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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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None
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Query Optimization
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If a specific index exists it is probably always better to use than the wildcard index. If not, we should probably warn or maybe even disallow adding the otherwise redundant index.
One way to do this would be to model the wildcard index purely as a fallback in the QueryPlanner so that when it would go to use an index on a field but there is no index it can use, it will instead make a wildcard scan as-if there was that index on that field. I think that would also solve SERVER-53927
> db.foo.ensureIndex({'$**': 1})
{
"createdCollectionAutomatically" : true,
"numIndexesBefore" : 1,
"numIndexesAfter" : 2,
"ok" : 1
}
> db.foo.ensureIndex({a: 1})
{
"createdCollectionAutomatically" : false,
"numIndexesBefore" : 2,
"numIndexesAfter" : 3,
"ok" : 1
}
> db.foo.find({a:1}).explain()
{
"queryPlanner" : {
"plannerVersion" : 1,
"namespace" : "test.foo",
"indexFilterSet" : false,
"parsedQuery" : {
"a" : {
"$eq" : 1
}
},
"queryHash" : "4B53BE76",
"planCacheKey" : "4281AFAD",
"winningPlan" : {
"stage" : "FETCH",
"inputStage" : {
"stage" : "IXSCAN",
"keyPattern" : {
"$_path" : 1,
"a" : 1
},
"indexName" : "$**_1",
"isMultiKey" : false,
"multiKeyPaths" : {
"$_path" : [ ],
"a" : [ ]
},
"isUnique" : false,
"isSparse" : false,
"isPartial" : false,
"indexVersion" : 2,
"direction" : "forward",
"indexBounds" : {
"$_path" : [
"[\"a\", \"a\"]"
],
"a" : [
"[1.0, 1.0]"
]
}
}
},
"rejectedPlans" : [
{
"stage" : "FETCH",
"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]"
]
}
}
}
]
},
"serverInfo" : {
"host" : "silversurfer-wsl",
"port" : 27017,
"version" : "4.4.3",
"gitVersion" : "913d6b62acfbb344dde1b116f4161360acd8fd13"
},
"ok" : 1
}