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Re: Reasons and drawbacks for unused item pointers

From: Martin Lesser <ml-pgsql(at)bettercom(dot)de>
To: pgsql-performance(at)postgresql(dot)org
Subject: Re: Reasons and drawbacks for unused item pointers
Date: 2005-10-27 22:09:46
Message-ID: 87u0f2zk5x.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:

> Martin Lesser <ml-pgsql(at)bettercom(dot)de> writes:
>> What causes this "unused item pointers" and which impact do they have
>> regarding performance?
> The direct performance impact is really pretty minimal (and none at
> all on indexscans, AFAIR).  The reason Denis' number drew my attention
> was that it implied that the table had gone un-vacuumed for awhile at
> some time in the past. [...]  To have 1905028 unused pointers in a
> table with only 5106307 live entries suggests that at some point there
> were 1.9 million (or so) dead but not-yet-vacuumed tuples, which
> suggests insufficient vacuuming.

Does each update of a single row result in an "unused item pointer"?
I.e. if I update one row 10 times between VACUUMing the table the result
are 10 unused pointers?

Some rows in some of my tables are updated much more frequently than
others so I'm not sure whether the number of unused pointers implie that
I should VACUUM more often than every 24 hours.

Martin

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