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Type:
Improvement
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Resolution: Unresolved
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Priority:
Major - P3
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None
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Affects Version/s: None
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Component/s: Querying
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Query Optimization
Compare these two queries and their plans:
db.full.explain().count({a:{$eq:1}})
{
"queryPlanner" : {
"plannerVersion" : 1,
"namespace" : "test.full",
"indexFilterSet" : false,
"parsedQuery" : {
"a" : {
"$eq" : 1
}
},
"winningPlan" : {
"stage" : "COUNT",
"inputStage" : {
"stage" : "COUNT_SCAN",
"keyPattern" : {
"a" : 1
},
"indexName" : "a_1",
"isMultiKey" : false,
"multiKeyPaths" : {
"a" : [ ]
},
"isUnique" : false,
"isSparse" : false,
"isPartial" : false,
"indexVersion" : 2,
"indexBounds" : {
"startKey" : {
"a" : 1
},
"startKeyInclusive" : true,
"endKey" : {
"a" : 1
},
"endKeyInclusive" : true
}
}
},
"rejectedPlans" : [ ]
},
"serverInfo" : {
"host" : "Asyas-MacBook-Pro.local",
"port" : 27017,
"version" : "3.6.4",
"gitVersion" : "d0181a711f7e7f39e60b5aeb1dc7097bf6ae5856"
},
"ok" : 1
}
With equality on "a" we get best possible plan.
But adding a "redundant" condition on "a" that by itself cannot be fulfilled using the same index, we now revert to the "worse" plan with additional FETCH:
test@127.0.0.1:27017(3.6.4) > db.full.explain().count({a:{$exists:true,$eq:1}})
{
"queryPlanner" : {
"plannerVersion" : 1,
"namespace" : "test.full",
"indexFilterSet" : false,
"parsedQuery" : {
"$and" : [
{
"a" : {
"$eq" : 1
}
},
{
"a" : {
"$exists" : true
}
}
]
},
"winningPlan" : {
"stage" : "COUNT",
"inputStage" : {
"stage" : "FETCH",
"filter" : {
"a" : {
"$exists" : true
}
},
"inputStage" : {
"stage" : "IXSCAN",
"keyPattern" : {
"a" : 1
},
"indexName" : "a_1",
"isMultiKey" : false,
"multiKeyPaths" : {
"a" : [ ]
},
"isUnique" : false,
"isSparse" : false,
"isPartial" : false,
"indexVersion" : 2,
"direction" : "forward",
"indexBounds" : {
"a" : [
"[1.0, 1.0]"
]
}
}
}
},
"rejectedPlans" : [ ]
},
"serverInfo" : {
"host" : "Asyas-MacBook-Pro.local",
"port" : 27017,
"version" : "3.6.4",
"gitVersion" : "d0181a711f7e7f39e60b5aeb1dc7097bf6ae5856"
},
"ok" : 1
}
This feels conceptually maybe related to SERVER-12281 (and even SERVER-34119) but seemed like it might be an easier fix on its own.
Same thing happens if $eq:1 is and'ed with $ne:null for instance.
- is related to
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SERVER-81792 Optimize $exists and $ne: null in the boolean expression simplifier
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- Backlog
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- Closed
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