https://postgrespro.ru/docs/postgresql/16/transaction-iso

mainly relevant to lock-based DB impls (while PG is an MVCC-one)

read uncommitted
    not supported by PG
read committed (default)
    (the same 2 selects can bring distinct results)
repeatable read
serializable
    (as if all transactions are chained one-after-another)

show default_transaction_isolation;
    default_transaction_isolation
    -----------------------------
    read committed

at distinct level we use distinct snapshoting strategies ...

2024
https://habr.com/ru/articles/845522/
https://habr.com/ru/articles/815323/