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POWhy is a single index faster than a compound index in a query for two keys? (MongoDB, multi-key)
 

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  1. POWhy is a single index faster than a compound index in a query for two keys? (MongoDB, multi-key)
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    <p>I have created 4 indexes to test query performance in my collection when quering for two fields of the same document, one of which is an array (needs a multi-key index). Two of the indexes are single and two compound.</p> <p>I am surpised because of getting better performance with one of the single indexes than with the compound ones. I was expecting to obtain the best performace with a compound index, because I understand that it indexes the two fields allowing for faster querying.</p> <p>These are my indexes:</p> <pre><code>{ "v" : 1, "key" : { "_id" : 1 }, "ns" : "bt_twitter.mallorca.mallorca", "name" : "_id_" }, { "v" : 1, "key" : { "epoch_creation_date" :1 }, "ns" : "bt_twitter.mallorca.mallorca", "name" : "epoch_creation_date_1" }, { "v" : 1, "key" : { "related_hashtags" : 1 }, "ns" : "bt_twitter.mallorca.mallorca", "name" : "related_hashtags_1" }, { "v" : 1, "key" : { "epoch_creation_date" : 1, "related_hashtags" : 1 }, "ns" : "bt_twitter.mallorca.mallorca", "name" : "epoch_creation_date_1_related_hashtags_1" } </code></pre> <p>My queries and performance indicators are (hint parameter shows the index used at each query):</p> <p>QUERY 1:</p> <pre><code>active_collection.find( {'epoch_creation_date': {'$exists': True}}, {"_id": 0, "related_hashtags":1} ).hint([("epoch_creation_date", ASCENDING)]).explain() </code></pre> <p>millis: 237</p> <p>nscanned: 101226</p> <p>QUERY 2:</p> <pre><code>active_collection.find( {'epoch_creation_date': {'$exists': True}}, {"_id": 0, "related_hashtags": 1} ).hint([("related_hashtags", ASCENDING)]).explain() </code></pre> <p>millis: 1131</p> <p>nscanned: 306715</p> <p>QUERY 3:</p> <pre><code>active_collection.find( {'epoch_creation_date': {'$exists': True}}, {"_id": 0, "related_hashtags": 1} ).hint([("epoch_creation_date", ASCENDING), ("related_hashtags", ASCENDING)]).explain() </code></pre> <p>millis: 935</p> <p>nscanned: 306715</p> <p>QUERY 4:</p> <pre><code>active_collection.find( {'epoch_creation_date': {'$exists': True}}, {"_id": 0, "related_hashtags": 1} ).hint([("related_hashtags", ASCENDING),("epoch_creation_date", ASCENDING)]).explain() </code></pre> <p>millis: 1165</p> <p>nscanned: 306715</p> <p>QUERY 1 scans less documents, what is probably the reason to be faster. Can somebody help me to understand why is it performing better than queries with compound indexes? Therefore, when is better to use a compound index than a single one?</p> <p>I am reading mongo documentation but these concepts are resulting hard for me to digest.</p> <p>Thanks in advance.</p> <p>UPDATED question (in response to Sammaye and Philipp)</p> <p>This is the result of a full explain()</p> <pre><code>"cursor" : "BtreeCursor epoch_creation_date_1", "isMultiKey" : false, "n" : 101226, "nscannedObjects" : 101226, "nscanned" : 101226, "nscannedObjectsAllPlans" : 101226, "nscannedAllPlans" : 101226, "scanAndOrder" : false, "indexOnly" : false, "nYields" : 0, "nChunkSkips" : 0, "millis" : 242, "indexBounds" : {u'epoch_creation_date': [[{u'$minElement': 1}, {u'$maxElement': 1}]] }, "server" : "vmmongodb:27017" </code></pre> <p>for the following query:</p> <pre><code>active_collection.find( {'epoch_creation_date': {'$exists': True}}, {"_id": 0, "related_hashtags":1}) .hint([("epoch_creation_date", ASCENDING)]).explain() </code></pre>
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    1. USOscar Moya
    1. USOscar Moya
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      1. POWhy is a single index faster than a compound index in a query for two keys? (MongoDB, multi-key)
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    1. COYou would need to tell us how these indexes are defined.
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      1. USPhilipp
    2. COHi Phillipp, do you mean how did I create the index? I used active_collection.create_index([("epoch_creation_date", ASCENDING),("related_hashtags", ASCENDING)]) for example
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      1. USOscar Moya
    3. COI mean the ensureIndex calls you used to create the indexes.
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      1. USPhilipp
 

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