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
