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  1. Core Server
  2. SERVER-13789

Some nested queries trigger an assertion error

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    Details

    • Type: Bug
    • Status: Closed
    • Priority: Major - P3
    • Resolution: Fixed
    • Affects Version/s: 2.6.0, 2.6.1
    • Fix Version/s: 2.6.2, 2.7.1
    • Component/s: Querying
    • Labels:
      None
    • Operating System:
      ALL
    • Backport Completed:
    • Steps To Reproduce:
      Hide

      in mongo shell:

      > use test
      > db.test.ensureIndex({'bar.baz': 1})
      > db.test.find({'foo': 'bar', '$and': [{'bar': {'$elemMatch': {'$or': [{'baz': 'foo'}]}}}]})
      error: { "$err" : "assertion src/mongo/db/query/planner_access.cpp:851" }
       
      > db.serverBuildInfo()
      {
      	"version" : "2.6.1-rc1-pre-",
      	"gitVersion" : "116d7d799a6edea172e612767d0605a913f7a9f5",
      	"OpenSSLVersion" : "",
      	"sysInfo" : "Darwin mci-osx108-5.build.10gen.cc 12.5.0 Darwin Kernel Version 12.5.0: Sun Sep 29 13:33:47 PDT 2013; root:xnu-2050.48.12~1/RELEASE_X86_64 x86_64 BOOST_LIB_VERSION=1_49",
      	"loaderFlags" : "-fPIC -pthread -Wl,-bind_at_load -mmacosx-version-min=10.6",
      	"compilerFlags" : "-Wnon-virtual-dtor -Woverloaded-virtual -fPIC -fno-strict-aliasing -ggdb -pthread -Wall -Wsign-compare -Wno-unknown-pragmas -Winvalid-pch -Werror -pipe -O3 -Wno-unused-function -Wno-deprecated-declarations -mmacosx-version-min=10.6",
      	"allocator" : "system",
      	"versionArray" : [
      		2,
      		6,
      		1,
      		-9
      	],
      	"javascriptEngine" : "V8",
      	"bits" : 64,
      	"debug" : false,
      	"maxBsonObjectSize" : 16777216,
      	"ok" : 1
      }

      Show
      in mongo shell: > use test > db.test.ensureIndex({'bar.baz': 1}) > db.test.find({'foo': 'bar', '$and': [{'bar': {'$elemMatch': {'$or': [{'baz': 'foo'}]}}}]}) error: { "$err" : "assertion src/mongo/db/query/planner_access.cpp:851" }   > db.serverBuildInfo() { "version" : "2.6.1-rc1-pre-", "gitVersion" : "116d7d799a6edea172e612767d0605a913f7a9f5", "OpenSSLVersion" : "", "sysInfo" : "Darwin mci-osx108-5.build.10gen.cc 12.5.0 Darwin Kernel Version 12.5.0: Sun Sep 29 13:33:47 PDT 2013; root:xnu-2050.48.12~1/RELEASE_X86_64 x86_64 BOOST_LIB_VERSION=1_49", "loaderFlags" : "-fPIC -pthread -Wl,-bind_at_load -mmacosx-version-min=10.6", "compilerFlags" : "-Wnon-virtual-dtor -Woverloaded-virtual -fPIC -fno-strict-aliasing -ggdb -pthread -Wall -Wsign-compare -Wno-unknown-pragmas -Winvalid-pch -Werror -pipe -O3 -Wno-unused-function -Wno-deprecated-declarations -mmacosx-version-min=10.6", "allocator" : "system", "versionArray" : [ 2, 6, 1, -9 ], "javascriptEngine" : "V8", "bits" : 64, "debug" : false, "maxBsonObjectSize" : 16777216, "ok" : 1 }

      Description

      Issue Status as of May 16, 2014

      ISSUE SUMMARY
      Queries which have the format below fail when there's an index reaching into the deepest subfield (y.z). The format is triggered by an $and operator that contains an $elemMatch with any indexed logical operator below it. This includes indexed $or, $not, and $ne below $elemMatch. For example:

      db.test.find({x: 1, $and: [{y: {'$elemMatch': {'$or': [{z: 3}]}}}]})

      USER IMPACT
      Queries of this type will fail with an assertion error.

      WORKAROUNDS
      Rewrite the query if possible to avoid $elemMatch over the logical operator (e.g.: $or):

      db.test.find({x: 1, $and: [{y: {'$elemMatch': {'$or': [{z: 3}, {z:4}]}}}]})

      can be rewritten as:

      db.test.find({x: 1, $and: [{$or: [{"y.z": 3}, {"y.z":4}]}]})

      AFFECTED VERSIONS
      MongoDB production releases 2.6.0 and 2.6.1 are affected.

      FIX VERSION
      The fix is included in the 2.6.2 production release.

      RESOLUTION DETAILS
      The special $elemMatch object handling now accounts for predicates tagged by the enumerator that are below non-bounds generating MatchExpression nodes. Previously, this code skipped these tagged predicates, producing zero index scan leafs as a result (causing us to hit the assertion which makes sure there is at least one index scan).

      Original description.

      We tried to upgrade to MongoDB 2.6 but encountered some queries that would not function anymore.

      I isolated a minimal test case that triggers an assertion error in the query planner. The error occurs on 2.6.0 (tested on OSX and Linux 64bit) and latest nightly build (mongodb-osx-x86_64-116d7d799a6edea172e612767d0605a913f7a9f5-2014-04-22).

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                • Created:
                  Updated:
                  Resolved: