From: Tadipathri Raghu <traghu.dba@gmail.com>
To: Gary Doades <gpd@gpdnet.co.uk>
Cc: pgsql-performance@postgresql.org, Szymon Guz <mabewlun@gmail.com>
Subject: Re: Optimizer showing wrong rows in plan
Date: Sun, 28 Mar 2010 14:37:13 +0530
Message-ID: <645d9d71003280207p14fa4628i91b51ca8ee954216@mail.gmail.com> (raw)
In-Reply-To: <4BAF14F2.20200@gpdnet.co.uk>
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<e4edc9361003280002o627b710dhe89c98c18b5d1db3@mail.gmail.com>
<645d9d71003280011q6cf2c6d6p7279605e6e0b4235@mail.gmail.com>
<e4edc9361003280029v7174f84bo72d073aced772206@mail.gmail.com>
<645d9d71003280033l16069edcp45d9bbb0e7ed5517@mail.gmail.com>
<4BAF14F2.20200@gpdnet.co.uk>
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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To: pgsql-performance@postgresql.org
Cc: traghu.dba@gmail.com, gpd@gpdnet.co.uk, mabewlun@gmail.com
Subject: Re: Optimizer showing wrong rows in plan
In-Reply-To: <645d9d71003280207p14fa4628i91b51ca8ee954216@mail.gmail.com>
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