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Date: Fri, 26 Aug 2011 17:31:29 +0000 (UTC)
From: "Paul Joseph Davis (JIRA)" <jira@apache.org>
To: dev@couchdb.apache.org
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Subject: [jira] [Commented] (COUCHDB-834) startkey_docid/endkey_docid don't
 work without an exact startkey/endkey match
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    [ https://issues.apache.org/jira/browse/COUCHDB-834?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=13091890#comment-13091890 ] 

Paul Joseph Davis commented on COUCHDB-834:
-------------------------------------------

startkey_docid and endkey_docid only come into effect when you have identical keys.

Internall, view rows are stored like this:

    [key1, docid1], value
    [key2, docid2], value
    [key3, docid3], value
    [key4, docid4], value
    [key5, docid5], value

Sorting with docids is the same as if you emit an array key. If the first elements of the array are different it will never consult the second element. Its similar to sorting strings. You only need to look at the prefix that's shared to figure out which comes first.

> startkey_docid/endkey_docid don't work without an exact startkey/endkey match
> -----------------------------------------------------------------------------
>
>                 Key: COUCHDB-834
>                 URL: https://issues.apache.org/jira/browse/COUCHDB-834
>             Project: CouchDB
>          Issue Type: Bug
>          Components: JavaScript View Server
>    Affects Versions: 1.0
>            Reporter: Mathias Meyer
>
> This issue popped up when I wanted to paginate through a list of documents using a combined array key, using a startkey and endkey that's based solely on the first part of said key. First part is a reference to a different document, second part is a timestamp to keep the list sorted by creation time. The list of documents can be fetched using startkey=["key"] and endkey=["key", {}]
> Now, I wanted to add pagination to this list, only fetching so many documents starting at startkey_docid, which failed using this setup. It seems (and Jan validated that assumption by analyzing the source) that both startkey needs to be an exact match for startkey_docid to have any effect. If there's no exact match, CouchDB will silently ignore the startkey_docid, a behaviour that's undocumented and to be quite frank, unintuitive.
> Consider the following two documents, both pointing to the same other_id:
> {"_id": "one", "other_id": "other", "second_key": "one"}
> {"_id": "two", "other_id": "other", "second_key": "two"}
> And a simple map/reduce function that just emits the combined key:
> {
>    "other_documents": {
>        "reduce": "_sum",
>        "map": "          function(doc) { \n emit([doc.other_id, doc.second_key], 1);\n  }\n"
>    }
> }
> Querying the view like this gives the expected results:
> curl 'http://localhost:5984/startkey_bug/_design/other_documents/_view/other_documents?reduce=false&startkey=\["other"\]&endkey=\["other",\{\}\]'
> {"total_rows":2,"offset":0,"rows":[
> {"id":"one","key":["other","one"],"value":1},
> {"id":"two","key":["other","two"],"value":1}
> ]}
> If I add in a startkey_docid of two, I'd expect CouchDB to skip to the second result in the list, skipping the first, but it doesn't:
> curl 'http://localhost:5984/startkey_bug/_design/other_documents/_view/other_documents?reduce=false&startkey=\["other"\]&endkey=\["other",\{\}\]&startkey_docid=two'
> {"total_rows":2,"offset":0,"rows":[
> {"id":"one","key":["other","one"],"value":1},
> {"id":"two","key":["other","two"],"value":1}
> ]}
> However, it does what I'd expect when I specify an exact startkey (the endkey is still the same):
> curl 'http://localhost:5984/startkey_bug/_design/other_documents/_view/other_documents?reduce=false&startkey=\["other","one"\]&endkey=\["other",\{\}\]&startkey_docid=two'
> {"total_rows":2,"offset":1,"rows":[
> {"id":"two","key":["other","two"],"value":1}
> ]}
> If you add in an exact endkey, the situation doesn't change, and the result is as expected.
> Having an exact startkey is an acceptable workaround, but I'd still say this behaviour is not intuitive, and either should be fixed to work the same in all of the above situations. If not, at least the documentation should properly reflect these situation, explaining the proper workarounds.
> Update: I just checked how this works out when using descending=true, the same is true for the swapped endkey and startkey parameters. Specifying and endkey_docid requires to specify an exact endkey match.

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