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  1. Core Server
  2. SERVER-14228

Setting batchSize and sort on a cursor in sharded collection causes fewer than all documents to be returned

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Details

    • Bug
    • Status: Closed
    • Critical - P2
    • Resolution: Duplicate
    • 2.4.8, 2.4.10
    • None
    • Querying
    • None
    • ALL
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      This reproduces on a single instance:

      > db.foo.drop()
      > for (i=0; i<1000; i++) {db.foo.insert({x:i})}
      > db.foo.find().batchSize(100).itcount()
      1000
      > db.foo.find().sort({x:1}).batchSize(100).itcount()
      100

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      This reproduces on a single instance: > db.foo.drop() > for (i=0; i<1000; i++) {db.foo.insert({x:i})} > db.foo.find().batchSize(100).itcount() 1000 > db.foo.find().sort({x:1}).batchSize(100).itcount() 100

    Description

      Here is the correct number of documents:

      mongos> db.ad_campaigns.find({fs:2}).count()
      4587

      If you specify a batchSize and a sort, you get fewer than all the documents:

      mongos> function cursorCount(query, batchSize) { var count = 0; var cursor = db.ad_campaigns.find(query).batchSize(batchSize); while(cursor.hasNext()) { cursor.next(); count++; }; return count }
      mongos> cursorCount({fs:2}, 100)
      3504

      The number of documents returned appears to be K + batchSize, for some K:

      mongos> cursorCount({fs:2}, 101)
      3505
      mongos> cursorCount({fs:2}, 102)
      3506

      K (3404 in this example) happens to match the number of documents matching the query on one shard (ads1):

      ads1> db.ad_campaigns.find({fs:2}).count()
      3404

      This relationship between K and the number of records on ads1 is borne out on other queries:

      mongos> db.ad_campaigns.find({fs:5}).count()
      9764
      mongos> cursorCount({fs:5}, 100)
      5249
      ads1> db.ad_campaigns.find({fs:5}).count()
      5149

      If the batch size is not specified or the cursor is not sorted, the problem goes away:

      mongos> db.ad_campaigns.find({fs:2}).count()
      4587
      mongos> cursorCount({fs:5}, 2000)
      4587
      mongos> var count = 0; var cursor = db.ad_campaigns.find({fs:2}).batchSize(100); while(cursor.hasNext()) { cursor.next(); count++; }; count
      4587

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              david.storch@mongodb.com David Storch
              jliszka Jason Liszka
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                Created:
                Updated:
                Resolved: