浅析为何Oracle物化视图对distinct, group by不支持快速刷新

问题描述：我们知道distinct，group by 不能用于物化视图的快速刷新，而 group by + count 就可以，下面通过简单例子分析一下原因。
实验场景：Oracle 10 / Oracle Linux

一个物化视图定义语句
create materialized view mv_name
build immediate 
refresh fast on commit 
enable query rewrite 
as select distinct x,y,z from table_name 

物化视图三种刷新方式：complete, fast, force 其中complete 相当于清空当前mv所有的内容，重新执行一次创建语句，所以这种刷新方式不需要物化视图日志； fast是根据mv log更新的内容，增量刷新到mv中；force是个和事老，咱们先fast刷新，如果不支持，咱就complete。 所以如要搞清楚distinct,group by为什么不能应用于快速刷新，就要搞清楚如果不用基础表，光靠物化视图日志和物化视图，能不能完成distinct,group by操作。

我们先看一个正确的创建的物化视图，分析它快速刷新的过程：（所有测试均在HR 用户下完成）

 1. 创建物化视图日志
 SQL> select * from t1;

           X          Y          Z
 ---------- ---------- ----------
 
                  1          1          1
          1          1          1
          2          2          2
          2          2          2                   3          3          3
          3          3          3                    6          6          6
          6          6          6                   11         11         11
          7          7          7
          7          7          7                              8          8          8
          8          8          8

  SQL> create materialized view log on t1 with sequence, rowid(x,y,z) including new values;

 SQL> create materialized view mv_t1 build immediate refresh fast on commit enable query rewrite as select x,y,z,count(*)
      from t1 group by x,y,z;

 SQL> select * from mv_t1;             X          Y          Z   COUNT(*) ---------- ---------- ---------- ----------
          1          1          1          2          3          3          3          2          6          6          6          2         11         11         11          1          5          5          5          2          7          7          7          2          2          2          2          2          8          8          8          2

 8 rows selected.

2. 删除x=1的两列

 SQL> delete from t1 where x=1;

 2 rows deleted.

3. 查看物化视图日志

 SQL> select * from mlog$_t1;

          X          Y          Z M_ROW$$                        SEQUENCE$$ SNAPTIME$$          D O CHANGE_VECTOR$$
 ---------- ---------- ---------- ------------------------------ ---------- ------------------- - - ------------------
          1          1          1 AAACfYAAFAAABATAAK             20002      4000-01-01 00:00:00 D O 00
          1          1          1 AAACfYAAFAAABATAAA             20001      4000-01-01 00:00:00 D O 00

4. 从v$sqlarea视图中抓取Oracle内部是如何执行快速刷新的语句

 SQL> commit;

 Commit complete.

 SQL> ed

   1 select * from
   2 (
   3 select sql_text
   4 from v$sqlarea
   5 where sql_text like '%MV_REFRESH%'
   6 order by buffer_gets desc
   7 )
   8* where rownum < 5
   9 /

 SQL_TEXT
 ------------------------------------------------------------------------------------------------------------------------

 /* MV_REFRESH (UPD) */ UPDATE /*+ BYPASS_UJVC */ (SELECT /*+ NO_MERGE("AV$") */ "SNA$"."COUNT(*)" "C0", "AV$"."D0" FROM "HR"."MV_T1" "SNA$", (SELECT   /*+  OPT_ESTIMATE(QUERY_BLOCK MAX=1000)   */ "DLT$0"."X" "GB0", "DLT$0"."Y" "GB1", "DLT$0 "."Z" "GB2", SUM(-1) "D0" FROM (SELECT  /*+ CARDINALITY(MAS$ 2) */  CHARTOROWID("MAS$"."M_ROW$$") RID$  ,  "MAS$"."Z", " MAS$"."Y", "MAS$"."X"   FROM "HR"."MLOG$_T1" "MAS$"   WHERE "MAS$".SNAPTIME$$ > :1 ) "DLT$0"  GROUP BY "DLT$0"."X","DLT$ 0"."Y","DLT$0"."Z")"AV$" WHERE SYS_OP_MAP_NONNULL("SNA$"."X")=SYS_OP_MAP_NONNULL("AV$"."GB0") AND SYS_OP_MAP_NONNULL("SN A$"."Y")=SYS_OP_MAP_NONNULL("AV$"."GB1") AND SYS_OP_MAP_NONNULL("SNA$"."Z")=SYS_OP_MAP_NONNULL("AV$"."GB2")) UV$ SET "C0 "="C0"+"D0"

 /* MV_REFRESH (DEL) */ DELETE FROM \"HR\".\"MV_T1\" \"SNA$\" WHERE \"SNA$\".\"COUNT(*)\"=0

5. 我们发现这个对基础表的删除语句产生了两条刷新语句，整理格式，查看执行计划便于分析：

 SQL> explain plan for

 UPDATE /*+ BYPASS_UJVC */
  (SELECT /*+ NO_MERGE("AV$") */ "SNA$"."COUNT(*)" "C0", "AV$"."D0"
   FROM     "HR"."MV_T1" "SNA$",
     (SELECT /*+ OPT_ESTIMATE(QUERY_BLOCK MAX=1000) */ "DLT$0"."X" "GB0", "DLT$0"."Y" "GB1", "DLT$0"."Z" "GB2", SUM(-1) "D0"
      FROM
         (SELECT  /*+ CARDINALITY(MAS$ 2) */  CHARTOROWID("MAS$"."M_ROW$$") RID$  ,  "MAS$"."Z", "MAS$"."Y","MAS$"."X"
           FROM
             "HR"."MLOG$_T1" "MAS$"  WHERE "MAS$".SNAPTIME$$ > :1
          ) "DLT$0"
      GROUP BY "DLT$0"."X","DLT$0"."Y", "DLT$0"."Z"
      )"AV$"
   WHERE
      SYS_OP_MAP_NONNULL("SNA$"."X")=SYS_OP_MAP_NONNULL("AV$"."GB0") AND
      SYS_OP_MAP_NONNULL("SNA$"."Y")=SYS_OP_MAP_NONNULL("AV$"."GB1") AND
      SYS_OP_MAP_NONNULL("SNA$"."Z")=SYS_OP_MAP_NONNULL("AV$"."GB2")
   ) UV$

 SET "C0"="C0"+"D0"

 Explained.

 注释：
 -- SNA$ = "HR"."MV_T1"      代表物化视图
 -- MAS$ = "HR"."MLOG$_T1"   代表物化视图日志
 -- M_ROW$$                  物化视图日志中的列，代表rowid

 SQL> SELECT * FROM TABLE(DBMS_XPLAN.DISPLAY);

 PLAN_TABLE_OUTPUT
 -------------------------------------------------------------------------------------------------
 Plan hash value: 3886021645

 --------------------------------------------------------------------------------------------------
 | Id | Operation                         | Name          | Rows | Bytes | Cost (%CPU)| Time |
 --------------------------------------------------------------------------------------------------
 | 0 | UPDATE STATEMENT                   |               | 1    | 104   | 18 (6)    | 00:00:01 |
 | 1 |   UPDATE                           | MV_T1         |      |       |           |          |
 | 2 |     NESTED LOOPS                   |               | 1    | 104   | 18 (6)    | 00:00:01 |
 | 3 |      VIEW                          |               | 2    | 104   | 17 (6)    | 00:00:01 |
 | 4 |       SORT GROUP BY                |               | 2    | 96    | 17 (6)    | 00:00:01 |
 |* 5|        TABLE ACCESS FULL           | MLOG$_T1      | 2    | 96    | 16 (0)    | 00:00:01 |
 | 6 |      MAT_VIEW ACCESS BY INDEX ROWID| MV_T1         | 1    | 52    | 1 (0)     | 00:00:01 |
 |* 7|       INDEX UNIQUE SCAN            | I_SNAP$_MV_T1 | 1    |       | 0 (0)     | 00:00:01 |
 --------------------------------------------------------------------------------------------------

 Predicate Information (identified by operation id):
 ---------------------------------------------------

    5 - filter("MAS$"."SNAPTIME$$">:1)
    7 - access(SYS_OP_MAP_NONNULL("X")=SYS_OP_MAP_NONNULL("AV$"."GB0") AND
               SYS_OP_MAP_NONNULL("Y")=SYS_OP_MAP_NONNULL("AV$"."GB1") AND
               SYS_OP_MAP_NONNULL("Z")=SYS_OP_MAP_NONNULL("AV$"."GB2"))

 Note
 -----
    - dynamic sampling used for this statement

 26 rows selected.

SQL> explain plan for /* MV_REFRESH (DEL) */ DELETE FROM "HR"."MV_T1" "SNA$" WHERE "SNA$"."COUNT(*)"=0;

Explained.

SQL> select * from table(dbms_xplan.display);

PLAN_TABLE_OUTPUT
---------------------------------------------------------------------------------------------------------
Plan hash value: 583344350

-------------------------------------------------------------------------------
| Id  | Operation             | Name  | Rows  | Bytes | Cost (%CPU)| Time     |
-------------------------------------------------------------------------------
|   0 | DELETE STATEMENT      |       |     1 |    52 |    19   (0)| 00:00:01 |
|   1 |  DELETE               | MV_T1 |       |       |            |          |
|*  2 |   MAT_VIEW ACCESS FULL| MV_T1 |     1 |    52 |    19   (0)| 00:00:01 |
-------------------------------------------------------------------------------


Predicate Information (identified by operation id):
---------------------------------------------------

   2 - filter("SNA$"."COUNT(*)"=0)

Note
-----
   - dynamic sampling used for this statement

18 rows selected.

6. 语句分析结论：

经过对两条刷新语句的分析我们可以知道，步骤2 中的 delete 基础表数据的语句，对应的物化视图更新的SQL 有两个，
第一条复杂的update语句的作用是，从物化视图中的count字段减去delete操作删掉基础表的行数（从物化视图日志中得出），
如果相减等于0，就是说表中所有等于1的行已全部被删除，于是就产生了第二条更新，就是从物化视图中删除这条记录。
从这我们可以看出，如果没有count(*)字段，单凭现有的物化视图，以及物化视图日志，只知道删了几个，没有办法知道删除
之前的基础表相同的数据到底有几个，比如基础表原有 x=3,y=3,z=3 的行有3个，此时若物化视图没有count(*) 列，那么它将
只有一条3,3,3的记录。若delete语句对基础表删除了所有3,3,3的数据，物日志中将有3条记录，而物化视图因为缺少总共的条
目记录，将无法知道该条目是否应该从视图中删除

补充：以下是对第一条刷新操作SQL的拆分执行：

 SQL> create table AV$ as SELECT /*+ CARDINALITY(MAS$ 2) */ CHARTOROWID("MAS$"."M_ROW$$") RID$ , "MAS$"."Z", "MAS$"."Y","MAS$"."X"
         FROM "HR"."MLOG$_T1" "MAS$" 2 ;

 RID$               Z Y X
 ------------------ - --------- ---------- ----------
 AAACfYAAFAAABATAAC 3 3 3
 AAACfYAAFAAABATAAM 3 3 3

 SQL> select * from AV$

      GB0 GB1 GB2 D0
     ---------- ---------- ---------- ----------
      3 3 3 -2

 SQL>

   1 SELECT /*+ NO_MERGE(\"AV$\") */ SNA$.\"COUNT(*)\" \"C0\", \"AV$\".\"D0\"
   2 FROM
   3 \"HR\".\"T3\" \"SNA$\", \"AV$\"
   4 WHERE
   5 SYS_OP_MAP_NONNULL(\"SNA$\".\"X\")=SYS_OP_MAP_NONNULL(\"AV$\".\"GB0\") AND
   6 SYS_OP_MAP_NONNULL(\"SNA$\".\"Y\")=SYS_OP_MAP_NONNULL(\"AV$\".\"GB1\") AND
   7* SYS_OP_MAP_NONNULL(\"SNA$\".\"Z\")=SYS_OP_MAP_NONNULL(\"AV$\".\"GB2\")
   8
 SQL> /


         C0 D0
 ---------- ----------
          2 -2

结论：从上面的实验可以得出，对于delete 操作，物化视图语句中的distinct, group by 不查询基础表的情况下，通过物化视图本身以及物化视图日志是无法
进行刷新的，就是无法进行快速刷新功能。 对于其它对基础表的DML操作，可以用相同的方法加以测试。

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转载于:http://blog.itpub.net/20482996/viewspace-1251469/