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Date: Sun, 28 Mar 2010 14:37:13 +0530
Message-ID: <645d9d71003280207p14fa4628i91b51ca8ee954216@mail.gmail.com>
Subject: Re: Optimizer showing wrong rows in plan
From: Tadipathri Raghu <traghu.dba@gmail.com>
To: Gary Doades <gpd@gpdnet.co.uk>
Cc: pgsql-performance@postgresql.org, Szymon Guz <mabewlun@gmail.com>
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Hi All,

I want to give some more light on this by analysing more like this

1. In my example I have created a table with one column as INT( which
occupies 4 bytes)
2. Initially it occupies one page of  space on the file that is (8kb).

So, here is it assuming these many rows may fit in this page. Clarify me on
this Please.

Regards
Raghavendra


On Sun, Mar 28, 2010 at 2:06 PM, Gary Doades <gpd@gpdnet.co.uk> wrote:

>  On 28/03/2010 8:33 AM, Tadipathri Raghu wrote:
>
> Hi Guz,
>
>
>> It is assuming that there are 2400 rows in this table. Probably you've
>> deleted some rows from the table leaving just one.
>
>
> Frankly speaking its a newly created table without any operation on it as
> you have seen the example. Then how come it showing those many rows where we
> have only one in it.
> Thanks if we have proper explination on this..
>
> It's not *showing* any rows at all, it's *guessing* 2400 rows because
> you've never analyzed the table. Without any statistics at all, postgres
> will use some form of in-built guess for a table that produces reasonable
> plans under average conditions. As you've already seen, once you analyze the
> table, the guess get's much  better and therefore would give you a more
> appropriate plan.
>
> Regards,
> Gary.
>
>

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<div>Hi All,</div>
<div>=A0</div>
<div>I want to give some more light on this by analysing more like this</di=
v>
<div>=A0</div>
<div>1. In my example=A0I=A0have created a table with one column as INT( wh=
ich occupies 4 bytes)</div>
<div>2. Initially it occupies one page of=A0 space on the file that is (8kb=
).</div>
<div>=A0</div>
<div>So, here is it assuming these many rows may fit in this page. Clarify =
me on this Please.</div>
<div>=A0</div>
<div>Regards</div>
<div>Raghavendra</div>
<div><br>=A0</div>
<div class=3D"gmail_quote">On Sun, Mar 28, 2010 at 2:06 PM, Gary Doades <sp=
an dir=3D"ltr">&lt;<a href=3D"mailto:gpd@gpdnet.co.uk">gpd@gpdnet.co.uk</a>=
&gt;</span> wrote:<br>
<blockquote style=3D"BORDER-LEFT: #ccc 1px solid; MARGIN: 0px 0px 0px 0.8ex=
; PADDING-LEFT: 1ex" class=3D"gmail_quote">
<div text=3D"#000000" bgcolor=3D"#ffffff">
<div class=3D"im">On 28/03/2010 8:33 AM, Tadipathri Raghu wrote:=20
<blockquote type=3D"cite">
<div>Hi Guz,</div>
<div>=A0</div>
<blockquote style=3D"BORDER-LEFT: rgb(204,204,204) 1px solid; MARGIN: 0px 0=
px 0px 0.8ex; PADDING-LEFT: 1ex" class=3D"gmail_quote">It is assuming that =
there are 2400 rows in this table. Probably you&#39;ve deleted some rows fr=
om the table leaving just one.</blockquote>

<div>=A0</div>
<div>Frankly speaking its a newly created table without any operation on it=
 as you have seen the example. Then how come it showing those many rows whe=
re we have only one in it.<br></div>
<div>Thanks if we have proper explination on this..</div></blockquote></div=
>It&#39;s not *showing* any rows at all, it&#39;s *guessing* 2400 rows beca=
use you&#39;ve never analyzed the table. Without any statistics at all, pos=
tgres will use some form of in-built guess for a table that produces reason=
able plans under average conditions. As you&#39;ve already seen, once you a=
nalyze the table, the guess get&#39;s much=A0 better and therefore would gi=
ve you a more appropriate plan.<br>
<br>Regards,<br>Gary.<br><br></div></blockquote></div><br>

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