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From: Ron Johnson <ronljohnsonjr@gmail.com>
Date: Wed, 25 Sep 2024 13:50:33 -0400
Message-ID: <CANzqJaBjwtzLg_bZHkokUJbtA8gy6RpHjirfdBH9OJGfn5=tmw@mail.gmail.com>
Subject: Re: Repeatable Read Isolation Level "transaction start time"
To: "pgsql-generallists.postgresql.org" <pgsql-general@lists.postgresql.org>
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On Wed, Sep 25, 2024 at 1:45=E2=80=AFPM Adrian Klaver <adrian.klaver@aklave=
r.com>
wrote:

>
>
> On 9/25/24 10:22 AM, Greg Sabino Mullane wrote:
> > On Tue, Sep 24, 2024 at 10:28=E2=80=AFAM Tom Lane <tgl@sss.pgh.pa.us
> > <mailto:tgl@sss.pgh.pa.us>> wrote:
> >
> >     It's even looser than that, really: it's the first statement that
> >     requires an MVCC snapshot.
> >
> >
> > Hm....so why does "SELECT 1;" work as a transaction start marker then,
> > as opposed to "SHOW work_mem;", which does not? Do we simply consider
> > anything with a SELECT as needing a snapshot?
>
>
> SELECT some_func();
>
> Where some_func() does something that requires a snapshot.
>
>
But why does "SELECT 1;" need a snapshot?  Heck, why does "SELECT
<immutable>;" need a snapshot?

--=20
Death to <Redacted>, and butter sauce.
Don't boil me, I'm still alive.
<Redacted> crustacean!

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<div dir=3D"ltr"><div dir=3D"ltr">On Wed, Sep 25, 2024 at 1:45=E2=80=AFPM A=
drian Klaver &lt;<a href=3D"mailto:adrian.klaver@aklaver.com">adrian.klaver=
@aklaver.com</a>&gt; wrote:<br></div><div class=3D"gmail_quote"><blockquote=
 class=3D"gmail_quote" style=3D"margin:0px 0px 0px 0.8ex;border-left:1px so=
lid rgb(204,204,204);padding-left:1ex"><br>
<br>
On 9/25/24 10:22 AM, Greg Sabino Mullane wrote:<br>
&gt; On Tue, Sep 24, 2024 at 10:28=E2=80=AFAM Tom Lane &lt;<a href=3D"mailt=
o:tgl@sss.pgh.pa.us" target=3D"_blank">tgl@sss.pgh.pa.us</a> <br>
&gt; &lt;mailto:<a href=3D"mailto:tgl@sss.pgh.pa.us" target=3D"_blank">tgl@=
sss.pgh.pa.us</a>&gt;&gt; wrote:<br>
&gt; <br>
&gt;=C2=A0 =C2=A0 =C2=A0It&#39;s even looser than that, really: it&#39;s th=
e first statement that<br>
&gt;=C2=A0 =C2=A0 =C2=A0requires an MVCC snapshot.<br>
&gt; <br>
&gt; <br>
&gt; Hm....so why does &quot;SELECT 1;&quot; work as a transaction start ma=
rker then, <br>
&gt; as opposed to &quot;SHOW work_mem;&quot;, which does not? Do we simply=
 consider <br>
&gt; anything with a SELECT as needing a snapshot?<br>
<br>
<br>
SELECT some_func();<br>
<br>
Where some_func() does something that requires a snapshot.<br><br></blockqu=
ote><div><br></div><div>But why does &quot;SELECT 1;&quot; need a snapshot?=
=C2=A0 Heck, why does &quot;SELECT &lt;immutable&gt;;&quot; need a snapshot=
?=C2=A0</div></div><div><br></div><span class=3D"gmail_signature_prefix">--=
 </span><br><div dir=3D"ltr" class=3D"gmail_signature"><div dir=3D"ltr">Dea=
th to &lt;Redacted&gt;, and butter sauce.<div>Don&#39;t boil me, I&#39;m st=
ill alive.<br><div><div>&lt;Redacted&gt; crustacean!</div></div></div></div=
></div></div>

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