Uploaded image for project: 'Core Server'
  1. Core Server
  2. SERVER-1896

limit() clause is not being pushed down to the shard for evaluation, causing more documents to be returned than are needed

    XMLWordPrintable

Details

    • Bug
    • Status: Closed
    • Major - P3
    • Resolution: Fixed
    • 1.7.0
    • 1.7.3
    • None
    • None
    • 1.7.2-pre-
    • ALL

    Description

      Problem:
      It appears that the limit() clause is not being pushed down to the shard server for evaluation, since the explain() indicates more documents returned than the limit.

      For example, given the shard key "x" has values 0-100 and the split is at 50. Given the query

      db.test20101005.find( {x : { $lt : 60}} ).sort(

      { x:-1}

      ).limit(1).explain()

      produces

      > db.test20101005.find( {x : { $lt : 60}} ).sort(

      { x:-1}

      ).limit(1).explain()
      {
      "clusteredType" : "ParallelSort",
      "shards" : {
      "10.195.79.0:27000" : [
      {
      "cursor" : "BtreeCursor x_1 reverse",
      "nscanned" : 50,
      "nscannedObjects" : 50,
      "n" : 50,
      "millis" : 0,
      "nYields" : 0,
      "nChunkSkips" : 0,
      "indexBounds" :

      { "x" : [ [ 60, -1.7976931348623157e+308 ] ] }

      }
      ],
      "10.203.83.53:27000" : [
      {
      "cursor" : "BtreeCursor x_1 reverse",
      "nscanned" : 10,
      "nscannedObjects" : 10,
      "n" : 10,
      "millis" : 0,
      "nYields" : 0,
      "nChunkSkips" : 0,
      "indexBounds" :

      { "x" : [ [ 60, -1.7976931348623157e+308 ] ] }

      }
      ]
      },
      "n" : 60,
      "nChunkSkips" : 0,
      "nYields" : 0,
      "nscanned" : 60,
      "nscannedObjects" : 60,
      "millisTotal" : 0,
      "millisAvg" : 0,
      "numQueries" : 2,
      "numShards" : 2
      }

      The value of "n" indicates that shard0000 returns 50 documents, and shard0001 returns 10 before the final sort & limit is applied by the mongos.

      Reproduce:
      > for (i=0; i < 100; i++) { db.test20101005.insert(

      { x: i}

      ); }
      > db.test20101005.ensureIndex(

      {x:1}

      )
      > use admin
      switched to db admin
      > db.runCommand(

      { enablesharding : "test"}

      )

      { "ok" : 1 }

      > db.runCommand( { shardcollection : "test.test20101005", key :

      { x : 1 }

      } )

      { "collectionsharded" : "test.test20101005", "ok" : 1 }

      > db.runCommand( { split : "test.test20101005", middle : { x : 50 }} );

      { "ok" : 1 }

      > db.runCommand( { moveChunk: "test.test20101005", find :

      { x : 51}

      , to : "shard0001" })

      { "millis" : 2413, "ok" : 1 }

      > use test
      switched to db test
      > db.test20101005.find( {x : { $lt : 60}} ).sort(

      { x:-1}

      ).limit(1).explain()
      {
      "clusteredType" : "ParallelSort",
      "shards" : {
      "10.195.79.0:27000" : [
      {
      "cursor" : "BtreeCursor x_1 reverse",
      "nscanned" : 50,
      "nscannedObjects" : 50,
      "n" : 50,
      "millis" : 0,
      "nYields" : 0,
      "nChunkSkips" : 0,
      "indexBounds" :

      { "x" : [ [ 60, -1.7976931348623157e+308 ] ] }

      }
      ],
      "10.203.83.53:27000" : [
      {
      "cursor" : "BtreeCursor x_1 reverse",
      "nscanned" : 10,
      "nscannedObjects" : 10,
      "n" : 10,
      "millis" : 0,
      "nYields" : 0,
      "nChunkSkips" : 0,
      "indexBounds" :

      { "x" : [ [ 60, -1.7976931348623157e+308 ] ] }

      }
      ]
      },
      "n" : 60,
      "nChunkSkips" : 0,
      "nYields" : 0,
      "nscanned" : 60,
      "nscannedObjects" : 60,
      "millisTotal" : 0,
      "millisAvg" : 0,
      "numQueries" : 2,
      "numShards" : 2
      }

      Solution:
      Certainly in this case, the sort and limit could be applied to the data set returned by each shard. The final sort and limit will still need to happen in the mongos.

      Business Case:
      Performance.

      Attachments

        Activity

          People

            eliot Eliot Horowitz (Inactive)
            alvin Alvin Richards (Inactive)
            Votes:
            2 Vote for this issue
            Watchers:
            3 Start watching this issue

            Dates

              Created:
              Updated:
              Resolved: