在Oracle10g之后，提供了DBMS_ADVANCED_REWRITE包，具有强大的查询重写功能，可以让我们在数据库层面实现很多微妙的调整。假设我们有一个应用，但是现在无法直接修改应用程序的编码，但是又想能够让应用程序的某些SQL产生我们想要的变化，那么就可以使用DBMS_ADVANCED_REWRITE包。
drop table t;
create table t as select object_id,object_name from dba_objects;
drop table t1;
create table t1 as select object_id,object_name from dba_objects where 1=0;
SQL> select count(*) from t;
COUNT(*)
----------
16636
SQL> select count(*) from t1;
COUNT(*)
----------
0
现在我们有表T和T1，表结构相同，但是表T中有1.6万记录，而表T1中没有记录，如果说我们的应用中有一个SQL多次地查询表T的总记录数，占用了大量的CPU和逻辑读，而这样的count记录数又是完全没有用处的，但是我们无法修改应用程序去掉这个SQL，那么我们就可以通过DBMS_ADVANCED_REWRITE包来讲查询表T的SQL转变为查询表T1，这样就大大减少了这条SQL的逻辑读。
首先DBMS_ADVANCED_REWRITE包的执行权限必须显式赋给需要的用户。
CONN sys/password AS SYSDBA
GRANT EXECUTE ON DBMS_ADVANCED_REWRITE TO kamus;
CONN kamus/password
BEGIN
SYS.DBMS_ADVANCED_REWRITE.declare_rewrite_equivalence (
name => 't_rewrite',
source_stmt => 'SELECT count(*) FROM t',
destination_stmt => 'SELECT count(*) FROM t1',
validate => FALSE,
rewrite_mode => 'TEXT_MATCH');
END;
/
然后需要设置会话层面的QUERY_REWRITE_INTEGRITY参数，该参数默认值为ENFORCED，表示只有重写后的SQL输出结果跟原结果完全一样时，查询才会被真正重写，在这里需要修改为TRUSTED。
SQL> ALTER SESSION SET QUERY_REWRITE_INTEGRITY = TRUSTED;
SQL> set autot on
SQL> select count(*) from t;
COUNT(*)
----------
0
Execution Plan
----------------------------------------------------------
Plan hash value: 238181912
----------------------------------------------------------------------------
| Id | Operation | Name | Rows | Bytes | Cost (%CPU)| Time |
----------------------------------------------------------------------------
| 0 | SELECT STATEMENT | | 1 | 13 | 2 (0)| 00:00:01 |
| 1 | VIEW | | 1 | 13 | 2 (0)| 00:00:01 |
| 2 | SORT AGGREGATE | | 1 | | | |
| 3 | TABLE ACCESS FULL| T1 | 1 | | 2 (0)| 00:00:01 |
----------------------------------------------------------------------------
Note
-----
- dynamic sampling used for this statement
Statistics
----------------------------------------------------------
0 recursive calls
0 db block gets
3 consistent gets
0 physical reads
0 redo size
417 bytes sent via SQL*Net to client
416 bytes received via SQL*Net from client
2 SQL*Net roundtrips to/from client
0 sorts (memory)
0 sorts (disk)
1 rows processed
SQL> ALTER SESSION SET QUERY_REWRITE_INTEGRITY = ENFORCED;
SQL> select count(*) from t;
COUNT(*)
----------
16636
Execution Plan
----------------------------------------------------------
Plan hash value: 2966233522
-------------------------------------------------------------------
| Id | Operation | Name | Rows | Cost (%CPU)| Time |
-------------------------------------------------------------------
| 0 | SELECT STATEMENT | | 1 | 22 (0)| 00:00:01 |
| 1 | SORT AGGREGATE | | 1 | | |
| 2 | TABLE ACCESS FULL| T | 16639 | 22 (0)| 00:00:01 |
-------------------------------------------------------------------
Note
-----
- dynamic sampling used for this statement
Statistics
----------------------------------------------------------
0 recursive calls
0 db block gets
69 consistent gets
0 physical reads
0 redo size
420 bytes sent via SQL*Net to client
416 bytes received via SQL*Net from client
2 SQL*Net roundtrips to/from client
0 sorts (memory)
0 sorts (disk)
1 rows processed
SQL>
可以看到在重写之后，执行计划中显示直接去查询表T1，而consistent gets也从查询表T需要的69减少为3。
可以从[USER|ALL|DBA]_REWRITE_EQUIVALENCES视图中获得查询重写的信息。
SQL> select * from user_rewrite_equivalences;
OWNER NAME SOURCE_STMT DESTINATION_STMT REWRITE_MO
------ ---------- ------------------------- ------------------------- ----------
KAMUS T_REWRITE SELECT count(*) FROM t SELECT count(*) FROM t1 TEXT_MATCH
DBMS_ADVANCED_REWRITE包也有限制，不可以重写牵涉到SYS用户对象的SQL。
drop table t1;
create table t1 as select * from all_tables where 1=0;
SQL> BEGIN
2 SYS.DBMS_ADVANCED_REWRITE.declare_rewrite_equivalence (
3 name => 't_rewrite',
4 source_stmt => 'SELECT count(*) FROM all_tables',
5 destination_stmt => 'SELECT count(*) FROM t1',
6 validate => FALSE,
7 rewrite_mode => 'TEXT_MATCH');
8 END;
9 /
BEGIN
*
ERROR at line 1:
ORA-30354: Query rewrite not allowed on SYS relations
ORA-06512: at "SYS.DBMS_ADVANCED_REWRITE", line 29
ORA-06512: at "SYS.DBMS_ADVANCED_REWRITE", line 185
ORA-06512: at line 2
在测试中，如果destination_stmt中包含SYS用户对象，是可以成功创建查询重写的，但是在执行SQL的时候却会报ORA-03113错误，后台出现ORA-07445错误，无法正常执行。
SQL> BEGIN
2 SYS.DBMS_ADVANCED_REWRITE.declare_rewrite_equivalence (
3 name => 't_rewrite',
4 source_stmt => 'SELECT count(*) FROM t1',
5 destination_stmt => 'SELECT count(*) FROM all_tables',
6 validate => FALSE,
7 rewrite_mode => 'TEXT_MATCH');
8 END;
9 /
PL/SQL procedure successfully completed.
SQL> select * from user_rewrite_equivalences;
OWNER NAME SOURCE_STMT DESTINATION_STMT REWRITE_MO
------ ---------- ------------------------- ------------------------- ----------
KAMUS T_REWRITE SELECT count(*) FROM t1 SELECT count(*) FROM all_ TEXT_MATCH
tables
SQL> ALTER SESSION SET QUERY_REWRITE_INTEGRITY = TRUSTED;
Session altered.
SQL> select count(*) from t1;
select count(*) from t1
*
ERROR at line 1:
ORA-03113: end-of-file on communication channel
Process ID: 8360
Session ID: 135 Serial number: 22
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