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https://issues.apache.org/jira/browse/COUCHDB-403?page=com.atlassian.jira.plugin.system.issuetabpanels:all-tabpanel
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Paul Joseph Davis updated COUCHDB-403:
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Skill Level: Committers Level (Medium to Hard)
> User-defined GroupRowsFun
> -------------------------
>
> Key: COUCHDB-403
> URL: https://issues.apache.org/jira/browse/COUCHDB-403
> Project: CouchDB
> Issue Type: Wish
> Components: Database Core, HTTP Interface
> Reporter: Brian Candler
> Priority: Minor
>
> CouchDB has hard-coded functionality for grouping. From the user's point of
> view: group_level=N will truncate Array keys to the first N elements, and
> that's it. (*)
> It would be wonderful if application-specific grouping functions could be
> added. Useful examples include:
> * for string keys, truncate to the first N characters (e.g. group by first 3
> letters of surname)
> * for numeric keys, trunc(k/N) (e.g. divide by 100 would give you buckets of
> 0..99, 100..199, 200..299 etc)
> * combine with group_level: e.g. truncate array to first two elements plus
> the third element divided by 100
> ["string1","string2",Number,"rest"] =>
> ["string1","string2",trunc(Number/100)]
> * for numeric keys: use trunc(log(V) * N) for exponential buckets
> * for hexadecimal-string keys: right-shift N places
> * ...etc
> In each case N would be a parameter chosen at query time, like group_level is
> now.
> It would be sufficient just to have a hook to statically link Erlang
> functions to do this. There would then need to be two new HTTP parameters:
> one to choose the grouping function and one for any arguments it needs.
> Theoretically this function could also be handed off to the external view
> server so the logic could be written in Javascript or whatever, but I think
> it would be too slow in practice.
> Note: group truncation functions would have need to meet certain constraints
> to work with grouping logic. Something like:
> K1 <= K2 implies grouptrunc(K1) <= grouptrunc(K2)
> (*) It's not implemented exactly like that. As far as I can see, there's one
> function to compare keys for equality by looking at the first N elements
> (GroupRowsFun), and another function truncates them when emitting them
> (RespFun). For adding bolt-on functions it would be more convenient just to
> define a single group key truncation function.
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