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Re: Reasons and drawbacks for unused item pointers (was: Update using primary key slow)

From: Martin Lesser <ml-pgsql(at)bettercom(dot)de>
To: pgsql-performance(at)postgresql(dot)org
Cc: Denis <denis(dot)sailer(at)yellowbook(dot)com>,pgsql-performance(at)postgresql(dot)org
Subject: Re: Reasons and drawbacks for unused item pointers (was: Update using primary key slow)
Date: 2005-10-27 21:01:55
Message-ID: 87y84eznb0.fsf_-_@nb-aspire.bettercom.de (view raw or flat)
Thread:
Lists: pgsql-performance
Tom Lane <tgl(at)sss(dot)pgh(dot)pa(dot)us> writes:

> Denis <denis(dot)sailer(at)yellowbook(dot)com> writes:
>> There were 1905028 unused item pointers.
> The "unused item pointers" number seems a bit high, but otherwise that
> looks pretty reasonable.
>
> Is it possible that the particular row you were updating has been
> updated quite a lot of times since the last vacuum?  Or even quite
> a few times within a single transaction?

What causes this "unused item pointers" and which impact do they have
regarding performance?

If I understood your last posting correctly more than one update on a
single row between two vacuum's would i.e. result in one ore more
"unused item pointer". Does this slow down the vacuum process and/or
other processes?  Until now I could not find an answer what this number
implies.

Regards

Martin

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Subject: Re: Reasons and drawbacks for unused item pointers (was: Update using primary key slow)
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