User-defined GroupRowsFun
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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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