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POMongoDB runs alternate query plans all the time
 

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  1. POMongoDB runs alternate query plans all the time
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    <p>The MongoDB documentation <a href="http://docs.mongodb.org/manual/core/indexes/" rel="nofollow">says</a>:</p> <blockquote> <p>... The query optimizer selects the index empirically by occasionally running alternate query plans and by selecting the plan with the best response time for each query type...</p> </blockquote> <p>The problem I'm experiencing with MongoDB 2.0.2 is that it seems like Mongo runs alternate query plans all the time for certain queries. </p> <p>Not only that makes the whole process very slow (explain({verbose:true}) shows 10-15 additional tests in 'allPlans' array), Mongo also doesn't choose the best index. Providing a hint command with the index I created exactly for that query results in much faster result.</p> <p>Here is an example:</p> <pre><code>&gt; db.posts.find({"project.id" : 2, "project.sections" : 1, "reading" : {"$in" : [0,1]}, "publicate" : 1}).sort({"date":-1}).explain() "cursor" : "BtreeCursor project.id_1_reading_1_publicate_1_public_1_last_comment_time_1 multi", "nscanned" : 13347, "nscannedObjects" : 13346, "n" : 7619, "millis" : 2371, "nYields" : 7, "nChunkSkips" : 0, "isMultiKey" : false, "indexOnly" : false, </code></pre> <p>Same query with verbose:true shows these indexes in 'allPlans' array:</p> <pre><code>"BtreeCursor reading_1_publicate_1_last_comment_time_1 multi", "BtreeCursor date_-1_community_id_1", "BtreeCursor project.id_1_reading_1_publicate_1_public_1_last_comment_time_1 multi", "BtreeCursor project.id_1_reading_1_publicate_1_last_comment_time_1 multi", "BtreeCursor project.id_1_project.sections_1_reading_1_publicate_1_public_1_date_-1 multi", "BtreeCursor timeline_main_liked_all", "BtreeCursor timeline_main_liked_user", "BtreeCursor timeline_main_commented_all", "BtreeCursor timeline_main_commented_user", "BtreeCursor project.id_1_reading_1_publicate_1_public_1_project.sections_1 multi", "BtreeCursor publicate_1_public_1_date_-1_reading_1_type_1 multi", "BtreeCursor publicate_1_timeline_visibility_all_1_short.formatted_1_short.show_1_short.commercial_1_date_-1", "BtreeCursor publicate_1_timeline_visibility_user_1_short.formatted_1_short.show_1_short.commercial_1_date_-1", "BasicCursor", </code></pre> <ol> <li>How do I make Mongo stop running those index checks all the time?</li> <li>How do I make Mongo use and stick to the actual optimal index?</li> </ol>
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    1. PTQuestion
    1. USrakhim
    1. USrakhim
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    1. VO
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      1. VTUpMod
    1. COHave you tried with 2.2.2? Also, what's your question?
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      1. USWiredPrairie
    2. CONo, I haven't, couldn't recreate the conditions. Added the actual questions to the post, sorry.
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      1. USrakhim
    3. COWhat do you mean, you couldn't recreate the conditions? 2.2.2 is the current release.
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      1. USWiredPrairie
 

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