Small bug in GROUP BY

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POSTGRESQL BUG REPORT TEMPLATE
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Your name : Andriy I Pilipenko
Your email address : bamby(at)marka(dot)net(dot)ua

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

Operating System (example: Linux 2.0.26 ELF) : FreeBSD-3.x

PostgreSQL version (example: PostgreSQL-7.0): PostgreSQL-7.0.2
PostgreSQL-7.0beta5

Compiler used (example: gcc 2.8.0) : gcc 2.7.2.3

Please enter a FULL description of your problem:
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This problem is new to version 7. In PostgreSQL 6.x this problem not
present.

There is a bug where backend dumps core while executing such a request:

SELECT sum(a), b
FROM t
GROUP BY b
UNION SELECT sum(a), 1 as b
FROM t
GROUP BY b

I know, that last 'GROUP BY' clause is not necessary to use, but it's not
necessary for backend to dump core in such a harmless situation ;-) This
query works pretty well without last 'GROUP BY' clause.

Please describe a way to repeat the problem. Please try to provide a
concise reproducible example, if at all possible:
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First create the table:

CREATE TABLE t (a int, b int)

Next insert some data into the table. It's necessary part - without data
you cannot catch the bug.

INSERT INTO t VALUES (0, 0)

And finally:

SELECT sum(a), b
FROM t
GROUP BY b
UNION SELECT sum(a), 1 as b
FROM t
GROUP BY b

If you know how this problem might be fixed, list the solution below:
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Sorry, I don't :-(