bad.
Using an index, speed is several times greater.

Is this a bug? Or I'm missing something about the immutable functions
characteristics?

Thank you very much.