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From Brian Candler <B.Cand...@pobox.com>
Subject Re: Improving pagination for views
Date Wed, 25 Nov 2009 09:25:55 GMT
On Tue, Nov 24, 2009 at 08:33:43PM +0100, Dirkjan Ochtman wrote:
> On Couch, as far as I can see, pre-issuing a count query means
> actually retrieving all the row data, which doesn't seem to scale
> nicely.

Not true: you have a count reduce function. Because each node of the Btree
contains the already-calculated reduce value, this is very efficient. At
best, it just reads the value out of the root node. At worst, for a key
range, it will combine (re-reduce) count values for the subtrees within the
key range, and count a few loose values at each end of the range.

Where it's inefficient is when retrieving: if I understand it correctly,
using offset=50000 *will* require walking through 50,000 rows in the index.

That's unless each Btree node has a calculated 'number of rows' value which
would allow chunks of tree to be skipped over quickly. I haven't heard
anyone say that it does, but I could be wrong.

For now, the best advice seems to be use 'next' and 'prev' links, using
descending=true to get the values for the previous page.

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