Read with eventual consistency

This topic describes how to perform read operations with eventual consistency in OceanBase Database.

For applications that do not require real-time data, such as batch or statistical applications, you can configure weak consistency reads to avoid impacting the primary replica. You can configure session-level or weak consistency read hints to execute read operations on the standby replica:

• Add a hint to a SELECT statement

  SELECT /*+read_consistency(weak)*/ * FROM test WHERE c1=1;
  

• Set the system variable at the session level (1=FROZEN, 2=WEAK, 3=STRONG)

  SET @@ob_read_consistency= 2;
  

• Weak consistency reads are supported when the isolation level is set to Repeatable Read or Serializable. Example:

  obclient> SET transaction read only;
  Query OK, 0 rows affected (0.001 sec)
  obclient> SELECT /*+ read_consistency(weak)*/ * FROM t WHERE a=2;
  +------+------+
  | ID   | A    |
  +------+------+
  | NULL |    2 |
  +------+------+
  1 row in set (0.001 sec)