From: | Martijn van Oosterhout <kleptog(at)svana(dot)org> |
---|---|
To: | ITAGAKI Takahiro <itagaki(dot)takahiro(at)oss(dot)ntt(dot)co(dot)jp> |
Cc: | Satoshi Nagayasu <nagayasus(at)nttdata(dot)co(dot)jp>, pgsql-hackers(at)postgresql(dot)org |
Subject: | Re: pgstattuple extension for indexes |
Date: | 2006-08-17 12:23:48 |
Message-ID: | 20060817122348.GB31061@svana.org |
Views: | Raw Message | Whole Thread | Download mbox | Resend email |
Thread: | |
Lists: | pgsql-hackers pgsql-patches |
On Thu, Aug 17, 2006 at 12:55:28PM +0900, ITAGAKI Takahiro wrote:
> I think this condition should be regarded as full-fragmented,
> but pgstatindex reports that the leaf_fragmentation is 50%.
>
> Presently, fragmentation factor is computed as the code below:
>
> if (opaque->btpo_next != P_NONE && opaque->btpo_next < blkno)
> stat->fragments++;
>
> But the method has the above problem. So I suggest to use whether
> the right link points to the next adjacent page or not.
>
> if (opaque->btpo_next != P_NONE && opaque->btpo_next != blkno + 1)
> stat->fragments++;
>
> Do you think which method is better? Or do you have other ideas?
If we do it your way, then every index will probably get a
fragmentation of nearly 100%. That's not very useful. I don't agree
that your example is fully fragmented, since on the first read the OS
will read the next four (or more) blocks so all the others are
zero-cost.
A more useful definition of fragmentation would be: if you're scanning
forward through an index, how often do you have to jump backwards
again. The current calculation seems to get that fairly right...
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.
From | Date | Subject | |
---|---|---|---|
Next Message | Andrew Dunstan | 2006-08-17 12:52:42 | Re: Enum proposal / design |
Previous Message | Greg Stark | 2006-08-17 11:47:15 | Re: CREATE INDEX ... ONLINE |
From | Date | Subject | |
---|---|---|---|
Next Message | Tom Lane | 2006-08-17 12:54:00 | Re: [PATCHES] WIP: bitmap indexes |
Previous Message | Greg Stark | 2006-08-17 11:47:15 | Re: CREATE INDEX ... ONLINE |