Re: the big picture for index-only scans

On Sat, Aug 20, 2011 at 2:25 AM, Heikki Linnakangas <
heikki(dot)linnakangas(at)enterprisedb(dot)com> wrote:

> On 19.08.2011 21:06, Gokulakannan Somasundaram wrote:
>
>> If you are following the same design that Heikki put forward, then there
>> is
>> a problem with it in maintaining the bits in page and the bits in
>> visibility
>> map in sync, which we have already discussed.
>>
>
> Are you referring to this: http://archives.postgresql.**
> org/pgsql-hackers/2010-02/**msg02097.php<http://archives.postgresql.org/pgsql-hackers/2010-02/msg02097.php>? I believe Robert's changes to make the visibility map crash-safe covers
> that. Clearing the bit in the visibility map now happens within the same
> critical section as clearing the flag on the heap page and writing th WAL
> record.
>
> In that case, say a 100 sessions are trying to update records which fall
under the 8000*4 heap pages( i assume 2 bits per visibility map - 8 * 1024 *
4 exact) covered by one page of visibility map, won't it make the 99
sessions wait for that visibility map while holding the exclusive lock on
the 99 heap pages?
Are we going to say, that these kind of situations occur very rarely? Or
that the loss of scalability in these situations, is worth the performance
during the read-heavy workloads?

In any case, making a database going through all these extra overheads,
while they don't even have any index-only scans!!! That definitely should
be given a thought.

Gokul.