MongoDB not using my index(arrays, boolean) - mongodb

Question

I have collection full of the following documents, for example:
{
"_id" : ObjectId("2819738917238dgd21873"),
"mailIntid" : 10000000,
"mailCreated" : "2019-02-08",
"mailLastModified" : null,
"mailReceived" : "2019-02-08",
"mailSend" : "2019-02-08",
"hadAttachment" : false,
"subject" : "nieuwe vacature ",
"bodyPreview" : "Test Body",
"importance" : "normal",
"isDeliveryRequested" : null,
"isReadReceiptRequested" : false,
"isRead" : false,
"isDraft" : false,
"inferenceClassification" : "focused",
"bodyContentType" : "html",
"senderName" : "Jobs",
"senderEmail" : "noreply#test.nl",
"fromName" : "Jobs",
"fromEmail" : "noreply#mailing.test.nl",
"flagStatus" : "notFlagged",
"urls" : "https://test.nl/request-details?id=1337",
"insertDate" : "2019-02-09",
"modifiedDate" : "2019-05-05",
"parseComplete" : true,
"rawMailUrl" : [
{
"url" : "https://test.nl/request-details?id=1337",
"parsed" : false
}
]
}
I created a index with the following code:
db.testAB.createIndex(
{"rawMailUrl.parsed": 1},
{partialFilterExpression: { "rawMailUrl.parsed": false}}
)
But whenever I use the following query, it's not using the index from above.
db.testAB.find({"rawMailUrl.parsed": false})
Any ideas? Am I doing something wrong with creating a index with arrays and a true or false expression?

Answer 1

You defined an index for partialFilterExpression: { "rawMailUrl.parsed": false}, i.e. all records in your index have the same value false.
An index with low number of distinct values is a bad index, it does not improve the access path. Thus the index is not used, it is just a waste of disc space.