Hrmm I just grabbed the stack most familiar with. Node/express postgres.
Believe it or not I've actually never worked with MongoDB. I've done a lot of AWS dynamodb development though are they similar?
The npub data should be the same as a uuid. To your database it shouldnt make a difference but according to nip-1 of the protocol it should be HEX.
“pubkey": <32-bytes lowercase hex-encoded public key of the event creator>
https://github.com/nostr-protocol/nips/blob/master/01.md#events-and-signatures
I would imagine a native json db like mongodb or duckdb would have better performance than postgres
Discussion
Yeah dynamodb is similar to mongodb. Honestly, if you have experience with dynamodb then you should be good.
I find dynamo quirky and a pain to use their api/ db interface