The following bug has been logged online:

Bug reference: 2739
Logged by: Mason Hale
Email address: masonhale@gmail.com
PostgreSQL version: 8.1.5
Operating system: GNU/Linux 2.6.9-42.0.3.ELsmp
Description: INTERSECT ALL not working
Details:

'INTERSECT ALL' does not return duplicate rows in a query.

The query below should return 10 rows, but it returns 5 rows on my system:

(
SELECT tablename
FROM pg_tables
LIMIT 5
)
INTERSECT ALL
(
(
SELECT tablename
FROM pg_tables
LIMIT 5
)
UNION ALL
(
SELECT tablename
FROM pg_tables
LIMIT 5
)
)

Note, the above is a simplied query meant to demonstrate the problem. This
same behavior occurs (and was discovered) in real-world situations with
user-defined tables.

This is nearly a deal-stopper for our application. Please reply to let me
know the status of this report.

Thanks,
Mason Hale

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"Mason Hale" <masonhale@gmail.com> writes:
> The query below should return 10 rows,

Not by my reading of the spec. SQL92 7.10 saith:

b) If a set operator is specified, then the result of applying
the set operator is a table containing the following rows:

i) Let R be a row that is a duplicate of some row in T1 or of
some row in T2 or both. Let m be the number of duplicates
of R in T1 and let n be the number of duplicates of R in
T2, where m >= 0 and n >= 0.

....

iii) If ALL is specified, then

....


3) If INTERSECT is specified, then the number of duplicates
of R that T contains is the minimum of m and n.

You have m = 1, n = 2 for each distinct row at the INTERSECT step,
ergo you get one copy out.

regards, tom lane

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Tom -

Many thanks for the quick reply. I feel honored to receive email from you
after seeing your name so many times in my web searches on Postgres topics.

That's not how I understood INTERSECT ALL to work. But it's the clear the
spec is right and my understanding is wrong.
This is not a bug.

Unfortunately the INTERSECT ALL as spec'd and implemented doesn't quite give
me what I need. So back to the drawing board for me...

best regards,
Mason

On 11/6/06, Tom Lane <tgl@sss.pgh.pa.us> wrote:
>
> "Mason Hale" <masonhale@gmail.com> writes:
> > The query below should return 10 rows,
>
> Not by my reading of the spec. SQL92 7.10 saith:
>
> b) If a set operator is specified, then the result of applying
> the set operator is a table containing the following rows:
>
> i) Let R be a row that is a duplicate of some row in T1 or
> of
> some row in T2 or both. Let m be the number of duplicates
> of R in T1 and let n be the number of duplicates of R in
> T2, where m >= 0 and n >= 0.
>
> ...
>
> iii) If ALL is specified, then
>
> ...
>
>
> 3) If INTERSECT is specified, then the number of
> duplicates
> of R that T contains is the minimum of m and n.
>
> You have m = 1, n = 2 for each distinct row at the INTERSECT step,
> ergo you get one copy out.
>
> regards, tom lane
>