Что означает уровень изоляции транзакций, установленный по умолчанию в базе данных?

The transaction isolation level determines how much a transaction is isolated from modifications made by other transactions. The default isolation level depends on the specific DBMS, but most commonly it is "Read Committed".

This means that a transaction only sees data that has been committed by other transactions and does not see intermediate uncommitted changes. This level prevents "dirty reads" but allows "non-repeatable reads" and "phantom reads".

For example, in PostgreSQL, the default isolation level is Read Committed.