Is NoSQL faster than SQL?

Not as a category. A key-value read from Redis beats any SQL query, but a Postgres primary-key lookup takes a millisecond too. NoSQL stores win on horizontal write scaling and on the access patterns they were shaped for, and lose on ad hoc queries and multi-record transactions. Speed comparisons only mean something for one specific query at one specific scale.

Can I just use Postgres for JSON documents instead of MongoDB?

Often yes. Postgres jsonb columns store nested documents, support indexing into them, and you keep transactions and joins for everything else. Teams that want some schema flexibility inside a relational core usually do this. A dedicated document store starts making sense when nearly all data is document-shaped and write volume needs horizontal scaling.

How many databases is too many?

Count operational burdens, not technologies. Each store needs backups, monitoring, upgrades, and someone who understands its failure modes at 3am. Two or three with clear roles (truth, cache, search) is a normal production setup. If you cannot say in one sentence which store owns the truth for a given piece of data, that is the real problem.

Databaseschapter 1 of 5 · 4 lessons

SQL vs NoSQL

Choose based on access patterns, consistency needs, and query complexity, not hype.

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A team picks a database on a Tuesday afternoon, in about twenty minutes, mostly by asking which one people liked at their last job. Two years later that twenty minutes is the reason a reporting feature takes a quarter.

The choice deserves better, and it is not actually hard. It comes down to three questions with boring answers: what are the queries you will run most, how many writes per second do they add up to, and what should happen when two of them touch the same row at the same moment. Answer those and the database picks itself.

Lessons

4 in this chapter

What Relational Buys YouSchemas, transactions, and joins are features you miss most the day after you give them up.3 min
The NoSQL FamiliesNoSQL is not one thing. It is six different tools sharing a marketing label.3 min
How to Actually ChooseWrite down your top five queries first. The database falls out of the list.2 min
Polyglot PersistenceMost real systems run two or three stores, with exactly one of them holding the truth.3 min