The default isolation level for Oracle is read committed. This degree of isolation is
appropriate for environments where few transactions are likely to conflict. Oracle
causes each query to run with respect to its own materialized view time, thereby
permitting nonrepeatable reads and phantoms for multiple executions of a query, but
providing higher potential throughput. Read committed isolation is the appropriate
level of isolation for environments where few transactions are likely to conflict.

已提交读取隔离
1. 适合可能发生冲突的少量事务环境
2. 阻止不可重复读和幻象读取