Check Mongo query performance

performance_check.js

/*
db.setProfilingLevel(2)     Needs to be ran first
  0 - Off
  1 - Only slow
  2 - all
  
This enables query profiling, after enabling profiling, run the application for a bit to get some data.
*/
db.getCollection('system.profile')
  .find({
    op: 'query',
    $and: [
      { ns: { $ne: 'DATABASE-NAME.system.profile' }},
      { ns: { $ne: 'DATABASE-NAME.system.namespaces' }},
      { ns: { $ne: 'DATABASE-NAME.system.indexes' }}
    ]}, 
    { ns: 1, query: 1, nscannedObjects: 1, nreturned: 1, ts: 1 }
  )
  .sort({ ts: -1 })

// Looking for nscannedObjects & nreturned to be 1:1, or close to it
// If the number of scans is larger than the number of returned, you may need an index

/*
 Response will be somethig like:
 
 {
    "ns" : "DATABASE-NAME.collection-name",
    "query" : {
        "orderby" : {
            "type" : 1
        },
        "$query" : {
            "is_deleted" : false
        }
    },
    "nscannedObjects" : 34,
    "nreturned" : 34,
    "ts" : ISODate("2016-10-26T14:39:46.504Z")
}
*/


// In later versions of Mongo (3.2 i believe), this query changes slightly.

db.getCollection('system.profile')
  .find({
    op: 'query',
    $and: [
      { ns: { $ne: 'DATABASE-NAME.system.profile' }},
      { ns:{ $ne: 'DATABASE-NAME.system.namespaces' }},
      { ns: { $ne: 'DATABASE-NAME.system.indexes' }}
    ]},
    { ns: 1, query: 1, keysExamined: 1, docsExamined: 1, nreturned: 1, ts: 1 }
  )
  .sort({ ts: -1 })

// And you are comparing docsExamined and nreturned. This can also be used with...

db.getCollection('COLLECTION_NAME').aggregate( [ { $indexStats: { } } ] )

// To determine if a current index is actually needed (Mongo 3.2) by looking at the 'ops' field in the returned value.