[dragonwriter]

> Who really does understand how a SQL engine work?

Presumably, at a minimum, all the people who work on such engines, including committees to the various open-source ones.

But also lots of other people.

> Don't you usually require to understand how something work before starting using it?

No. Very few programmers understand how compilers work before they start using them. I'd say it's more common to require working on something to really understand how it works than the reverse.

> I think SQL was designed when RAM was scarce and expensive, so to speed up data access, it has to be properly indexed with a database engine. I really wonder who, today, have data that cannot fit in RAM, apart from big actors.

Indexing is no less important for in-memory data access.