Re: [PERFORM] encouraging index-only scans

On Sun, Sep 8, 2013 at 8:49 PM, Amit Kapila <amit(dot)kapila16(at)gmail(dot)com> wrote:
> On Mon, Sep 9, 2013 at 2:35 AM, Jeff Janes <jeff(dot)janes(at)gmail(dot)com> wrote:
>> I thought it was well known, but maybe I was overly optimistic. I've
>> considered IOS to be mostly useful for data mining work on read-mostly
>> tables, which you would probably vacuum manually after a bulk load.
>>
>> For transactional tables, I think that trying to keep the vm set-bit
>> density high enough would be a losing battle. If we redefined the
>> nature of the vm so that doing a HOT update would not clear the
>> visibility bit, perhaps that would change the outcome of this battle.
>
> Wouldn't it make the Vacuum bit in-efficient in the sense that it will
> skip some of the pages in which there are only
> HOT updates for cleaning dead rows.

Maybe. But anyone is competent to clean up dead rows from HOT
updates, it is not exclusively vacuum that can do it, like it is for
non-HOT tuples. So I think any inefficiency would be very small.

Cheers,

Jeff