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    <p>1) The easiest way to do this is via the aggregation framework. This takes two "$group" commands: the first one groups by distinct values, the second one counts all of the distinct values</p> <pre><code>pipeline = [ { $group: { _id: "$myIndexedNonUniqueField"} }, { $group: { _id: 1, count: { $sum: 1 } } } ]; // // Run the aggregation command // R = db.runCommand( { "aggregate": "myCollection" , "pipeline": pipeline } ); printjson(R); </code></pre> <p>2) If you want to do this with Map/Reduce you can. This is also a two-phase process: in the first phase we build a new collection with a list of every distinct value for the key. In the second we do a count() on the new collection.</p> <pre><code>var SOURCE = db.myCollection; var DEST = db.distinct DEST.drop(); map = function() { emit( this.myIndexedNonUniqueField , {count: 1}); } reduce = function(key, values) { var count = 0; values.forEach(function(v) { count += v['count']; // count each distinct value for lagniappe }); return {count: count}; }; // // run map/reduce // res = SOURCE.mapReduce( map, reduce, { out: 'distinct', verbose: true } ); print( "distinct count= " + res.counts.output ); print( "distinct count=", DEST.count() ); </code></pre> <p>Note that you cannot return the result of the map/reduce inline, because that will potentially overrun the 16MB document size limit. You <em>can</em> save the calculation in a collection and then count() the size of the collection, or you can get the number of results from the return value of mapReduce().</p>
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    1. POMongoDB select count(distinct x) on an indexed column - count unique results for large data sets
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    1. USWilliam Z
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    1. COI downloaded Mongo 2.2 RC0, and used your 1st suggestion, and it works! and fast! thank you (well done 10gen...) Created a gist here (used the shortcut aggregate command and put it in one line) https://gist.github.com/3241616
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    2. CO@EranMedan I should warn you though, I didn't suggest the aggregation framework because 2.2 rc0 is still not really ready for full deployment, just something to bare in mind, I would wait until the full release of 2.2 before recommending deployment of the aggregation framework.
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    3. CO@Sammaye yes, thanks I'm aware of it, will not go to production yet, I needed that for internal statistics and wanted to avoid moving data to SQL if possible (and quench my curiosity)
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