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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: Aggregation Framework, Querying
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Labels:
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Query Optimization
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(copied to CRM)
Imagine you have an aggregation pipeline that looks something like:
db.collection.aggregate([
{$facet: {
xs: [
{$match: {x: "SOMETHING"}},
{$group: {...}}
],
ys: [
{$match: {y: "SOMETHING ELSE"}},
{$unwind: "$y"},
...
]
}}
])
Such a pipeline would not provide any predicates for the query system to use to begin the pipeline. This pipeline might benefit from being optimized to something more like:
db.collection.aggregate([
{$match: {$or: [{x: "SOMETHING"}, {y: "SOMETHING ELSE"}]}},
{$facet: {
xs: [
{$match: {x: "SOMETHING"}},
{$group: {...}}
],
ys: [
{$match: {y: "SOMETHING ELSE"}},
{$unwind: "$y"},
...
]
}}
])
Original Description
$facet is not smart enough to use an index. if the only stage in a pipeline is $facet, the planning system will simply default to a COLLSCAN.
- is related to
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SERVER-34428 $facet aggregation stage does not use index created for the sub pipelines in it and perform COLLSCAN instead.
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- Closed
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- related to
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DOCS-13879 Document that $facet cannot use indexes
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- Closed
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