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Subject: Re: Subselect Possible?
From: Doug Reeder <reeder.29@gmail.com>
In-Reply-To: <AANLkTi=L7GXm70G069=hB9knUEpxCK0Fs+q3=n-nEfd8@mail.gmail.com>
Date: Tue, 28 Sep 2010 09:04:11 -0400
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 <AANLkTi=L7GXm70G069=hB9knUEpxCK0Fs+q3=n-nEfd8@mail.gmail.com>
To: user@couchdb.apache.org

Sorry, my earlier response seems to have gone astray.

Thank you, but this map and reduce function produce exactly the same =
results as mine.

Is there a general method of structuring data and map-reduce functions =
to do the same thing that a subselect does?


On Sep 26, 2010, at 8:16 PM, Paul Davis wrote:

> Hrm, I haven't thought this through to the end, but something that
> might get you started:
>=20
> Change your emit to emit(key, 1);
>=20
> Change your reduce to:
>=20
> function(keys, vals) {
>    return sum(vals);
> }
>=20
> Then query like such:
>=20
> ?key=3Dquery_key&limit=3D1
> ?group_level=3Dlength(query_key)&key=3Dquery_key
>=20
> It seems like there should be a fancy way to get the reduce call to
> tell you if the original node had < 0 work but I'm distracted by other
> code right now.
>=20
> HTH,
> Paul Davis
>=20
> On Sun, Sep 26, 2010 at 7:46 PM, Doug Reeder <reeder.29@gmail.com> =
wrote:
>> I've searched the wiki and googled for "subselect" and "subquery", =
and not found anything relevelant.
>>=20
>> I'm evaluating whether CouchDB can handle an app which currently =
stores a tree structure in a relational database using the "materialized =
path" represenation.  For example, the item with path "QG" is a child of =
the item with path "Q" and a parent of the item with path "QGC".
>>=20
>> The trickiest query is, in English, "find all items with work less =
than zero and whether they have any children with work less than zero".  =
The SQL for this is
>> SELECT item.*, EXISTS (SELECT work FROM item AS d WHERE d.path > =
item.path AND d.path <=3D item.path || '\ufffd' AND d.work < 0) AS =
undonePrereq, FROM item WHERE work < 0
>>=20
>> Can this be done in CouchDB, using a limited, small number of =
queries?
>>=20
>> My closest sally is the map function
>> function(doc) {
>>  if (doc.work < 0) {
>>    var key =3D [];
>>    var i;
>>    for (i=3D0; i<doc._id.length; ++i)
>>      key.push(doc._id.charCodeAt(i));
>>    emit(key, doc.work);
>>  }
>> }
>>=20
>> and the reduce function
>> function(keys, values, rereduce) {
>>  var shortestKeyLength =3D Infinity;
>>  for (var i=3D0; i<keys.length; ++i) {
>>    if (keys[i][0].length < shortestKeyLength)
>>        shortestKeyLength =3D keys[i][0].length;
>>  }
>>=20
>>  return [keys.length, shortestKeyLength, =
keys[0][0].slice(0,shortestKeyLength), values.length];
>> }
>>=20
>> A query such as
>> curl -X GET =
'http://127.0.0.1:5984/tracker1/_design/trackerA/_view/undoneArrayNum?star=
tkey=3D\[45,65,65\]&endkey=3D\[45,65,9999\]&group_level=3D2'
>> will return the data necessary for items at the second level, but the =
desired items could be at any level.

Doug Reeder
reeder.29@gmail.com
http://reeder29.livejournal.com/
https://twitter.com/reeder29

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