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2008-07-16 on #couchdb
11:19 < beppu> I have a really simple question about reduce functions. I see that they take keys and
values as parameters. What kind of data should I expect in those 2 params?
11:19 < tilgovi> jan____, randall <dizzot> leeds <atface> gmail
11:20 < jchris> beppu: arrays
11:20 < jan____> beppu: the values are the ones you emit in the map function, only without duplicates
and values are all values that have the same key in an array
11:21 < beppu> when you call the emit function, the first param ends up being the key and the second one
is a value, right?
11:21 < jeffd> jan____, how's that move/copy stuff coming? :)
11:21 < beppu> (or wrong?)
11:21 < jchris> beppu: correct
11:21 < beppu> ok.
11:21 < beppu> jchris: oh hey, you're the couchrest guy.
11:22 < beppu> I like your client library.
18:13 < beppu> In a reduce function that takes keys and values as params, is keys.length ==
values.length all the time?
18:14 < beppu> If so, is there a 1:1 correspondence between what's in the keys array and what's in the
values array?
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19:12 < jchris> beppu, the best way to learn all this stuff is via the log() function
19:12 < jchris> i think the answer is yes, at least when the third paramater is false
19:13 < beppu> there's a 3rd param?
19:13 < jchris> it's true on combine aka rereduce
19:14 < beppu> whoa... does this mean there are effectively 3 stages: map, reduce, and combine (but
reduce and combine get lumped into the same function and it's up to you to look at the
3rd param and act accordingly)?
19:15 < jchris> sorta
19:15 < beppu> let me ask another way: what does it mean to "combine"
19:15 < beppu> the wiki is sorely lacking these important little details.
19:16 < jchris> it's complex to describe
19:17 < jchris> but the idea is that on reduce (when group=false) Couch wants to roll everything up into
a single value
19:17 < beppu> (sorry to put you on the spot)
19:17 < beppu> ok.
19:17 < jchris> so it runs the map
19:17 < jchris> and then the reduce function at least once for every map row
19:17 < jchris> and then again on its own output (in groups)
19:17 < jchris> until the final answer is 42
19:18 < beppu> hehe
19:18 < jchris> or whatever
19:18 < jchris> not like the reduce you might know from the map/reduce paper, hadoop, etc
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19:18 < beppu> ok.
19:18 < jchris> which would be like sorting the results of reduce group=true on their values...
19:19 < jchris> (and would be mighty handy if you ask me) :)
19:19 < beppu> I'm trying to imagine the possibilities... like what can I do w/ this....
19:20 < jchris> my markov chain blog post is the only really imaginative thing I've done
19:20 < beppu> Yeah, I just pulled that page up, actually.
19:21 < jchris> damien did a std-deviation calculation using reduce
19:22 < beppu> do you have a url for that?
19:22 < jchris> the final value can be something complex like {total: 25255326, count: 2487, avg:
441.32, stddev: 21.4}
19:22 < jchris> just not a list, i guess
19:22 < jchris> I'm fuzzy there
19:22 < jchris> damien's thing was just in the channel
19:22 < beppu> doh
19:22 < jchris> I used to have a pastie.org for it, but the link is lost
19:22 < beppu> right...
19:22 < beppu> ephemeral data
19:23 < damienkatz_> the stddev stuff is in the tests
19:24 < damienkatz_> another use from reduce (this is from an email)
19:24 < damienkatz_> :
19:24 < damienkatz_> Anyway, I was thinking about a question that came up, which was how to do a
drill-down, guided categorized search, ala-Endeca.
19:24 < damienkatz_> I think this sort of thing with CouchDB is now possible using our key arrays and
the group_level query option for reduce. Essentially, you'd permute the relevant
fields in keys into arrays and emit them into the view. Then, given any number of
arbitrary key constraints, we can instantly retrieve the number of documents
meeting those constraints, and the counts of the relevant sub constraints.
19:24 < damienkatz_> The cost of this is for N of keys you want to permute is N!. So if a document has 5
fields relevant to a guided search, it would require 120 key arrays (each a 5
element array of keys) to be emitted. This would significant slow down the view
indexing speed, however, once built cost of the querying of the view during the
drill downs is small and can be done in a view single query.
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