Find slow queries in mongo DB

README.md

A few show tricks to find slow queries in mongodb

Enable profiling

First, you have to enable profiling

> db.setProfilingLevel(1)

Now let it run for a while. It collects the slow queries ( > 100ms) into a capped collections, so queries go in and if it's full, old queries go out, so don't be surprised that it's a moving target...

Histogram of the slowest collections (collections with the slowest queries) - number of queries per collection

This presents a histogram of slow collections

> db.system.profile.group({key: {ns: true}, initial: {count: 0}, reduce: function(obj,prev){ prev.count++;}})

Histogram of the slowest collections (collections with the slowest queries) - number of millies spent in each collection

> db.system.profile.group({key: {ns: true}, initial: {millis: 0}, reduce: function(obj, prev){ prev.millis += obj.millis;}})

Find the most recent slow query

> db.system.profile.find().sort({$natural: -1}).limit(1)

Find the single slowest query in the capped system.profile collection right now

> db.system.profile.find().sort({millis: -1}).limit(1)

db.collection.group() has been deprecated since Mongo 3.4 so db.system.profile.group queries won't work if you have newer version.
You have to use aggregate operations, for instance

db.system.profile.aggregate(
{ $group : {
_id :"$op",
count:{$sum:1},
"max response time":{$max:"$millis"},
"avg response time":{$avg:"$millis"}
}});