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@svana.org> http://svana.org/kleptog/ > From each according to his ability. To each according to his ability to > litigate.
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