A few million rows should take at most, on the most awful networked storage available, maybe 10 seconds. I just built an index locally on 10,000,000 rows in 4 seconds. Moreover, though, there are vanishingly few cases where you wouldn't want to use CONCURRENTLY in prod - you shouldn't need to run a test to tell you that.
IMO branching can be a cool feature, but the use I keep seeing touted (indexes) doesn't seem like a good one for it. You should have a pretty good idea how an index is going to behave before you build it, just from understanding the RDBMS. There are also tools like hypopg [0], which are also available on cloud providers.
A better example would be showing testing a large schema change, like normalizing a JSON blob into proper columns or something, where you need to validate performance before committing to it.
Looking at Xata’s technical deep dive, the site claims that we need an additional Postgres instance per replica and proposes a network file system to work around that. But I don’t really understand why that’s needed. Can someone explain to me my misunderstanding here?
At the same time Postgres people don't seem comfortable with the idea in practice so I'm not sure if this is actually ok to do.
Not disputing that Oracle might have had something like this built-in, but it sounds like something that I could have whipped up in a day or so as a custom solution. I actually proposed a similar system to create anonymized datasets for researchers when I worked at a national archive institute.
I actually built my own immutable database which does support branching (see profile), so it seems like a huge miss that these ones don't. It's pretty much the main reason I would want an immutable database.
That said, I’m adding xitdb to the list of tech to try out. Thank you for building it!
Oh, and thanks for linking to my article :-)