> Even with indexes, joins are costly, especially under load at scale with millions of simultaneous users. You can avoid a lot of this by simply having that sub-table information inside a JSON field with the row in question.

They’re really not that bad. Even on large-ish tables (hundreds of millions of rows), the typical query time I see for a query with 1-2 inner joins is 1-2 msec. That can of course vary with result set size, but in general it’s going to be dwarfed by network RTT.

If you’ve tested your schema with normalized and denormalized versions and found a significant difference that justifies it, by all means, but IME query speed for any non-trivial query is generally dominated by query shape, index design, and schema design (specifically for clustering indices, not taking advantage of it to have physical and logical tuple correlation).