Re: Performance penalty of visibility info in indexes?

On Thu, Feb 01, 2007 at 11:57:41PM -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 thought the main problem was locking. If you change the visibility of
an existing row, you have to update the index in a way that won't kill
concurrant scans, either by returning the row twice, or skipping it.

I think it hinges on what exactly falls under "visibility info". Maybe
with the page-at-a-time index scans, the problem is easier now.

Have a nice day,
--
Martijn van Oosterhout <kleptog(at)svana(dot)org> http://svana.org/kleptog/
> From each according to his ability. To each according to his ability to litigate.