From: "Ilsa Loving" <ilsa@ilsadee.com>
To: "Alvaro Herrera" <alvherre@alvh.no-ip.org>
Cc: "Michael Paquier" <michael.paquier@gmail.com>, "Tom Lane"
 <tgl@sss.pgh.pa.us>, pgsql-docs@lists.postgresql.org
Subject: Re: What does "Table rewrite" mean?
Date: Mon, 22 Jan 2018 15:59:07 -0500
Message-ID: <63910968-9247-4112-842A-F7063CD4A4C7@ilsadee.com>
In-Reply-To: <20180121034750.cgfugcqbgnxxm3lg@alvherre.pgsql>
References: <20180121034750.cgfugcqbgnxxm3lg@alvherre.pgsql>
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That=E2=80=99s all that=E2=80=99s needed, really.  It=E2=80=99s impossibl=
e to make an =

informed decision if there is no way for someone to know what a table =

rewrite actually does and how it does it.


/*-----------------------------------------------------------------------=
----------*/
float o=3D0.075,h=3D1.5,T,r,O,l,I;/* Ilsa Loving =C2=A0 =C2=A0 =C2=A0 =C2=
=A0*/int =

_,L=3D80,s=3D3200; int main()
{for(;s%L||(h-=3Do,T=3D-2),s;4-(r=3D/* IT Manager =C2=A0 =C2=A0 =C2=A0 =C2=
=A0 =

*/O*O)<(l=3DI*I)|++ _=3D=3DL&&
write(1,(--s%L?_<L?--_%6:6:7) /* The Jonah Group =C2=A0 =C2=A0*/+"Ilsa =

L.\n",1)&&(O=3DI=3Dl=3D_=3Dr=3D0,
T+=3Do /2))O=3DI*2*O+h,I=3Dl+T-r;} =C2=A0/* 416-304-0860x227 =C2=A0 */


On 20 Jan 2018, at 22:47, Alvaro Herrera wrote:

> Michael Paquier wrote:
>> On Fri, Jan 19, 2018 at 11:33:43AM -0500, Tom Lane wrote:
>>> It means reading the whole table and writing it out in some modified
>>> form (for instance, with some column transformed into a new =

>>> datatype).
>>> It's not "dangerous" in any way ... but if you've got many GB of =

>>> data in
>>> the table and you can't afford to have the table locked for a long =

>>> time,
>>> then it's something to avoid.
>>
>> Yeah that can be costly. Note that WAL corresponding to this data =

>> needs
>> to be generated as well.
>
> Maybe we need to document this somewhere, particularly now that we =

> have
> a "table_rewrite" event item.
>
> -- =

> =C3=81lvaro Herrera                https://www.2ndQuadrant.com/
> PostgreSQL Development, 24x7 Support, Remote DBA, Training & Services

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<div style=3D"font-family:sans-serif"><div style=3D"white-space:normal"><=
p dir=3D"auto">That=E2=80=99s all that=E2=80=99s needed, really.  It=E2=80=
=99s impossible to make an informed decision if there is no way for someo=
ne to know what a table rewrite actually does and how it does it.</p>
<br></div>
<pre>
/*-----------------------------------------------------------------------=
----------*/
float o=3D0.075,h=3D1.5,T,r,O,l,I;/* Ilsa Loving &#xA0; &#xA0; &#xA0; &#x=
A0;*/int _,L=3D80,s=3D3200; int main()
{for(;s%L||(h-=3Do,T=3D-2),s;4-(r=3D/* IT Manager &#xA0; &#xA0; &#xA0; &#=
xA0; */O*O)&#x3C;(l=3DI*I)|++ _=3D=3DL&#x26;&#x26;
write(1,(--s%L?_&#x3C;L?--_%6:6:7) /* The Jonah Group &#xA0; &#xA0;*/+&#x=
22;Ilsa L.\n&#x22;,1)&#x26;&#x26;(O=3DI=3Dl=3D_=3Dr=3D0,
T+=3Do /2))O=3DI*2*O+h,I=3Dl+T-r;} &#xA0;/* 416-304-0860x227 &#xA0; */
</pre>
<div class=3D"plaintext">

<p dir=3D"auto">On 20 Jan 2018, at 22:47, Alvaro Herrera wrote:</p>
<blockquote style=3D"border-left:2px solid #777; color:#777; margin:0 0 5=
px; padding-left:5px"><p dir=3D"auto">Michael Paquier wrote:</p>
<blockquote style=3D"border-left:2px solid #777; color:#999; margin:0 0 5=
px; padding-left:5px; border-left-color:#999"><p dir=3D"auto">On Fri, Jan=
 19, 2018 at 11:33:43AM -0500, Tom Lane wrote:</p>
<blockquote style=3D"border-left:2px solid #777; color:#BBB; margin:0 0 5=
px; padding-left:5px; border-left-color:#BBB"><p dir=3D"auto">It means re=
ading the whole table and writing it out in some modified<br>
form (for instance, with some column transformed into a new datatype).<br=
>
It's not "dangerous" in any way ... but if you've got many GB of data in<=
br>
the table and you can't afford to have the table locked for a long time,<=
br>
then it's something to avoid.</p>
</blockquote><p dir=3D"auto">Yeah that can be costly. Note that WAL corre=
sponding to this data needs<br>
to be generated as well.</p>
</blockquote><p dir=3D"auto">Maybe we need to document this somewhere, pa=
rticularly now that we have<br>
a "table_rewrite" event item.<br>
<br>
-- <br>
=C3=81lvaro Herrera                <a href=3D"https://www.2ndQuadrant.com=
/" style=3D"color:#777">https://www.2ndQuadrant.com/</a><br>
PostgreSQL Development, 24x7 Support, Remote DBA, Training &amp; Services=
</p>
</blockquote></div>
</div>
</body>
</html>

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