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  1. Core Server
  2. SERVER-8300

Covered multi-key query still scans

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Details

    • Improvement
    • Status: Closed
    • Critical - P2
    • Resolution: Duplicate
    • 2.2.2
    • None
    • Querying
    • None
    • RHEL5

    Description

      index exists:

      { t:1, lc:1, wpm:1}

      through the Java library I use the
      com.mongodb.DBCollection,find(DBObject ref, DBObject keys)

      with keys =

      {lc: 1, t:1, wpm:1, _id: 0}

      and this query is logged as

      Wed Jan 23 17:31:09 [conn5342] query jerome5.Unit query: { wpm:

      { $gte: 0.008999999612569809 }

      , t:

      { $in: [ "Central", "Commission", "Kenya", "CKRC", "East", "Mandera", "Constituency", "Report", "North", "Province", "Rhamu" ] }

      , lc:

      { $in: [ "por", "est", "pol", "vie", "isl", "nor", "ita", "deu", "ces", "eng", "hun", "sqi", "slk", "hin", "slv", "nld", "tur", "dan", "cat", "fra", "fin", "esl", "sve", "lav" ] }

      } ntoreturn:0 ntoskip:0 nscanned:210 keyUpdates:0 numYields: 1 locks(micros) r:106836 nreturned:83 reslen:3694 126ms

      I then ran an explain on a similar query:

      db.Unit.find({ wpm:

      { $gte: 0.008999999612569809 }

      , t:

      { $in: [ "knocking", "witnesses", "care", "off", "you", "systematically", "and" ] }

      , lc:

      { $in: [ "ukr", "slk", "cat", "zho", "deu", "fra", "por", "fin", "hin", "ces", "slv", "nld", "kor", "est", "jap", "rus", "ara", "pol", "eng", "vie", "sve", "esl", "hun", "isl", "lav", "ell", "tur", "bel", "nor", "bul", "dan", "sqi", "ita", "mkd" ] }

      },

      {t: 1, lc: 1, wpm: 1, _id: 0}

      ).explain();
      {
      "cursor" : "BtreeCursor t_1_lc_1_wpm_1 multi",
      "isMultiKey" : false,
      "n" : 63,
      "nscannedObjects" : 63,
      "nscanned" : 142,
      "nscannedObjectsAllPlans" : 480,
      "nscannedAllPlans" : 559,
      "scanAndOrder" : false,
      "indexOnly" : true,
      "nYields" : 6,
      "nChunkSkips" : 0,
      "millis" : 3566,

      I don't understand why nscannedObjects is > 0 especially is indexOnly is true. Since i'm not asking for _id, everthing to return is already in the index, so why are any object scans happening?

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              aaron Aaron Staple
              niccottrell Nic Cottrell (Personal)
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                Created:
                Updated:
                Resolved: