Thread: BUG #17671: Query returns unexpected result

The following bug has been logged on the website:

Bug reference:      17671
Logged by:          Nikola Mirkov
Email address:      niko@aku.mx
PostgreSQL version: 12.12
Operating system:   Ubuntu 12.12-0ubuntu0.20.
Description:

Hey guys,
When I run the following query:
SELECT foo, bar
FROM table1
WHERE fk_id = 637
AND utc > '2022-10-16'
UNION
SELECT foo, bar
FROM table2
WHERE fk_id = 637
AND utc > '2022-10-16'

I'm getting only one row per table. If I add the column "utc" to the list in
SELECT, I get the expected result.

Why is this happening?

Thank you so much!
Niko

On Mon, 31 Oct 2022 at 13:15, PG Bug reporting form <noreply@postgresql.org> wrote:
> The following bug has been logged on the website:
>
> Bug reference:      17671
> Logged by:          Nikola Mirkov
> Email address:      niko@aku.mx
> PostgreSQL version: 12.12
> Operating system:   Ubuntu 12.12-0ubuntu0.20.
> Description:        
>
> Hey guys,
> When I run the following query:
> SELECT foo, bar
> FROM table1
> WHERE fk_id = 637
> AND utc > '2022-10-16'
> UNION
> SELECT foo, bar
> FROM table2
> WHERE fk_id = 637
> AND utc > '2022-10-16'
>
> I'm getting only one row per table. If I add the column "utc" to the list in
> SELECT, I get the expected result.
>
> Why is this happening?
>

The UNION eliminates duplicate rows from its result, in the same way as
DISTINCT [1].

[1] https://www.postgresql.org/docs/current/queries-union.html


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