On Wed, Feb 24, 2010 at 11:05 AM, Gokulakannan Somasundaram
<gokul007@gmail.com> wrote:
>> I think you're a barking up the wrong tree. AFAIUI, the need for the
>> visibility map has not very much to do with whether the table has
>> indices, and everything to do with avoiding unnecessary VACUUMs. In
>> any event, you've not shown that the visibility map HAS any overhead,
>> so talking about skipping it seems entirely premature. Keep in mind
>> that the visibility map is quite small.
>
> OK! i am not saying to remove the visibility map, if i am misunderstood. All
> i am saying here is to remove the index only scan processing of visibility
> map. If it is being used only for vacuums, you need not make it crash safe
> and no WAL comes into picture.
So basically you want to have index-only scans, but you want them to
be really slow?
>> The point of the visibility map as far as index-only scans are
>> concerned is that if all the needed column values can be extracted
>> from the index, we still need to read the heap page to check tuple
>> visibility - unless, of course, we already know from the visibility
>> map that all the tuples on that heap page are guaranteed to be visible
>> to all transactions. On a read-only or read-mostly table, this will
>> reduce the cost of checking tuple visibility by several orders of
>> magnitude.
>>
> I understand that. As i suggested above, if you have no indexes for a table,
> why do you need to spend the extra effort in making it crash safe for that
> table? Hope i am clear.
Tables without indices don't need to be crash safe? News to me.
...Robert