[saurik]

If you implement ACID using the "normal" locking semantics (such as the ones the SQL standard authors who defined the isolation levels used in the language were assuming) you can tell the difference because old values are not preserved.

Instead, we would have had contention: in the case of my example walkthrough, to implement the repeatable read semantics that I requested, the first connection would have taken a share lock on the rows it queried, causing the second connection to block on the update until after the first connection committed.

This means that you would not have been able to have the semantics where the first connect and the second connection were seeing different things at the same time (which, to be again clear, is due to none of the data being destroyed: MVCC is providing the semantics of a snapshot).

(As for your text analytics work, I am curious: are you using gin and trigrams at all? There are a bunch of things I dislike about PostgreSQL's implementation of both, but if you haven't looked at them yet you really should: if your use case fits into them they are amazing, and if not the entire point of PostgreSQL is to let you build your own data types and indexes using the ones it comes with as examples.)

[fauigerzigerk]

I don't use gin or trigrams because I don't do much general purpose text search. I do things like named entity recognition, anaphora resolution, collecting statistics about the usage of terms over time, etc.

But you're right, it might be a good idea to look into the postgres extension mechanism. I've never seriously done that.