Unused index detection

Access count per index; zero ops = candidate to drop and reclaim write cost

mongosh --eval 'db.<name>.aggregate([{$indexStats:{}}]).forEach(i=>print(i.name,i.accesses.ops))'

More mongodb commands

Profile slow queries

mongosh --eval 'db.setProfilingLevel(1,{slowms:100}); db.system.profile.find().sort({millis:-1}).limit(5)'

Capture ops slower than 100ms then read top offenders from system.profile
Connect with mongosh

mongosh 'mongodb://user:pass@mongo:27017/dbname'

Open interactive shell to a MongoDB instance
Replica set status

mongosh --eval 'rs.status()' mongodb://mongo:27017

Show replica set members, lag, and election status
Database stats

mongosh --eval 'db.stats()' mongodb://mongo:27017/mydb

Data size, storage size, collection count for a database
Collection stats

mongosh --eval 'db.myCollection.stats()' mongodb://mongo:27017/mydb

Document count, index sizes, storage for a collection