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Type:
Bug
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Resolution: Duplicate
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Priority:
Major - P3
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None
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Affects Version/s: 2.6.0
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Component/s: Index Maintenance, Querying
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Labels:None
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ALL
I need to find the biggest value for the field "tourney" in the collection "games". I have added an appropriate index which was automatically used by all MongoDB versions prior to 2.6. In 2.6. MongoDB doesn't use the index but does a full table scan instead. Given the right hint the query uses the index and works as expected. Here are more details.
I have a sparse index in the "games" collection:
{
"v" : 1,
"key" : {
"tourney" : 1,
"tourneyRd" : 1
},
"ns" : "server.games",
"name" : "tourney_1_tourneyRd_1",
"sparse" : true
}
This query does a full table scan in MongoDB 2.6.0:
db.games.find({},{tourney:1}).sort({tourney:-1}).limit(1).explain()
{
"clauses" : [
{
"cursor" : "BasicCursor",
"isMultiKey" : false,
"n" : 1,
"nscannedObjects" : 12619952,
"nscanned" : 12619952,
"scanAndOrder" : true,
"indexOnly" : false,
"nChunkSkips" : 0
},
{
"cursor" : "BasicCursor",
"isMultiKey" : false,
"n" : 0,
"nscannedObjects" : 0,
"nscanned" : 0,
"scanAndOrder" : true,
"indexOnly" : false,
"nChunkSkips" : 0
}
],
"cursor" : "QueryOptimizerCursor",
"n" : 1,
"nscannedObjects" : 12619952,
"nscanned" : 12619952,
"nscannedObjectsAllPlans" : 12619952,
"nscannedAllPlans" : 12619952,
"scanAndOrder" : false,
"nYields" : 98593,
"nChunkSkips" : 0,
"millis" : 77564,
"server" : "test:27017",
"filterSet" : false
}
Adding a hint does the correct query:
db.games.find({},{tourney:1}).sort({tourney:-1}).limit(1).hint({tourney:1,tourneyRd:1}).explain()
{
"cursor" : "BtreeCursor tourney_1_tourneyRd_1 reverse",
"isMultiKey" : false,
"n" : 1,
"nscannedObjects" : 1,
"nscanned" : 2,
"nscannedObjectsAllPlans" : 1,
"nscannedAllPlans" : 2,
"scanAndOrder" : false,
"indexOnly" : false,
"nYields" : 0,
"nChunkSkips" : 0,
"millis" : 0,
"indexBounds" : {
"tourney" : [
[
{
"$maxElement" : 1
},
{
"$minElement" : 1
}
]
],
"tourneyRd" : [
[
{
"$maxElement" : 1
},
{
"$minElement" : 1
}
]
]
},
"server" : "test:27017",
"filterSet" : false
}
A work-around for me right now is to avoid the empty query with:
db.games.find({tourney:{$gt:0}},{tourney:1}).sort({tourney:-1}).limit(1).explain()
{
"cursor" : "BtreeCursor tourney_1_tourneyRd_1 reverse",
"isMultiKey" : false,
"n" : 1,
"nscannedObjects" : 1,
"nscanned" : 2,
"nscannedObjectsAllPlans" : 1,
"nscannedAllPlans" : 2,
"scanAndOrder" : false,
"indexOnly" : false,
"nYields" : 0,
"nChunkSkips" : 0,
"millis" : 0,
"indexBounds" : {
"tourney" : [
[
Infinity,
0
]
],
"tourneyRd" : [
[
{
"$maxElement" : 1
},
{
"$minElement" : 1
}
]
]
},
"server" : "test:27017",
"filterSet" : false
}
- duplicates
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- Closed
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