using mongodb case insentive regex with case insentive index - regex

Question

is mongo regex ignoring my index? I have a case insentive index, but by the look of things my regex search recognize it and ignores it.
db.getCollection("myCol").find({ value: /^mysearchVal/i }}).explain(...)
I have 95, 708 docs total.
output:
{
"queryPlanner": {
"plannerVersion": 1,
"namespace": "myDb.myCol",
"indexFilterSet": false,
"parsedQuery": {
"Value": {
"$regex": "^mysearchVal",
"$options": "i"
}
},
"winningPlan": {
"stage": "FETCH",
"filter": {
"Value": {
"$regex": "^mysearchVal",
"$options": "i"
}
},
"inputStage": {
"stage": "IXSCAN",
"keyPattern": {
"Value": 1
},
"indexName": "value_case_insensitive_and_unique",
"collation": {
"locale": "en",
"caseLevel": false,
"caseFirst": "off",
"strength": 2,
"numericOrdering": false,
"alternate": "non-ignorable",
"maxVariable": "punct",
"normalization": false,
"backwards": false,
"version": "57.1"
},
"isMultiKey": false,
"multiKeyPaths": {
"Value": []
},
"isUnique": true,
"isSparse": false,
"isPartial": false,
"indexVersion": 2,
"direction": "forward",
"indexBounds": {
"Value": [
"[\"\", {})",
"[/^mysearchVal/i, /^mysearchVal/i]"
]
}
}
},
"rejectedPlans": []
},
"executionStats": {
"executionSuccess": true,
"nReturned": 1,
"executionTimeMillis": 1447,
"totalKeysExamined": 95708,
"totalDocsExamined": 95708,
"executionStages": {
"stage": "FETCH",
"filter": {
"Value": {
"$regex": "^mysearchVal",
"$options": "i"
}
},
"nReturned": 1,
"executionTimeMillisEstimate": 1270,
"works": 95709,
"advanced": 1,
"needTime": 95707,
"needYield": 0,
"saveState": 785,
"restoreState": 785,
"isEOF": 1,
"invalidates": 0,
"docsExamined": 95708,
"alreadyHasObj": 0,
"inputStage": {
"stage": "IXSCAN",
"nReturned": 95708,
"executionTimeMillisEstimate": 596,
"works": 95709,
"advanced": 95708,
"needTime": 0,
"needYield": 0,
"saveState": 785,
"restoreState": 785,
"isEOF": 1,
"invalidates": 0,
"keyPattern": {
"Value": 1
},
"indexName": "value_case_insensitive_and_unique",
"collation": {
"locale": "en",
"caseLevel": false,
"caseFirst": "off",
"strength": 2,
"numericOrdering": false,
"alternate": "non-ignorable",
"maxVariable": "punct",
"normalization": false,
"backwards": false,
"version": "57.1"
},
"isMultiKey": false,
"multiKeyPaths": {
"Value": []
},
"isUnique": true,
"isSparse": false,
"isPartial": false,
"indexVersion": 2,
"direction": "forward",
"indexBounds": {
"Value": [
"[\"\", {})",
"[/^mysearchVal/i, /^mysearchVal/i]"
]
},
"keysExamined": 95708,
"seeks": 1,
"dupsTested": 0,
"dupsDropped": 0,
"seenInvalidated": 0
}
},
"allPlansExecution": []
},
"ok": 1.0
}
the output shows 95,708 keys and docs examined, 1 doc returned. really? did the index apply in this case or am I missing a point or two?

Answer 1

Case insensitive regular expression queries generally cannot use
indexes effectively. The $regex implementation is not collation-aware
and is unable to utilize case-insensitive indexes.
https://docs.mongodb.com/manual/reference/operator/query/regex/#index-use