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  1. Core Server
  2. SERVER-32758

shard key and planSummary IDHACK

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    • Type: Question
    • Status: Closed
    • Priority: Major - P3
    • Resolution: Done
    • Affects Version/s: None
    • Fix Version/s: None
    • Component/s: None
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      Description

      I'm using the following shard key and document in the collection, sh.status()

      1. shard key:

      { "_id": 1 }

      ,

      { _id: "AH_25a55ce50b8dce490000000001" }


      > using the following shard key and document in the collection, no problem?

      2. planSummary: IDHACK:
      > it is just informing that mongod has choose to use _id index. right?
      no problem?

       
      //sh.status()
      test.sos
          shard key: { "_id": 1 }
          chunks:
            shard1: 20
            shard3: 20
            shard2: 20
       
      //document in the collection      
      { _id: "AH_25a55ce50b8dce490000000001" } 
      { _id: "BH_25a55ce50b8dce490000000002" } 
      { _id: "CH_25a55ce50b8dce490000000003" } 
      { _id: "DH_25a55ce50b8dce490000000004" } 
      { _id: "EH_25a55ce50b8dce490000000005" } 
      { _id: "FH_25a55ce50b8dce490000000006" } 
      { _id: "GH_25a55ce50b8dce490000000007" } 
      { _id: "HH_25a55ce50b8dce490000000008" } 
       
      //mongod.log
      $query: { _id: "AH_25a55ce50b8dce490000000001" } } 
      planSummary: IDHACK ntoskip:0 nscanned:1 nscannedObjects:1 idhack:1 keyUpdates:0 writeConflicts:0 numYields:0 nreturned:1 reslen:1277 locks:{ Global: { acquireCount: { r: 2 } }, Database: { acquireCount: { r: 1 } }, Collection: { acquireCount: { r: 1 } } } 0ms 
      > IDHACK means that normal query plan path (evaluation, caching, re-evaluation) was bypassed for the query shape {_id: <value>} which is supported by the required _id index. 
        this is a performance optimization for the common use case of fetching a document by primary key.
      

      Thanks,
      hyeongwa

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                Updated:
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