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  1. Core Server
  2. SERVER-12952

Regex query performance regression in v2.6.0-rc0

    • Type: Icon: Bug Bug
    • Resolution: Duplicate
    • Priority: Icon: Major - P3 Major - P3
    • None
    • Affects Version/s: 2.6.0-rc0
    • Component/s: Querying
    • ALL
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      See QA-401.

      Show
      See QA-401.

      The following regex query takes roughly 8x longer in v2.6.0-rc0 vs. v2.4.9:

      db.QA401.find({ _id: /Big(ww)i/ }).sort({a:1})
      

      The test collection has 920 documents (see attached) with the following indices:

      db.QA401.ensureIndex({a:-1})
      db.QA401.ensureIndex({'a.a':-1})
      db.QA401.ensureIndex({'a.a.a':-1})
      

      v2.6.0-rc0:

      > db.QA401.find({ _id: /Big(ww)i/ }).sort({a:1}).explain();
      {
              "cursor" : "BtreeCursor _id_",
              "isMultiKey" : false,
              "n" : 0,
              "nscannedObjects" : 0,
              "nscanned" : 1,
              "nscannedObjectsAllPlans" : 1,
              "nscannedAllPlans" : 2,
              "scanAndOrder" : true,
              "indexOnly" : false,
              "nYields" : 0,
              "nChunkSkips" : 0,
              "millis" : 0,
              "indexBounds" : {
                      "_id" : [
                              [
                                      "",
                                      {
      
                                      }
                              ],
                              [
                                      /Big(ww)i/,
                                      /Big(ww)i/
                              ]
                      ]
              },
              "server" : "zzyzx:27017",
              "filterSet" : false
      }
      

      v2.4.9:

      > db.QA401.find({ _id: /Big(ww)i/ }).sort({a:1}).explain();
      {
              "cursor" : "BtreeCursor _id_ multi",
              "isMultiKey" : false,
              "n" : 0,
              "nscannedObjects" : 0,
              "nscanned" : 1,
              "nscannedObjectsAllPlans" : 0,
              "nscannedAllPlans" : 3,
              "scanAndOrder" : true,
              "indexOnly" : false,
              "nYields" : 0,
              "nChunkSkips" : 0,
              "millis" : 0,
              "indexBounds" : {
                      "_id" : [
                              [
                                      "",
                                      {
      
                                      }
                              ],
                              [
                                      /Big(ww)i/,
                                      /Big(ww)i/
                              ]
                      ]
              },
              "server" : "zzyzx:27018"
      }
      

      The query times are sub-millisecond; here is the output from QA-401:

      {
              "_id" : ObjectId("530f6bad7e6c53525919dfb6"),
              "deltaRatio" : "8.40",
              "deltaUs" : 741,
              "v249Avg" : "100.17",
              "v249Results" : [
                      100,
                      102,
                      101,
                      99,
                      100,
                      99
              ],
              "rc0Avg" : "841.17",
              "rc0Results" : [
                      870,
                      882,
                      839,
                      817,
                      815,
                      824
              ],
              "testId" : "GeneratedTests::GeneratedQuery750[Descending]",
              "qry" : "{ query: { _id: //Big(ww)i// }, orderby: { a: 1 } }",
              "proj" : "{ NONE: 1 }",
              "idx" : "Descending",
              "rc0Count" : 0,
              "v249Count" : 0,
              "countsMatch" : true
      }
      

      I'm having a tough time distilling this down to something which can be reproduced in the shell (with measurable results).

        1. QA401.export
          120 kB
          Ben Becker
        2. SERVER12952.cpp
          169 kB
          Ben Becker
        3. SERVER-12952-master-with-patches.svg
          106 kB
          Ben Becker
        4. SERVER12952-v249.svg
          99 kB
          Ben Becker

            Assignee:
            benety.goh@mongodb.com Benety Goh
            Reporter:
            benjamin.becker Ben Becker
            Votes:
            1 Vote for this issue
            Watchers:
            5 Start watching this issue

              Created:
              Updated:
              Resolved: