Oracle 12.2 使用Database Link优化Standby Database Workload

从Oracle 12.2开始，可以通过在database_link_to参数来指定一个数据库链路来优化备库workload。由于安全原因，Oracle建议使用私有数据库链路。这个链路必须属于sys用户并且能让有权限的用户来访问。Oracle数据库包含一个缺省的权限用户名为SYS$UMF。

在主库中使用sys用户创建表t1与配置sys$umf用户，这个用户默认是被锁定的，所以需要先解锁这个用户并设置密码:

SQL> alter user sys$umf account unlock identified by xxzx7817600;

User altered.

SQL>create table t1 as select * from user_tables;
Table created.

SQL> create index idx_t1_tablename on t1(table_name);

Index created.

SQL> exec dbms_stats.gather_table_stats('sys','t1',degree => 4,cascade => true);

PL/SQL procedure successfully completed.

创建一个备库(cs_dg)到主库的链路，由于ADG是只读的，所以创建数据库链路操作都需要在主库中执行:

SQL> create database link cs_dg2_to_cs2 connect to sys$umf identified by abcd1234 using   'CS';

Database link created.

在备库中验证数据库链路是否创建成功

 col owner for a20
SQL> col object_name for a50
SQL> select owner,object_name from dba_objects where object_type='DATABASE LINK';

OWNER                OBJECT_NAME
-------------------- --------------------------------------------------
SYS                  CS_DG2_TO_CS2
SYS                  SYS_HUB

SQL>  select table_name from user_tables@cs_dg2_to_cs2;

no rows selected

SQL> select sysdate from dual@cs_dg2_to_cs2;

SYSDATE
------------
24-JAN-19

在备库中对表t1执行查询，database_link_to参数用来指定备库到主库的数据链路名cs_dg2_to_cs2

SQL> variable tname varchar2(30);
SQL> variable query varchar2(500);
SQL> exec :tname := 'jy_task';

PL/SQL procedure successfully completed.

SQL> exec :query := 'select /*+ full(t)*/ col1 from t1 t where table_name=''T1''';

PL/SQL procedure successfully completed.

SQL> begin
  2  :tname := dbms_sqltune.create_tuning_task
  3  (
  4  sql_text => :query,
  5  task_name => :tname,
  6  database_link_to => 'CS_DG2_TO_CS2',
  7  scope=>'COMPREHENSIVE',
  8  time_limit=>60,
  9  description=>'Task to tune a query_on adg');
 10  end;
 11  /

PL/SQL procedure successfully completed.

SQL> exec dbms_sqltune.execute_tuning_task(:tname);

PL/SQL procedure successfully completed.

SQL> select dbms_sqltune.report_tuning_task(:tname) from dual;

DBMS_SQLTUNE.REPORT_TUNING_TASK(:TNAME)
--------------------------------------------------------------------------------
GENERAL INFORMATION SECTION
----------------------------------------------------