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POSimple MongoDB query very slow although index is set
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  1. POSimple MongoDB query very slow although index is set
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    copied!<p>I've got a MongoDB collection that holds about 100M documents.</p> <p>The documents basically look like this:</p> <pre><code>_id : ObjectId("asd1234567890") _reference_1_id : ObjectId("fgh4567890123") _reference_2_id : ObjectId("jkl7890123456") name : "Test1" id : "4815162342" created_time : Date( 1331882436000 ) _contexts : ["context1", "context2"] ... </code></pre> <p>There are some indexes set, here's the output of <em>db.mycoll.getIndexes();</em></p> <pre><code>[ { "v" : 1, "key" : { "_id" : 1 }, "ns" : "mydb.mycoll", "name" : "_id_" }, { "v" : 1, "key" : { "_reference_1_id" : 1, "_reference_2_id" : 1, "id" : 1 }, "unique" : true, "ns" : "mydb.mycoll", "name" : "_reference_1_id_1__reference_2_id_1_id_1" }, { "v" : 1, "key" : { "_reference_1_id" : 1, "_reference_2_id" : 1, "_contexts" : 1, "created_time" : 1 }, "ns" : "mydb.mycoll", "name" : "_reference_1_id_1__reference_2_id_1__contexts_1_created_time_1" } ] </code></pre> <p>When I execute a query like</p> <pre><code>db.mycoll.find({"_reference_2_id" : ObjectId("jkl7890123456")}) </code></pre> <p>it takes over an hour (!) until it's finished, no matter if there are results or not. Any ideas?</p> <p><strong>Update:</strong> Here's what the output of</p> <pre><code>db.mycoll.find({"_reference_2_id" : ObjectId("jkl7890123456")}).explain(); </code></pre> <p>looks like:</p> <pre><code>{ "cursor" : "BasicCursor", "nscanned" : 99209163, "nscannedObjects" : 99209163, "n" : 5007, "millis" : 5705175, "nYields" : 17389, "nChunkSkips" : 0, "isMultiKey" : false, "indexOnly" : false, "indexBounds" : { } } </code></pre>
 

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