Github used to publish some pretty interesting postmortems. Maybe they still do. IIRC that they were struggling with scaling their SQL db and were starting to hit the limits. It's a tough position to be in because you have to either to a massive migration to a data layer with much different semantics, or you have to keep desperately squeezing performance and skirting on the edge of outages with a DB that wasn't really meant to handle what you're doing with it now. The OpenAI blog post on "scaling" Postgres to their current scale has much the same flavor, although I think they're doing it better than Github appears to be doing.