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Type:
Improvement
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Resolution: Unresolved
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Priority:
Major - P3
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Affects Version/s: None
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Component/s: None
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Query Optimization
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QO 2025-02-17
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In classic a class of queries could be simplified into a single interval scan. Instead the plan enumerator generates redundant index union plans. Some of the plans union index scans over the same field, while others also union index scans that produce no result.
Test cases:
const dataset = [ { "a" : 1, "b" : 1, "c" : 3 }, { "a" : 5, "b" : 2, "c" : 4 }, { "a" : 1, "b" : 1, "c" : 3 }, { "a" : 2, "b" : 3, "c" : 5 }, { "a" : 2, "b" : 2, "c" : 5 }, { "a" : 4, "b" : 3, "c" : 2 }, { "a" : 3, "b" : 2, "c" : 5 }, { "a" : 2, "b" : 2, "c" : 1 }, { "a" : 1, "b" : 5, "c" : 3 }, { "a" : 1, "b" : 3, "c" : 4 }, { "a" : 3, "b" : 3, "c" : 5 }, { "a" : 3, "b" : 1, "c" : 1 }, { "a" : 2, "b" : 2, "c" : 4 }, { "a" : 1, "b" : 2, "c" : 1 }, { "a" : 1, "b" : 5, "c" : 3 }, { "a" : 5, "b" : 2, "c" : 3 }, { "a" : 1, "b" : 1, "c" : 5 }, { "a" : 5, "b" : 5, "c" : 2 }, { "a" : 1, "b" : 3, "c" : 3 }, { "a" : 4, "b" : 2, "c" : 5 }, { "a" : 2, "b" : 4, "c" : 5 }, { "a" : 4, "b" : 2, "c" : 3 }, { "a" : 4, "b" : 5, "c" : 4 }, { "a" : 2, "b" : 3, "c" : 2 }, { "a" : 2, "b" : 1, "c" : 2 }, { "a" : 5, "b" : 4, "c" : 5 }, { "a" : 1, "b" : 5, "c" : 3 }, { "a" : 5, "b" : 2, "c" : 2 }, { "a" : 1, "b" : 1, "c" : 4 }, { "a" : 1, "b" : 3, "c" : 1 }, { "a" : 3, "b" : 3, "c" : 1 }, { "a" : 3, "b" : 4, "c" : 1 }, { "a" : 1, "b" : 4, "c" : 4 }]; db.foo.drop(); db.foo.insert(dataset); db.foo.createIndex({ "a" : 1}); const p1 = {$and : [{$or : [{a : {$gte : 5}}, {a : {$gte : 2}}]}, {$or : [{$and : [ {a : {$eq : 3}}]}, {$and : [{a : {$gte : 2}}, {a : {$eq : 4}}]}]}]}; const p1_cnf = { $and: [ { $or: [{ a: { $gte: 5 } }, { a: { $gte: 2 } }] }, { $or: [{ a: { $eq: 3 } }] }, { $or: [{ a: { $gte: 2 }, a: { $eq: 4 } }] } ] } const p1_dnf = { $or: [ { $and: [{ a: { $gte: 5 } }, { a: { $eq: 3 } }] }, { $and: [{ a: { $gte: 5 } }, { a: { $gte: 2 }, a: { $eq: 4 } }] }, { $and: [{ a: { $gte: 2 } }, { a: { $eq: 3 } }] }, { $and: [{ a: { $gte: 2 } }, { a: { $gte: 2 }, a: { $eq: 4 } }] } ] }
The first query is equivalent to {a: {$in: [3,4]}}, however the enumerator produces these plans:
winningPlan: {
isCached: false,
stage: 'FETCH',
filter: {
'$or': [ { a: { '$gte': 5 } }, { a: { '$gte': 2 } } ]
},
inputStage: {
stage: 'OR',
inputStages: [
{
stage: 'IXSCAN',
keyPattern: { a: 1 },
indexName: 'a_1',
isMultiKey: false,
multiKeyPaths: { a: [] },
isUnique: false,
isSparse: false,
isPartial: false,
indexVersion: 2,
direction: 'forward',
indexBounds: { a: [ '[4, 4]' ] }
},
{
stage: 'IXSCAN',
keyPattern: { a: 1 },
indexName: 'a_1',
isMultiKey: false,
multiKeyPaths: { a: [] },
isUnique: false,
isSparse: false,
isPartial: false,
indexVersion: 2,
direction: 'forward',
indexBounds: { a: [ '[3, 3]' ] }
}
]
}
},
rejectedPlans: [
{
isCached: false,
stage: 'FETCH',
filter: {
'$or': [
{
'$and': [ { a: { '$eq': 4 } }, { a: { '$gte': 2 } } ]
},
{ a: { '$eq': 3 } }
]
},
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: [ '[2, inf.0]' ] }
}
}
]
},
For the second query the enumerator does even worse job:
db.foo.find(p1_cnf).explain()
{
winningPlan: {
isCached: false,
stage: 'FETCH',
filter: {
'$or': [ { a: { '$gte': 5 } }, { a: { '$gte': 2 } } ]
},
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: [] }
}
},
rejectedPlans: [
{
isCached: false,
stage: 'FETCH',
inputStage: {
stage: 'OR',
inputStages: [
{
stage: 'IXSCAN',
keyPattern: { a: 1 },
indexName: 'a_1',
isMultiKey: false,
multiKeyPaths: { a: [] },
isUnique: false,
isSparse: false,
isPartial: false,
indexVersion: 2,
direction: 'forward',
indexBounds: { a: [] }
},
{
stage: 'IXSCAN',
keyPattern: { a: 1 },
indexName: 'a_1',
isMultiKey: false,
multiKeyPaths: { a: [] },
isUnique: false,
isSparse: false,
isPartial: false,
indexVersion: 2,
direction: 'forward',
indexBounds: { a: [] }
}
]
}
}
]
},
The third DNF form of the original query is has no better plan either:
winningPlan: {
isCached: false,
stage: 'SUBPLAN',
inputStage: {
stage: 'FETCH',
inputStage: {
stage: 'OR',
inputStages: [
{
stage: 'IXSCAN',
keyPattern: { a: 1 },
indexName: 'a_1',
isMultiKey: false,
multiKeyPaths: { a: [] },
isUnique: false,
isSparse: false,
isPartial: false,
indexVersion: 2,
direction: 'forward',
indexBounds: { a: [] }
},
{
stage: 'IXSCAN',
keyPattern: { a: 1 },
indexName: 'a_1',
isMultiKey: false,
multiKeyPaths: { a: [] },
isUnique: false,
isSparse: false,
isPartial: false,
indexVersion: 2,
direction: 'forward',
indexBounds: { a: [] }
},
{
stage: 'IXSCAN',
keyPattern: { a: 1 },
indexName: 'a_1',
isMultiKey: false,
multiKeyPaths: { a: [] },
isUnique: false,
isSparse: false,
isPartial: false,
indexVersion: 2,
direction: 'forward',
indexBounds: { a: [ '[3, 3]' ] }
},
{
stage: 'IXSCAN',
keyPattern: { a: 1 },
indexName: 'a_1',
isMultiKey: false,
multiKeyPaths: { a: [] },
isUnique: false,
isSparse: false,
isPartial: false,
indexVersion: 2,
direction: 'forward',
indexBounds: { a: [ '[4, 4]' ] }
}
]
}
}
},