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From: Andrii Novikov <adnyre@gmail.com>
Date: Thu, 9 Jan 2025 18:49:38 +0200
Message-ID: <CAOS4yi3eT2M-Wx576xRHEZnvHp9C_PrDP1NdXxmyZGB_sJwnMA@mail.gmail.com>
Subject: Re: Unexpected results from a query with UNION ALL
To: Tom Lane <tgl@sss.pgh.pa.us>
Cc: pgsql-general@postgresql.org
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Thanks for the update. Actually I was able to rewrite the query to get rid
of the union. But anyway it would be good to have this issue handled
somehow, it's tricky and it was hard to spot and reproduce.

=D1=81=D1=80, 8 =D1=81=D1=96=D1=87. 2025=E2=80=AF=D1=80. =D0=BE 19:01 Tom L=
ane <tgl@sss.pgh.pa.us> =D0=BF=D0=B8=D1=88=D0=B5:

> I wrote:
> > Andrey <adnyre@gmail.com> writes:
> >> ... but I would expect to get the same result as previously. Is it a b=
ug
> >> or am I doing something wrong here?
>
> > It's a surprising result for sure, but I believe it's explained by
> > the algorithm for READ COMMITTED [1], specifically the bit about
>
> Actually, on further thought I believe this really is a bug,
> because if you change the UNION ALL to UNION it works fine.
> It probably used to work with UNION ALL too, but that was a few
> decades ago before we started adding optimizations of UNION ALL :-(
>
> I've been poking at this off and on for the last few days, and I've
> found three different things that will need to be changed to make
> it work again.  At least one of them looks too invasive to consider
> for back-patch.  So don't hold your breath for a proper fix, but
> perhaps you could use UNION as a workaround?
>
>                         regards, tom lane
>

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<div dir=3D"ltr">Thanks for the update. Actually I was able to rewrite the =
query to get rid of the union. But anyway it would be good to have this iss=
ue handled somehow, it&#39;s tricky and it was hard to spot and reproduce.<=
/div><br><div class=3D"gmail_quote gmail_quote_container"><div dir=3D"ltr" =
class=3D"gmail_attr">=D1=81=D1=80, 8 =D1=81=D1=96=D1=87. 2025=E2=80=AF=D1=
=80. =D0=BE 19:01 Tom Lane &lt;<a href=3D"mailto:tgl@sss.pgh.pa.us">tgl@sss=
.pgh.pa.us</a>&gt; =D0=BF=D0=B8=D1=88=D0=B5:<br></div><blockquote class=3D"=
gmail_quote" style=3D"margin:0px 0px 0px 0.8ex;border-left:1px solid rgb(20=
4,204,204);padding-left:1ex">I wrote:<br>
&gt; Andrey &lt;<a href=3D"mailto:adnyre@gmail.com" target=3D"_blank">adnyr=
e@gmail.com</a>&gt; writes:<br>
&gt;&gt; ... but I would expect to get the same result as previously. Is it=
 a bug<br>
&gt;&gt; or am I doing something wrong here?<br>
<br>
&gt; It&#39;s a surprising result for sure, but I believe it&#39;s explaine=
d by<br>
&gt; the algorithm for READ COMMITTED [1], specifically the bit about<br>
<br>
Actually, on further thought I believe this really is a bug,<br>
because if you change the UNION ALL to UNION it works fine.<br>
It probably used to work with UNION ALL too, but that was a few<br>
decades ago before we started adding optimizations of UNION ALL :-(<br>
<br>
I&#39;ve been poking at this off and on for the last few days, and I&#39;ve=
<br>
found three different things that will need to be changed to make<br>
it work again.=C2=A0 At least one of them looks too invasive to consider<br=
>
for back-patch.=C2=A0 So don&#39;t hold your breath for a proper fix, but<b=
r>
perhaps you could use UNION as a workaround?<br>
<br>
=C2=A0 =C2=A0 =C2=A0 =C2=A0 =C2=A0 =C2=A0 =C2=A0 =C2=A0 =C2=A0 =C2=A0 =C2=
=A0 =C2=A0 regards, tom lane<br>
</blockquote></div>

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