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Re: Much Ado About COUNT(*)

From: "Jonah H(dot) Harris" <jharris(at)tvi(dot)edu>
To: Tom Lane <tgl(at)sss(dot)pgh(dot)pa(dot)us>
Cc: pgsql-hackers(at)postgresql(dot)org
Subject: Re: Much Ado About COUNT(*)
Date: 2005-01-12 19:52:53
Message-ID: 41E58015.7060903@tvi.edu (view raw or flat)
Thread:
Lists: pgsql-announcepgsql-hackerspgsql-patches
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 ...
>
>			regards, tom lane
>  
>
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.



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