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Comment by sascha_sl | Hacker News Reader

If you just need a key-value store, a database like MongoDB is rather oversized. Redis or and a bunch of object storage system (now that they actually include locking) are much easier to operate.

As soon as you're back to needing complex queries (with or without relations, Postgres it is again.

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Redis is an in-memory datastore first and foremost (with optional persistence). With that said, it's probably not the right tool if you need to store and persist TB-worth of data even if it is key-value. Something like Cassandra is probably a better fit if you have a lot of non-relational data that needs to be persisted and you need it to scale.

But you need more than a few TiB of stored data for Cassandra to be a net win. I've been part of two inhouse software projects with database in the single digit TiB scale based on Cassandra. Both projects where switched to PostgreSQL and the performance improved for almost everything. Plus all the other advantages of mature relational database. Hosting Cassandra is a whole different league of complication compared to PostgreSQL. So you should be very sure that you actually gain something before choosing it over PostgreSQL.

Absolutely - pick the right tool for the job. Generally speaking I'd go for PostgreSQL in most cases but there are definitely scenarios where Cassandra would perform better and be more appropriate.

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