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POmongodb 'count' with query is very slow
 

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  1. POmongodb 'count' with query is very slow
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    <p>everyone,im use a mongodb 2.4.6 version and in windows 2008 64-bit.</p> <p>i have a collection that have two million records and need to search and paging in client.</p> <pre><code>db.products.find({"catalogs":1205}).skip().limit() is very fast . </code></pre> <p>but when calculate total record count:</p> <pre><code>db.products.find({"catalogs":1205},{"_id":1}).count() is too slow. &gt;&gt; 442312 records. &gt;&gt;[log] Sat Sep 28 00:20:01.566 [conn10] command products.$cmd command: { count: "products", query: { catalogs: 1205.0 }, fields: { _id: 1.0 } } ntoreturn:1 keyUpdates:0 locks(micros) r:460681 reslen:48 460ms </code></pre> <p>this count command elapsed time is <code>460ms</code>,is too slow.if we have a lot of request that very terrible.</p> <p>i created a index for a 'catalogs' field and can't use <code>$inc</code> command because query could be very complex.</p> <p>im googling some like this problem and found this 'count' performance bug already fixed in mongodb 2.4 version.</p> <p>from <a href="http://docs.mongodb.org/manual/release-notes/2.4-overview/">http://docs.mongodb.org/manual/release-notes/2.4-overview/</a> </p> <pre><code>Improvements to count provide dramatically faster count operations. Counting is now up to 20 times faster for low cardinality index based counts. </code></pre> <p>what ways can improve count?thanks.</p> <p><strong>update some information</strong></p> <pre><code>&gt; db.products.getIndexes() [ { "v" : 1, "key" : { "_id" : 1 }, "ns" : "products.products", "name" : "_id_" }, { "v" : 1, "key" : { "catalogs" : 1, "created" : -1 }, "ns" : "products.products", "name" : "catalogs_1_created_-1" } ] </code></pre> <p>the query and elapsed time:</p> <pre><code>&gt;db.products.find({"catalogs":1205},{"_id":1}).limit(20) &gt;&gt;Tue Oct 01 15:39:19.160 [conn2] query products.products query: { catalogs: 1205.0 } cursorid:277334670708253 ntoreturn:20 ntoskip:0 nscanned:21 keyUpdates:0 locks(micros) W:5045 r:1017 nreturned:20 reslen:704 1ms </code></pre> <p>the query exaplin:</p> <pre><code>&gt;db.products.find({"catalogs":1205},{"_id":1}).explain() { "cursor" : "BtreeCursor catalogs_1_created_-1", "isMultiKey" : true, "n" : 451466, "nscannedObjects" : 451466, "nscanned" : 451466, "nscannedObjectsAllPlans" : 451466, "nscannedAllPlans" : 451466, "scanAndOrder" : false, "indexOnly" : false, "nYields" : 2, "nChunkSkips" : 0, "millis" : 2969, "indexBounds" : { "catalogs" : [ [ 1205, 1205 ] ], "created" : [ [ { "$maxElement" : 1 }, { "$minElement" : 1 } ] ] }, "server" : "WIN-O47CO6C2WXY:27017" </code></pre> <p>}</p>
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    1. PTQuestion
    1. USzhengchun
    1. USzhengchun
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      1. POmongodb 'count' with query is very slow
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      1. VTUpMod
    1. COI don't understand why do you collect `_id`s before counting? Why not just use `db.products.count({catalogs:1205})` ?
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      1. POmongodb 'count' with query is very slow
      1. USYevgeniy Anfilofyev
    2. COhi,`db.products.find({"catalogs":1205}).count()` and `db.products.find({"catalogs":1205},{"catalogs":1}).count()` also slow.thanks
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      1. POmongodb 'count' with query is very slow
      1. USzhengchun
    3. COCould you run it without `find` as I wrote above?
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      1. POmongodb 'count' with query is very slow
      1. USYevgeniy Anfilofyev
 

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