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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: 7.3.0-rc1
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Component/s: None
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None
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Query Execution
Experiment:
Collection0 contains 10,000 docs of the form:
> db.Collection0.find().limit(3)
{ "_id" : ObjectId("65badbe9a940789d3800c3c1"), "arr" : [ 5, 6, 7, 0, 4 ] }
{ "_id" : ObjectId("65badbe9a940789d3800c3c2"), "arr" : [ 8, 4, 3, 7, 0 ] }
{ "_id" : ObjectId("65badbe9a940789d3800c3c3"), "arr" : [ 4, 6, 5, 5, 0 ] }
Query:
db.Collection0.aggregate([
{"$unwind": {"path": "$arr"}},
{"$group": {"_id": "$arr"}}
]);
Per explain execution stats, this runs in
- 58 msec - Classic
- 642 msec (11x as long) - SBE
Individual stage time estimates (e) and total actual times from db.Collection0.explain('executionStats'), all in milliseconds:
Classic millisecs per stage
| Cumul | Indiv | Stage |
| 58 | TOTAL (actual) | |
| 52e | 5e | $group |
| 47e | 7e | $unwind |
| 40e | 40e | $cursor |
SBE millisecs per stage
| Cumul | Indiv | Stage |
| 642 | TOTAL (actual) | |
| 640e | 0e | project |
| 640e | 540e | group |
| 100e | 10e | project |
| 90e | 41e | project |
| 49e | 0e | unwind |
| 49e | 0e | project |
| 49e | 30e | project |
| 19e | 19e | scan |
I will add Genny-based repro instructions in a comment.