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Type:
Improvement
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Resolution: Gone away
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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: None
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None
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Query Optimization
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QO 2024-02-19, QO 2024-03-04, QO 2024-03-18
In the bestbuy high-value workload distinct_types_with_in_predicate-useAgg, we run an aggregation
db.products.aggregate([{$match: {type: {$in: ["Game", "Movie", "Music", "HardGood"]}}}, {$group: {_id: "$type"}}])
which results in a plan
stages: [
{
'$cursor': {
queryPlanner: {
namespace: 'bestbuy.products',
indexFilterSet: false,
parsedQuery: { type: { '$in': [ 'Game', 'HardGood', 'Movie', 'Music' ] } },
queryHash: '24FB8A6A',
planCacheKey: '6DBEACC2',
optimizationTimeMillis: 0,
maxIndexedOrSolutionsReached: false,
maxIndexedAndSolutionsReached: false,
maxScansToExplodeReached: false,
winningPlan: {
isCached: false,
stage: 'PROJECTION_COVERED',
transformBy: { type: 1, _id: 0 },
inputStage: {
stage: 'DISTINCT_SCAN',
keyPattern: { type: 1, subclass: 1, genre: 1, format: 1 },
indexName: 'type_1_subclass_1_genre_1_format_1',
isMultiKey: false,
multiKeyPaths: { type: [], subclass: [], genre: [], format: [] },
isUnique: false,
isSparse: false,
isPartial: false,
indexVersion: 2,
direction: 'forward',
indexBounds: {
type: [
'["Game", "Game"]',
'["HardGood", "HardGood"]',
'["Movie", "Movie"]',
'["Music", "Music"]'
],
subclass: [ '[MinKey, MaxKey]' ],
genre: [ '[MinKey, MaxKey]' ],
format: [ '[MinKey, MaxKey]' ]
}
}
},
rejectedPlans: []
}
}
},
{ '$groupByDistinctScan': { newRoot: { _id: '$type' } } }
],
However, there is a shorter index which can satisfy the query that the planner does not consider,
db.products.aggregate([{$match: {type: {$in: ["Game", "Movie", "Music", "HardGood"]}}}, {$group: {_id: "$type"}}], {hint: {type: 1, name: 1}})
stages: [
{
'$cursor': {
queryPlanner: {
namespace: 'bestbuy.products',
indexFilterSet: false,
parsedQuery: { type: { '$in': [ 'Game', 'HardGood', 'Movie', 'Music' ] } },
queryHash: '24FB8A6A',
planCacheKey: '6DBEACC2',
optimizationTimeMillis: 0,
maxIndexedOrSolutionsReached: false,
maxIndexedAndSolutionsReached: false,
maxScansToExplodeReached: false,
winningPlan: {
isCached: false,
stage: 'PROJECTION_COVERED',
transformBy: { type: 1, _id: 0 },
inputStage: {
stage: 'DISTINCT_SCAN',
keyPattern: { type: 1, name: 1 },
indexName: 'type_1_name_1',
isMultiKey: false,
multiKeyPaths: { type: [], name: [] },
isUnique: false,
isSparse: false,
isPartial: false,
indexVersion: 2,
direction: 'forward',
indexBounds: {
type: [
'["Game", "Game"]',
'["HardGood", "HardGood"]',
'["Movie", "Movie"]',
'["Music", "Music"]'
],
name: [ '[MinKey, MaxKey]' ]
}
}
},
rejectedPlans: []
}
}
},
{ '$groupByDistinctScan': { newRoot: { _id: '$type' } } }
],
We should use the shorter index in this case.