That was the section I stalled at --- the author doesn't explain why the transactions don't help, just states that they don't, and provides a diagram with no co

That was the section I stalled at --- the author doesn't explain why the transactions don't help, just states that they don't, and provides a diagram with no context. I don't understand what they're getting at here. Surely as soon as one transaction tries to modify the database, it'll lock out all other reads to the affected rows from other transactions, so as to prevent them observing changed data until the transaction's committed? So avoiding the entire problem? Isn't this the entire reason transactions exist? Edit: I haven't actually used PostgreSQL; I grew up on SQLite. I see from the docs that SQLite transactions are all serializable, which from the linked article seems to be what you get with PostgreSQL's BEGIN ISOLATION LEVEL SERIALIZABLE. So... if by default PostreSQL's transactions aren't like this, what are they? If they're not serializable, then aren't they simply broken, i.e. not enforcing the transactions properly?