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Re: Much Ado About COUNT(*)

From: "Andrew Dunstan" <andrew(at)dunslane(dot)net>
To: <jharris(at)tvi(dot)edu>
Cc: <tgl(at)sss(dot)pgh(dot)pa(dot)us>, <pgsql-hackers(at)postgresql(dot)org>
Subject: Re: Much Ado About COUNT(*)
Date: 2005-01-12 20:14:51
Message-ID: 1219.68.221.103.55.1105560891.squirrel@www.dunslane.net (view raw or flat)
Thread:
Lists: pgsql-announcepgsql-hackerspgsql-patches
Jonah H. Harris said:
> Tom Lane wrote:
>
>>The fundamental problem is that you can't do it without adding at least
>>16 bytes, probably 20, to the size of an index tuple header.  That
>>would double the physical size of an index on a simple column (eg an
>>integer or timestamp).  The extra I/O costs and extra maintenance costs
>>are unattractive to say the least.  And it takes away some of the
>>justification for the whole thing, which is that reading an index is
>>much cheaper than reading the main table.  That's only true if the
>>index is much smaller than the main table ...
>>
> I recognize the added cost of implementing index only scans.  As
> storage  is relatively cheap these days, everyone I know is more
> concerned about  faster access to data.  Similarly, it would still be
> faster to scan the  indexes than to perform a sequential scan over the
> entire relation for  this case.  I also acknowledge that it would be a
> negative impact to  indexes where this type of acces isn't required, as
> you suggested and  which is more than likely not the case.  I just
> wonder what more people  would be happier with and whether the added
> 16-20 bytes would be
> extremely noticable considering most 1-3 year old hardware.
>
>


Monetary cost is not the issue - cost in time is the issue.

cheers

andrew



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