Weak-consistency read

This topic describes how to perform weak-consistency reads in OceanBase Database.

For applications that do not require high real-time performance, such as batch processing and statistical analysis, you can enable weak-consistency reads to avoid affecting 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)