How Mongo Query planner choose his index - mongodb

Question

Mongo version : 3.4.3-10-g865d2fb
I have a request like this :
db.getCollection('c_zop_operations').find({
"a":{
"$in":[
"O",
"S",
"P"
]
},
"$or":[
{
"b":"008091",
"c":"1187",
"d":"F",
"e":ISODate("2018-07-22T22:00:00.000Z")
},
... x 39 elements in $or statement
]
}).explain("executionStats")
The request during 16 seconds and explain returns these results :
155769 documents parse in index !!!
{
"queryPlanner" : {
"plannerVersion" : 1,
...
"indexFilterSet" : false,
"parsedQuery" : {
...
},
"winningPlan" : {
"stage" : "FETCH",
"filter" : {
"$or" : [
{
"$and" : [
{
"c" : {
"$eq" : "1187"
}
},
{
"d" : {
"$eq" : "F"
}
},
{
"b" : {
"$eq" : "008091"
}
},
{
"e" : {
"$eq" : ISODate("2018-07-22T22:00:00.000Z")
}
}
]
},
x 39 times
...
},
"inputStage" : {
"stage" : "IXSCAN",
"keyPattern" : {
"a" : 1
},
"indexName" : "a",
"isMultiKey" : false,
"isUnique" : false,
"isSparse" : false,
"isPartial" : false,
"indexVersion" : 1,
"direction" : "forward",
"indexBounds" : {
"a" : [
"[\"O\", \"O\"]",
"[\"P\", \"P\"]",
"[\"S\", \"S\"]"
]
}
}
},
"rejectedPlans" : [
...
]
},
"executionStats" : {
"executionSuccess" : true,
"nReturned" : 0,
"executionTimeMillis" : 16010,
"totalKeysExamined" : 155769,
"totalDocsExamined" : 155769,
...
}
...
}
In my collection, I have a lot of indexes (65) and my collection contains 3 millions of documents.
Only two indexes interest me here :
aIndex : { "a" : 1 }
And
beIndex: { "b" : 1, "e" : 1 }
By default, mongo use { "a" : 1 } and the request takes 16 seconds.
If I use hint(beIndex), the request takes 0,011 second and totalKeysExamined = 0 and totalDocsExamined = 0.
Why MongoDB don't use the beIndex that is more effective ?

Answer 1

This behavior is a known bug. See SERVER-13732. This was fixed in MongoDB 3.6.
As a workaround, distributing the top-level filter on a into each $or clause should allow the query planner to make a better index choice.