在Oracle中创建包含部分主键的实例化视图时，无法设置ON COMMIT刷新属性

Question

我需要将作为主键一部分的列的唯一值从表中提取到物化视图中。如果使用“刷新完成”，我可以创建物化视图，但在尝试使用“提交时快速刷新”时，我就没有机会创建物化视图。有没有人能指出我是否遗漏了什么或者Oracle不支持这样的操作。

下面列出了示例输出。谢谢。

SQL> create table TEST( col1 number, col2 number, col3 varchar(32), CONSTRAINT test_pk Primary Key (col1, col2));

Table created.

SQL> create materialized view test_mv build immediate refresh fast on commit as select distinct col2 from test;
create materialized view test_mv build immediate refresh fast on commit as select distinct col2 from test
                                                                                                     *
ERROR at line 1:
ORA-12054: cannot set the ON COMMIT refresh attribute for the materialized view


SQL> create materialized view test_mv build immediate refresh complete as select distinct col2 from test;

Materialized view created.

SQL> drop materialized view test_mv;

Materialized view dropped.

SQL> create materialized view log on test;

Materialized view log created.

SQL> create materialized view test_mv build immediate refresh fast on commit as select distinct col2 from test;
create materialized view test_mv build immediate refresh fast on commit as select distinct col2 from test
                                                                                                     *
ERROR at line 1:
ORA-12054: cannot set the ON COMMIT refresh attribute for the materialized view

Answer 1

您的观点的主要问题是DISTINCT子句。提交时，快速刷新对底层查询非常敏感。要支持快速刷新，实例化视图必须满足许多规则。DISTINCT可以防止这种情况。

可以使用DBMS_MVIEW.EXPLAIN_MVIEW过程检查实体化视图的功能：

DECLARE
    result SYS.EXPLAINMVARRAYTYPE := SYS.EXPLAINMVARRAYTYPE();
BEGIN
    DBMS_MVIEW.EXPLAIN_MVIEW('TEST_MV', result);

    FOR i IN result.FIRST..result.LAST LOOP
        DBMS_OUTPUT.PUT_LINE(result(i).CAPABILITY_NAME || ': ' || CASE WHEN result(i).POSSIBLE = 'T' THEN 'Yes' ELSE 'No' || CASE WHEN result(i).RELATED_TEXT IS NOT NULL THEN ' because of ' || result(i).RELATED_TEXT END || '; ' || result(i).MSGTXT END);
    END LOOP;
END;

您可以在documentation http://docs.oracle.com/cd/B28359_01/server.111/b28313/basicmv.htm#i1007007中找到更多信息

Answer 2

快速刷新视图是挑剔的。此解决方案需要具有特定属性的实体化视图日志，以及具有一些额外功能和不同语法的实体化视图。

似乎不支持单独的DISTINCT。但也有支持GROUP BY的aggregate materialized views。如果实例化视图是使用ENABLE QUERY REWRITE创建，则Oracle可以在DISTINCT查询中使用它。还有一个额外的计数，因为“COUNT(*) (*)必须始终存在，以保证所有类型的快速刷新。”

Create table、实体化视图日志和实体化视图。

create table test(col1 number, col2 number, col3 varchar(32)
  ,constraint test_pk primary key (col1, col2));
create materialized view log on test with rowid (col2) including new values;
create materialized view test_mv
  build immediate
  refresh fast on commit
  enable query rewrite as
  select col2, count(*) total from test group by col2;

查询可以使用物化视图。

这些解释计划表明，实体化视图同时适用于GROUP BY和DISTINCT查询。

explain plan for select col2 from test group by col2;
select * from table(dbms_xplan.display);

explain plan for select distinct col2 from test;
select * from table(dbms_xplan.display);


Plan hash value: 1627509066

----------------------------------------------------------------------------------------
| Id  | Operation                    | Name    | Rows  | Bytes | Cost (%CPU)| Time     |
----------------------------------------------------------------------------------------
|   0 | SELECT STATEMENT             |         |     1 |    13 |     2   (0)| 00:00:01 |
|   1 |  MAT_VIEW REWRITE ACCESS FULL| TEST_MV |     1 |    13 |     2   (0)| 00:00:01 |
----------------------------------------------------------------------------------------