Intersect with null fields

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POSTGRESQL BUG REPORT TEMPLATE
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Your name : Marko Mikulicic
Your email address : mmikulicic(at)full-moon(dot)com

System Configuration
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Architecture (example: Intel Pentium) : Intel Pentium

Operating System (example: Linux 2.0.26 ELF) : Linux 2.2.13 ELF

PostgreSQL version (example: PostgreSQL-6.5.2): PostgreSQL-6.5.2

Compiler used (example: gcc 2.8.0) : pgcc-2.91.66

Please enter a FULL description of your problem:
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The SQL "intersect" construct returns an empty query in some cases
when some fields are null.

Please describe a way to repeat the problem. Please try to provide a
concise reproducible example, if at all possible:
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create table buggy ( a int, b int);
insert into buggy values ( 1);
insert into buggy values ( 2);
insert into buggy values ( 3);
insert into buggy values ( 4);

-- this one is ok
select a,b from buggy where a = 2 intersect select a,b from buggy
where a = 2;

-- this one gets an empty row
select a,b from buggy where a = 2 intersect select a,b from buggy
where a != 3;

-- this also
select a,b from buggy intersect select a,b from buggy where a = 3;

...