Query plan with stats

Show docs examined vs returned and index usage; COLLSCAN means missing index

mongosh --eval 'db.<name>.find({status:"active"}).explain("executionStats").executionStats'

More mongodb commands

Unused index detection

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

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