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PO'AVG' and 'SUM' functionality in MongoDB, any tips?
 

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    <p>I'm a relative newbie to MongoDB, but from what I've read there are various methods to going about finding averages and sums of values in a MongoDB database, with various benefits and drawbacks for each.</p> <p>I'm primarily asking for a method of finding the sum of a selection of values, and the average of a selection of values, in an as efficient (fast) method possible.</p> <p>The documents in the collection being queried resemble this structure (with a lot of other fields):</p> <pre><code>{ "_id": ObjectId('4e650107580fd649e5000005'), "date_added": ISODate("2011-09-05T00:00:00Z"), "value": 1500 } </code></pre> <p>Precalculating things like sums is, in my application, not always possible, because the selection of values to be summed can change (based on date ranges - e.g. between a start date and an end date, what is the average). This is a similar problem with precalculating averages.</p> <p>From what I've read, MapReduce is definitely not ideal for real-time (i.e. on demand) lookup, so that seems to be out of the question too.</p> <p>At the moment I'm querying the collection in this way: (note: this is using <code>pymongo</code>)</p> <pre><code>response = request.db['somecollection'].find( { 'date_added': { '$gte': date_start, '$lte': date_end } }, { 'value':1 } ).limit(500) </code></pre> <p>Then doing the calculation in Python using a <code>for</code> loop over the response. The limit of 500 results is arbitrary, to keep it from become too slow. I'm only retrieving the value, and none of the other fields.</p> <p>Is this the most efficient method of doing this calculcation, or are there other methods to accomplish what I need?</p> <p><strong>Caveats:</strong></p> <ul> <li>I can't use the <code>group</code> function because I will probably be using sharding in the future</li> <li>I can't use MapReduce because it's a function which will be used on-the-fly by users</li> <li>I can't precalculate a lot of my sums/averages because the selection of values to sum/average is almost always different</li> <li>I have looked around stackoverflow and the web to try and find recommendation on how to do this kind of thing, and it's fairly open-ended</li> </ul> <p><strong>EDIT:</strong></p> <p>I should point out that the number of documents returned from the query I posted above could be anything from 1 document to hundreds, but will probably have a maximum number of returned documents of about 150 (average of about 60 or 70)</p>
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