Details
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Type:
Improvement
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Status: Closed
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Priority:
Major - P3
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Resolution: Duplicate
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Affects Version/s: None
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Fix Version/s: None
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Component/s: Querying
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Labels:None
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Case:
Description
Some query shape optimizations are not reflected in an obvious way by the cached plans. For example, the parser appears to transform a single value $in list into an equality when parsing the query:
> c.find({x:{$in:[3]}}).explain().queryPlanner.parsedQuery
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{ "x" : { "$eq" : 3 } }
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> c.find({x:{$in:[6,7]}}).explain().queryPlanner.parsedQuery
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{ "x" : { "$in" : [ 6, 7 ] } }
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However, the shape reported by the plan cache does not visually reflect those optimizations. This means that the following situations can occur:
- Two entries will appear to have the same shape - one with a single value for the $in list and the other with a larger number of values.
- A single entry will be present reflecting both a direct equality query and an optimized single value $in list query. The representation will depend on variant of the shape present at the time it is cached.
> db.version()
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3.6.2
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> c.getPlanCache().listQueryShapes()
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[ ]
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> c.find({x:{$in:[3]}})
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> c.getPlanCache().listQueryShapes()
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[
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{
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"query" : {
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"x" : {
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"$in" : [
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3
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]
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}
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},
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"sort" : {
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},
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"projection" : {
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}
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}
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]
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> c.find({x:5})
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> c.getPlanCache().listQueryShapes()
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[
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{
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"query" : {
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"x" : {
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"$in" : [
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3
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]
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}
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},
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"sort" : {
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},
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"projection" : {
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}
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}
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]
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> c.find({x:{$in:[6,7]}})
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> c.getPlanCache().listQueryShapes()
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[
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{
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"query" : {
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"x" : {
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"$in" : [
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6,
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7
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]
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}
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},
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"sort" : {
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},
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"projection" : {
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}
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},
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{
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"query" : {
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"x" : {
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"$in" : [
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3
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]
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}
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},
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"sort" : {
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},
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"projection" : {
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}
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}
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]
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> c.getPlanCache().clear()
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> c.getPlanCache().listQueryShapes()
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[ ]
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> c.find({x:5})
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> c.getPlanCache().listQueryShapes()
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[
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{
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"query" : {
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"x" : 5
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},
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"sort" : {
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},
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"projection" : {
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}
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}
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]
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> c.find({x:{$in:[3]}})
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> c.getPlanCache().listQueryShapes()
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[
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{
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"query" : {
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"x" : 5
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},
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"sort" : {
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},
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"projection" : {
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}
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}
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]
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> c.find({x:{$in:[6,7]}})
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> c.getPlanCache().listQueryShapes()
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[
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{
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"query" : {
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"x" : {
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"$in" : [
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6,
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7
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]
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}
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},
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"sort" : {
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},
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"projection" : {
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}
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},
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{
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"query" : {
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"x" : 5
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},
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"sort" : {
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},
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"projection" : {
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}
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}
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]
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For completeness for this specific example, other changes in the length of the $in list appear to have the same query shape as intuitively expected:
>c.getPlanCache().clear()
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> c.find({x:{$in:[6,7]}})
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> c.getPlanCache().listQueryShapes()
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[
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{
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"query" : {
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"x" : {
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"$in" : [
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6,
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7
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]
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}
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},
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"sort" : {
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},
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"projection" : {
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}
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}
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]
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> c.find({x:{$in:[8,9,10]}})
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> c.getPlanCache().listQueryShapes()
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[
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{
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"query" : {
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"x" : {
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"$in" : [
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6,
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7
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]
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}
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},
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"sort" : {
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},
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"projection" : {
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}
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}
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]
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Attachments
Issue Links
- duplicates
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- Closed
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- related to
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- Closed
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