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Date: Mon, 15 Mar 2010 12:37:57 -0400
Message-ID: <600ad6df1003150937t68b6c1d4k976fb5504e448f3c@mail.gmail.com>
Subject: Re: Deleting bytea, autovacuum, and 8.2/8.4 differences
From: VJK <vjkmail@gmail.com>
To: Tom Lane <tgl@sss.pgh.pa.us>, pgsql-performance@postgresql.org
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On Mon, Mar 15, 2010 at 10:53 AM, Tom Lane <tgl@sss.pgh.pa.us> wrote:

> Greg Smith <greg@2ndquadrant.com> writes:
> > VJK wrote:
> >> Since Pg does not use the concept of rollback segments,  it is unclear
> >> why deletion produces so much disk IO (4GB).
>
> For an example like this one, you have to keep in mind that the
> toast-table rows for the large bytea value have to be marked deleted,
> too.  Also, since I/O happens in units of pages, the I/O volume to
> delete a tuple is just as much as the I/O to create it.
>

That makes sense.


>                        regards, tom lane
>

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<br><br><div class=3D"gmail_quote">On Mon, Mar 15, 2010 at 10:53 AM, Tom La=
ne <span dir=3D"ltr">&lt;<a href=3D"mailto:tgl@sss.pgh.pa.us">tgl@sss.pgh.p=
a.us</a>&gt;</span> wrote:<br><blockquote class=3D"gmail_quote" style=3D"bo=
rder-left: 1px solid rgb(204, 204, 204); margin: 0pt 0pt 0pt 0.8ex; padding=
-left: 1ex;">
<div class=3D"im">Greg Smith &lt;<a href=3D"mailto:greg@2ndquadrant.com">gr=
eg@2ndquadrant.com</a>&gt; writes:<br>
&gt; VJK wrote:<br>
&gt;&gt; Since Pg does not use the concept of rollback segments, =A0it is u=
nclear<br>
&gt;&gt; why deletion produces so much disk IO (4GB).<br>
<br>
</div>For an example like this one, you have to keep in mind that the<br>
toast-table rows for the large bytea value have to be marked deleted,<br>
too. =A0Also, since I/O happens in units of pages, the I/O volume to<br>
delete a tuple is just as much as the I/O to create it. <br></blockquote><d=
iv><br>That makes sense.<br>=A0<br></div><blockquote class=3D"gmail_quote" =
style=3D"border-left: 1px solid rgb(204, 204, 204); margin: 0pt 0pt 0pt 0.8=
ex; padding-left: 1ex;">

 =A0 =A0 =A0 =A0 =A0 =A0 =A0 =A0 =A0 =A0 =A0 =A0regards, tom lane<br>
</blockquote></div><br>

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