Problems with select chaining using INTERSECT

Version: PostgreSQL 7.1.2 on sparc-sun-solaris2.5.1, compiled by GCC 2.95

Hi, when using SELECT queries chained by INTERSECT i get confused about the
behavour of the INTERSECT operation. I try to outline this with the following
examples.

Example (Field query_desc is of type TEXT):

select query_desc FROM blasthits_obj_3_q3_db3;
Returning: 4114 rows

select distinct query_desc FROM blasthits_obj_3_q3_db3 ;
Returning: 333 rows

Now intersecting this table with itself:
select query_desc FROM blasthits_obj_3_q3_db3
INTERSECT
select query_desc FROM blasthits_obj_3_q3_db3;
Returning: 333 rows   thats ok till now

Adding a second INTERSECT:
select query_desc FROM blasthits_obj_3_q3_db3
INTERSECT
select query_desc FROM blasthits_obj_3_q3_db3
INTERSECT
select query_desc FROM blasthits_obj_3_q3_db3;
Returning: 157 rows  hmm..

ok lets try it with brackets:

select query_desc FROM blasthits_obj_3_q3_db3
INTERSECT
(select query_desc FROM blasthits_obj_3_q3_db3
INTERSECT
select query_desc FROM blasthits_obj_3_q3_db3);
Returning: 333 rows    ok...and now with a third INTERSECT

select query_desc FROM blasthits_obj_3_q3_db3
INTERSECT
((select query_desc FROM blasthits_obj_3_q3_db3
INTERSECT
select query_desc FROM blasthits_obj_3_q3_db3)
INTERSECT
SELECT query_desc FROM blasthits_obj_3_q3_db3);
Returning: 157 rows.   WHY ?

I tried to reproduce this on a small table containing one column and 7 integer
entries (2 duplicates). But in this case it works as expected, even without
 brackets. So mybe there is a problem with the data type TEXT and INTERSECT ?

Any idea ?

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Dipl.-Inform. Michael Beckstette        Office: M3-129
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