Bug #470: INTERSECT fails

Tamas Vincze (vincze(at)neb(dot)com) reports a bug with a severity of 1
The lower the number the more severe it is.

Short Description
INTERSECT fails

Long Description
I've used INTERSECT to get the common rows in 3 simple SELECTs.
If I write the query without grouping parentheses, PG reports 0 rows
which is incorrect. If I use parentheses to group the last two
SELECTs then it gives a better answer. If I reorder the SELECTs
then it gives another different answer. (see examples)
The result should be independent of the order and grouping of
the SELECTs, so I think it's a serious bug.
I use version 7.1.2 on solaris/sparc.

Sample Code
sites=# select recseq from frags where len>2222 and len<2406
sites-# intersect select recseq from frags where len>1503 and len<1627
sites-# intersect select recseq from frags where len>752 and len<814;
recseq
--------
(0 rows)

sites=# select recseq from frags where len>2222 and len<2406
sites-# intersect (select recseq from frags where len>1503 and len<1627
sites(# intersect select recseq from frags where len>752 and len<814);
recseq
--------
CACTG
CAGTG
CASTG
RGCGCY
YAACCT
(5 rows)

sites=# select recseq from frags where len>1503 and len<1627
sites-# intersect select recseq from frags where len>752 and len<814
sites-# intersect select recseq from frags where len>2222 and len<2406;
recseq
--------
CACTG
CAGTG
RGCGCY
YAACCT
(4 rows)

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