Overview

Isolation levels are defined based on phenomena that must be prevented during the concurrent execution of transactions.

The following phenomena are preventable:

• Dirty read: A transaction reads the data that has not been committed by other transactions.

• Non-repeatable read: When you query a row of data that you have read, you find that this row of data has been modified or deleted. For example, for select c2 from test where c1=1;, the value of c2 queried for the first time is 1. When you query c2 again, its value becomes 2 because other transactions have modified the value.

• Phantom read: A read-only request returns a group of rows that meet query conditions. The next time the request is executed, you find that other rows that meet the conditions have been inserted by a committed transaction.

Based on the preceding three phenomena, the ANSI and ISO/IEC standards define the following isolation levels:

• Read uncommitted

• Read committed

• Repeatable read

• Serializable

The following table compares the four isolation levels.

OceanBase Database supports the following isolation levels:

• Oracle mode

  • Read committed

  • Repeatable read

  • Serializable

• MySQL mode

  • Read committed

  • Repeatable read

  • Serializable

    It is similar to the serializable isolation level in Oracle Database, but is not strictly a serializable isolation level.

By default, OceanBase Database uses the read committed isolation level.