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Re: Performance penalty of visibility info in indexes?

From: "Simon Riggs" <simon(at)2ndquadrant(dot)com>
To: "Jim Nasby" <decibel(at)decibel(dot)org>
Cc: "PostgreSQL Development" <pgsql-hackers(at)postgresql(dot)org>
Subject: Re: Performance penalty of visibility info in indexes?
Date: 2007-02-02 19:41:26
Message-ID: (view raw, whole thread or download thread mbox)
Lists: pgsql-hackers
On Thu, 2007-02-01 at 23:57 -0600, Jim Nasby wrote:
> Has anyone actually measured the performance overhead of storing  
> visibility info in indexes? I know the space overhead sounds  
> daunting, but even if it doubled the size of the index in many cases  
> that'd still be a huge win over having to scan the heap as well as  
> the index (esp. for things like count(*)). There would also be  
> overhead from having to update the old index tuple, but for the case  
> of updates you're likely to need that page for the new index tuple  
> anyway.
> I know this wouldn't work for all cases, but ISTM there are many  
> cases where it would be a win.

It would prevent any optimization that sought to avoid inserting rows
into the index each time we perform an UPDATE. Improving UPDATE
performance seems more important than improving count(*), IMHO.

  Simon Riggs             

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