Details
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Bug
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Status: Closed
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Major - P3
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Resolution: Duplicate
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2.6.0
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None
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None
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ALL
Description
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:
{
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"v" : 1,
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"key" : {
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"tourney" : 1,
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"tourneyRd" : 1
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},
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"ns" : "server.games",
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"name" : "tourney_1_tourneyRd_1",
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"sparse" : true
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}
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This query does a full table scan in MongoDB 2.6.0:
db.games.find({},{tourney:1}).sort({tourney:-1}).limit(1).explain()
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{
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"clauses" : [
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{
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"cursor" : "BasicCursor",
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"isMultiKey" : false,
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"n" : 1,
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"nscannedObjects" : 12619952,
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"nscanned" : 12619952,
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"scanAndOrder" : true,
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"indexOnly" : false,
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"nChunkSkips" : 0
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},
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{
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"cursor" : "BasicCursor",
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"isMultiKey" : false,
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"n" : 0,
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"nscannedObjects" : 0,
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"nscanned" : 0,
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"scanAndOrder" : true,
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"indexOnly" : false,
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"nChunkSkips" : 0
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}
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],
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"cursor" : "QueryOptimizerCursor",
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"n" : 1,
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"nscannedObjects" : 12619952,
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"nscanned" : 12619952,
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"nscannedObjectsAllPlans" : 12619952,
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"nscannedAllPlans" : 12619952,
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"scanAndOrder" : false,
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"nYields" : 98593,
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"nChunkSkips" : 0,
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"millis" : 77564,
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"server" : "test:27017",
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"filterSet" : false
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}
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Adding a hint does the correct query:
db.games.find({},{tourney:1}).sort({tourney:-1}).limit(1).hint({tourney:1,tourneyRd:1}).explain()
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{
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"cursor" : "BtreeCursor tourney_1_tourneyRd_1 reverse",
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"isMultiKey" : false,
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"n" : 1,
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"nscannedObjects" : 1,
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"nscanned" : 2,
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"nscannedObjectsAllPlans" : 1,
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"nscannedAllPlans" : 2,
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"scanAndOrder" : false,
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"indexOnly" : false,
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"nYields" : 0,
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"nChunkSkips" : 0,
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"millis" : 0,
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"indexBounds" : {
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"tourney" : [
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[
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{
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"$maxElement" : 1
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},
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{
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"$minElement" : 1
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}
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]
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],
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"tourneyRd" : [
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[
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{
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"$maxElement" : 1
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},
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{
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"$minElement" : 1
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}
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]
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]
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},
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"server" : "test:27017",
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"filterSet" : false
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}
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A work-around for me right now is to avoid the empty query with:
db.games.find({tourney:{$gt:0}},{tourney:1}).sort({tourney:-1}).limit(1).explain()
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{
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"cursor" : "BtreeCursor tourney_1_tourneyRd_1 reverse",
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"isMultiKey" : false,
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"n" : 1,
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"nscannedObjects" : 1,
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"nscanned" : 2,
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"nscannedObjectsAllPlans" : 1,
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"nscannedAllPlans" : 2,
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"scanAndOrder" : false,
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"indexOnly" : false,
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"nYields" : 0,
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"nChunkSkips" : 0,
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"millis" : 0,
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"indexBounds" : {
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"tourney" : [
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[
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Infinity,
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0
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]
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],
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"tourneyRd" : [
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[
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{
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"$maxElement" : 1
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},
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{
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"$minElement" : 1
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}
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]
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]
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},
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"server" : "test:27017",
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"filterSet" : false
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}
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Attachments
Issue Links
- duplicates
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- Closed
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