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POMap-Reduce Query to Count Tags
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  1. POMap-Reduce Query to Count Tags
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    copied!<p>I have a database of documents which are tagged with keywords. I am trying to find (and then count) the unique tags which are used alongside each other. So for any given tag, I want to know what tags have been used alongside that tag.</p> <p>For example, if I had one document which had the tags <code>[fruit, apple, plant]</code> then when I query <code>[apple]</code> I should get <code>[fruit, plant]</code>. If another document has tags <code>[apple, banana]</code> then my query for <code>[apple]</code> would give me <code>[fruit, plant, banana]</code> instead.</p> <p>This is my map function which emits all the tags and their neighbours:</p> <pre><code>function(doc) { if(doc.tags) { doc.tags.forEach(function(tag1) { doc.tags.forEach(function(tag2) { emit(tag1, tag2); }); }); } } </code></pre> <p>So in my example above, it would emit</p> <pre><code>apple -- fruit apple -- plant apple -- banana fruit -- apple fruit -- plant ... </code></pre> <p>My question is: what should my reduce function be? The reduce function should essentially filter out the duplicates and group them all together.</p> <p>I have tried a number of different attempts, but my database server (CouchDB) keeps giving me a <strong>Error: reduce_overflow_error. Reduce output must shrink more rapidly</strong>.</p> <hr> <p><strong>EDIT: I've found something that seems to work, but I'm not sure why.</strong> I see there is an optional "rereduce" parameter to the reduce function call. If I ignore these special cases, then it stops throwing reduce_overflow_errors. Can anyone explain why? And also, should I just be ignoring these, or will this bite me in the ass later?</p> <pre><code>function(keys, values, rereduce) { if(rereduce) return null; // Throws error without this. var a = []; values.forEach(function(tag) { if(a.indexOf(tag) &lt; 0) a.push(tag); }); return a; } </code></pre>
 

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