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Re: UNION removes almost all rows (not duplicates) - in fresh build of pg17! - Mailing list pgsql-bugs

From Tom Lane
Subject Re: UNION removes almost all rows (not duplicates) - in fresh build of pg17!
Date
Msg-id 3538123.1716225084@sss.pgh.pa.us
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In response to UNION removes almost all rows (not duplicates) - in fresh build of pg17!  (hubert depesz lubaczewski <depesz@depesz.com>)
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hubert depesz lubaczewski <depesz@depesz.com> writes:
> $ select count(*) from ( select * from pg_class where relkind = 'r' union select * from pg_class where relkind = 'i'
);
>  count
> ───────
>      1
> (1 row)

Ugh, that is awful.  Bisecting shows it broke at

commit 66c0185a3d14bbbf51d0fc9d267093ffec735231
Author: David Rowley <drowley@postgresql.org>
Date:   Mon Mar 25 14:31:14 2024 +1300

    Allow planner to use Merge Append to efficiently implement UNION

I've not looked at the patch, but somehow it's totally forgetting
what it's supposed to be unique-ifying on.

            regards, tom lane

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