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> a transaction doesn't guarantee that all statements in it see the data at the same point in time

This depends on the transaction isolation level. If you use snapshot or serializable this should be the case (but you may have aborted transactions due to optimistic concurrency).

You are right, but note that the default isolation level is "Read committed" in postgres.

Doesn’t it guarantee consistency from the time the transaction started (assuming read committed isolation)? It guarantees you won’t see something “later” than when your transaction began.

https://www.postgresql.org/docs/current/transaction-iso.html

I’m likely misunderstanding what you mean by time.

Read committed (which is the default), doesn't guarantee that. See "Nonrepeatable Read" and "Phantom Read" which are both possible in your documentation page.

SSI scales fine for most workloads as long as you correctly mark your read-only transactions.