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From m.@hudora.de
Subject Re: API suggestions
Date Sun, 28 Dec 2008 14:12:08 GMT
While writing something about using CouchDB I came across the issue of "slice indexes" (called
startkey and endkey in CouchDB lingo). 

I found no exact definition of startkey and endkey anywhere in the documentation. Testing
reveals that access on _all_docs and on views documents are retuned in the interval

[startkey, endkey] = (startkey <= k <= endkey).

I don't know if this was a conscious design decision. But I like to promote a slightly different
interpretation (and thus API change):

[startkey, endkey[ = (startkey <= k < endkey).


Both approaches are valid and used in the real world. Ruby uses the inclusive ("right-closed"
in math speak) first approach:

>> l = [1,2,3,4]
>> l.slice(1,2)
=> [2, 3]


Python uses the exclusive ("right-open" in math speak) second approach:

>>> l = [1,2,3,4]
>>> l[1,2]
[2]


For array indices both work fine and which one to prefer is mostly an issue of habit. In spoken
language both approaches are used: "Have the Software done until saturday" probably means
right-open to the client and right-closed to the coder.

But if you are working with keys that are more than array indexes, then right-open is much
easier to handle. That is because you have to *guess* the biggest value you want to get. The
Wiki at http://wiki.apache.org/couchdb/View_collation contains an example of that problem:

It is suggested that you use
startkey="_design/"&endkey="_design/ZZZZZZZZZ" 
or
startkey="_design/"&endkey="_design/\u9999" 
to get a list of all design documents

This breaks if a design document is named "ZZZZZZZZZTop" or "\9999Iñtërnâtiônàlizætiøn".
Such names might be unlikely but we are computer scientists; "unlikely" is a bad approach
to software engineering.

The think what we really want to ask CouchDB is to "get all documents with keys starting with
'_design/'".

This is basically impossible to do with right-closed intervals. We could use startkey="_design/"&endkey="_design0"
('0' is the ASCII character after '/') and this will work fine ... until there is actually
a document with the key "_design0" in the system. Unlikely, but ...

To make selection by intervals reliable currently clients have to guess the last key (the
ZZZZ approach) or use the fist key not to include (the _design0 approach) and then post process
the result to remove the last element returned if it exactly matches the given endkey value.


If couchdb would change to a right-open interval approach post processing would go away in
most cases. See http://blogs.23.nu/c0re/2008/12/building-a-track-and-trace-application-with-couchdb/
for two real world examples.

At least for string keys and float keys changing the meaning to [startkey, endkey[ would allow
selections like

* "all strings starting with 'abc'"
* all numbers between 10.5 and 11

It also would hopefully break not to much existing code. Since the notion of endkey seems
to be already considered "fishy" (see the ZZZZZ approach) most code seems to try to avoid
that issue. For example 'startkey="_design/"&endkey="_design/ZZZZZZZZZ"' still would work
unless you have a design document being named exactly "ZZZZZZZZZ".

Regards

Maximillian Dornseif


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