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As someone that fairly recently spent many months trying to shard a large mongodb system, this is patently false. Mongodb is not sharded “by default”, and sharding it is a lot of work and comes with a very heavy performance impact.

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absolutely true that this is an area of improvement for MongoDB - moving from a "replica set" (a single shard cluster but without the config server cluster) to a sharded cluster can be hard. Atlas does it for you, but even then it can be hard. My info says they are working very hard on this.

Live resharding indeed helps for a sharded cluster that already exists.

Yes you need to enable sharding but its provided for you, if you want to enable it is what I meant. And adding shards as well. All configuration options in Mongo - vs you need to do this in a more manual way in Postgres (and change your app code more)

Of course it haa a perf impact - nothing is free.

You can even do live resharding since 5.0.

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