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Type:
Improvement
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Resolution: Unresolved
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Priority:
Minor - P4
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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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Due to e.g. SERVER-106829 the plan enumerator can generate the following two plans together:
- a stand-alone IXSCAN plan
- an AND_SORTED plan where the two children are two identical IXSCANs
Due to exponential back-off
the AND_SORTED plan will have a lower cost and win, causing double the index keys read when evaluating the query.
To reproduce:
1. Start the plan_stability test so that the dataset is populated:
buildscripts/resmoke.py run --installDir bazel-bin/install-dist-test/bin --suites=query_golden_classic '--mongodSetParameters={internalQueryFrameworkControl: forceClassicEngine, planRankerMode: samplingCE}' jstests/query_golden/plan_stability.js --pauseAfterPopulate
2. Wait until the following log line:
[js_test:plan_stability] [jsTest] ---- [js_test:plan_stability] [jsTest] TestData.pauseAfterPopulate is set. Pausing indefinitely ... [js_test:plan_stability] [jsTest] ----
3. Run the following manually:
pipeline = [{'$match': {'$and': [{'$or': [{'a_compound': {'$all': [8,
5,
2]}},
{'a_compound': {'$nin': [5,
14]}}]},
{'k_idx': {'$ne': 17}}],
'$or': [{'d_noidx': {'$gt': 11}},
{'$nor': [{'a_compound': {'$all': [6,
13]}},
{'a_noidx': {'$nin': [7,
6,
1,
6,
6]}},
{'d_compound': {'$nin': [7,
18]}}]},
{'a_idx': {'$gte': 11}}],
'a_compound': {'$all': [13, 13]}}}];
db.plan_stability.aggregate(pipeline).explain().queryPlanner.winningPlan;
and you will get the following winning plan:
{
isCached: false,
stage: 'FETCH',
costEstimate: 1.4775613933600216,
cardinalityEstimate: 33.96450911617378,
estimatesMetadata: { ceSource: 'Sampling' },
filter: {
'$and': [
{ a_compound: { '$eq': 13 } },
{ a_compound: { '$eq': 13 } },
{
'$or': [
{ '$and': [ [Object], [Object], [Object] ] },
{ a_compound: { '$not': [Object] } }
]
},
{
'$or': [
{ '$nor': [ [Object], [Object], [Object] ] },
{ a_idx: { '$gte': 11 } },
{ d_noidx: { '$gt': 11 } }
]
},
{ k_idx: { '$not': { '$eq': 17 } } }
]
},
inputStage: {
stage: 'AND_SORTED',
costEstimate: 1.2859183333333333,
cardinalityEstimate: 106.31465897496435,
estimatesMetadata: { ceSource: 'Sampling' },
inputStages: [
{
stage: 'IXSCAN',
costEstimate: 0.43457583333333333,
cardinalityEstimate: 1041.6666666666667,
numKeysEstimate: 1041.6666666666667,
estimatesMetadata: { ceSource: 'Sampling' },
keyPattern: { a_compound: 1 },
indexName: 'a_compound_1',
isMultiKey: true,
multiKeyPaths: { a_compound: [ 'a_compound' ] },
isUnique: false,
isSparse: false,
isPartial: false,
indexVersion: 2,
direction: 'forward',
indexBounds: { a_compound: [ '[13, 13]' ] }
},
{
stage: 'IXSCAN',
costEstimate: 0.43457583333333333,
cardinalityEstimate: 1041.6666666666667,
numKeysEstimate: 1041.6666666666667,
estimatesMetadata: { ceSource: 'Sampling' },
keyPattern: { a_compound: 1 },
indexName: 'a_compound_1',
isMultiKey: true,
multiKeyPaths: { a_compound: [ 'a_compound' ] },
isUnique: false,
isSparse: false,
isPartial: false,
indexVersion: 2,
direction: 'forward',
indexBounds: { a_compound: [ '[13, 13]' ] }
}
]
}
}
This plan will have two completely identical IXSCANS under AND_SORTED. It has a cost of 1.4775613933600216 . At the same time, the single IXSCAN plan:
nterprise test> db.plan_stability.aggregate(pipeline).explain().queryPlanner.rejectedPlans[10];
{
isCached: false,
stage: 'FETCH',
costEstimate: 2.2339512,
cardinalityEstimate: 1041.6666666666667,
estimatesMetadata: { ceSource: 'Sampling' },
filter: {
'$and': [
{
'$or': [
{ '$and': [ [Object], [Object], [Object] ] },
{ a_compound: { '$not': [Object] } }
]
},
{
'$or': [
{ '$nor': [ [Object], [Object], [Object] ] },
{ a_idx: { '$gte': 11 } },
{ d_noidx: { '$gt': 11 } }
]
},
{ k_idx: { '$not': { '$eq': 17 } } },
{ a_compound: { '$eq': 13 } }
]
},
inputStage: {
stage: 'IXSCAN',
costEstimate: 0.43457583333333333,
cardinalityEstimate: 1041.6666666666667,
numKeysEstimate: 1041.6666666666667,
estimatesMetadata: { ceSource: 'Sampling' },
keyPattern: { a_compound: 1 },
indexName: 'a_compound_1',
isMultiKey: true,
multiKeyPaths: { a_compound: [ 'a_compound' ] },
isUnique: false,
isSparse: false,
isPartial: false,
indexVersion: 2,
direction: 'forward',
indexBounds: { a_compound: [ '[13, 13]' ] }
}
}
has a cost of 2.2339512 and thus loses.
- depends on
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- Needs Scheduling
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- is related to
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SERVER-106829 Plan enumerator generates an AND_SORTED plan with two identical IXSCANs
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- Backlog
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- related to
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- Backlog
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