On Wed, Jun 3, 2009 at 12:03 PM, Justin Balthrop <[email protected]> wrote: > Nice! That sounds like exactly what I'm looking for. I don't think it will > address the performance issues with reduce, but it's definitely a start. > > Do you mind sending a diff of your changes to couch_view_updater.erl? I > diffed your file with trunk and there are a bunch of unrelated changes, of > course.
There's also a Paul Davis's Cascade: http://github.com/davisp/cascade/tree/master I'm planning on writing something with Hovercraft that takes a group reduce query and copies it to another database on demand. It wouldn't try to be incremental, just provide for easy chaining. I think chaining by copying to a db is a good way to work, because it lets you experiment with other views on top of your reduce rows, without regenerating the whole thing. Chris > > Thanks > > > On Jun 3, 2009, at 1:42 AM, Viacheslav Seledkin wrote: > >> Justin Balthrop wrote: >>> >>> Hi everyone, >>> >>> I've been reading the dev and user mailing lists for the past month or >>> so, but haven't posted yet. I've fallen in love with couchdb, its >>> power and simplicity, and I tell everyone who will listen why it is so >>> much better than a relational db for most applications. I now have >>> most of the engineering team at our company on board, and I'm in the >>> process of converting our rails site from postgres to couchdb. >>> >>> So, after spending a few weeks converting models over to using >>> couchdb, there is one feature that we are desperately missing: >>> >>> Multi-level map-reduce in views. >>> >>> We need a way to take the output of reduce and pass it back through >>> another map-reduce step (multiple times in some cases). This way, we >>> could build map-reduce flows that compute (and cache) any complex data >>> computation we need. >>> >>> Our specific use case isn't incredibly important, because multi-level >>> map-reduce could be useful in countless ways, but I'll include it >>> anyway just as illustration. The specific need for us arose from the >>> desire to slice up certain very large documents to make concurrent >>> editing by a huge number of users feasible. Then we started to use a >>> view step to combine the data back into whole documents. This worked >>> really well at first, but we soon found that we needed to run >>> additional queries on those documents. So we were stuck with either: >>> >>> 1) do the queries in the client - meaning we lose all the power and >>> caching of couchdb views; or >>> 2) reinsert the combined documents into another database - meaning we >>> are storing the data twice, and we still have to deal with contention >>> when modifying the compound documents in that database. >>> >>> Multi-level map-reduce would solve this problem perfectly! >>> >>> Multi-level views could also simplify and improve performance for >>> reduce grouping. The reduce itself would work just like Google's map- >>> reduce by only reducing values that have the exact same map key. Then >>> if you want to reduce further, you can just use another map-reduce >>> step on top of that with the map emitting a different key so the >>> reduce data will be grouped differently. For example, if you wanted a >>> count of posts per user and total posts, you would implement it as a >>> two-level map-reduce with the key=user_id for map1 and the key=null >>> for map2. >>> >>> This way, you only calculate reduce values for groupings you care >>> about, and any particular reduce value is immediately available from >>> the cached B+tree values without further computation. There is more >>> burden on the user to specify ahead of time which groupings they need, >>> but the performance and flexibility would be well worth it. This >>> eliminates the need to store reduce values internally in the map B >>> +tree. But it does mean that you would need a B+tree for each reduce >>> grouping to keep incremental reduce updates fast. The improved >>> performance comes from the fact that view queries would never need to >>> aggregate reduce values across multiple nodes or do any re-reducing. >>> >>> Does this make sense? What do you guys think? Have you discussed the >>> possibility of such a feature? >>> >>> I'd be happy to discuss it further and even help with the >>> implementation, though I've only done a little bit of coding in >>> Erlang. I'm pretty sure this would mean big changes to the couchdb >>> internals, so I want to get your opinions and criticisms before I get >>> my hopes up or dive into any coding. >>> >>> Cheers, >>> Justin Balthrop >>> >>> . >>> >>> >> Possible solution, I use it in my production ... >> https://issues.apache.org/jira/browse/COUCHDB-249 > > -- Chris Anderson http://jchrisa.net http://couch.io
