Oracle-引數學習_no_or_expansion
阿新 • • 發佈：2021-08-05
一、需求，某客戶問_no_or_expansion=TRUE 這個引數幹啥用的？是否建議在11.2.0.4環境上設定？
需求如上，那麼我們首先了解一下這個引數是幹啥的？ 另外就是探索為什麼客戶以前的DBA設定這個引數？ 非預設隱含引數
二、引數說明
2.1 引數含義
參考一個網站的論壇
https://community.oracle.com/mosc/discussion/3058861/no-or-expansion-parameter
說的很清楚！
Or expansion is a kind of query transformation that converts OR to UNION. Say you have a queryselect * from emp where id=1 OR name='Vishal' Oracle may choose to expand this OR condition , break this SQL into two pieces using a UNION clause. New transformed query would look like select * from emp where id=1 UNION select * from emp where name='Vishal'
就是Oracle內部是否會將Or 轉換為union 進行展開！ 預設展開，True引數設定後，則不允許展開
2.2 引數測試對比
未建立索引前，觀察設定引數後，執行計劃和引數預設false一樣，沒啥子區別！
-------------------------------------------------------------------------------- _no_or_expansion FALSE OR expansion during optimization disabled alter session set "_no_or_expansion"=true; set linesize 500 set termout off alter session set statistics_level=all;select * from scott.emp where EMPNO=7782 OR ename='FORD'; select * from table(dbms_xplan.display_cursor(null,null,'ADVANCED ALLSTATS LAST PEEKED_BINDS')); -------------------------------------------------------------------------------------------------------------------- | Id | Operation | Name | Starts | E-Rows |E-Bytes| Cost (%CPU)| E-Time | A-Rows | A-Time | Buffers | -------------------------------------------------------------------------------------------------------------------- | 0 | SELECT STATEMENT | | 1 | | | 3 (100)| | 2 |00:00:00.01 | 7 | |* 1 | TABLE ACCESS FULL| EMP | 1 | 2 | 76 | 3 (0)| 00:00:01 | 2 |00:00:00.01 | 7 | Query Block Name / Object Alias (identified by operation id): ------------------------------------------------------------- 1 - SEL$1 / EMP@SEL$1 ----------------------------------------------------------------- BEGIN_OUTLINE_DATA IGNORE_OPTIM_EMBEDDED_HINTS OPTIMIZER_FEATURES_ENABLE('11.2.0.4') DB_VERSION('11.2.0.4') ALL_ROWS OUTLINE_LEAF(@"SEL$1") FULL(@"SEL$1" "EMP"@"SEL$1") END_OUTLINE_DATA */ 1 - filter(("EMPNO"=7782 OR "ENAME"='FORD')) Column Projection Information (identified by operation id): ----------------------------------------------------------- 1 - "EMPNO"[NUMBER,22], "ENAME"[VARCHAR2,10], "EMP"."JOB"[VARCHAR2,9], "EMP"."MGR"[NUMBER,22], "EMP"."HIREDATE"[DATE,7], "EMP"."SAL"[NUMBER,22], "EMP"."COMM"[NUMBER,22], "EMP"."DEPTNO"[NUMBER,22]
再次觀察這個引數啥子作用？
"或"的擴張。
使用"or"連線由不同欄位構成的查詢條件下，按照查詢條件將整個查詢分為多個獨立的查詢，為各個獨立查詢制定最優查詢路徑，然後查詢完的結果再組合起來。這叫“或的擴張”。當然，只有當使用or的各個查詢條件為驅動查詢條件時，並且or連線的是不同的欄位，才能制定這樣的執行計劃，否則，會走全表掃然後然後將or的查詢條件當過濾用途，當然還可能選擇將rowid走bitmap or，後面有介紹。
_no_or_expansion引數，預設false，true的話會禁止“或的擴張” /*+use_concat*/是想使用或擴張的hint
建立索引！！！ or兩個欄位都建立索引，在觀察對比
SQL> create index scott.id on scott.emp(empno); create index scott.id on scott.emp(empno) * ERROR at line 1: ORA-01408: such column list already indexed SQL> create index scott.name_ind on scott.emp(ename);
引數調整為非預設true,這種情況引數如果有效果，則無法使用兩個索引進行彙總聚合返回結果
alter system set "_no_or_expansion"=true; set linesize 500 set termout off alter session set statistics_level=all; select * from scott.emp where EMPNO=7782 OR ename='FORD'; select * from table(dbms_xplan.display_cursor(null,null,'ADVANCED ALLSTATS LAST PEEKED_BINDS')); --------------------------------------------------------------------------------------------------------------------------------------- | Id | Operation | Name | Starts | E-Rows |E-Bytes| Cost (%CPU)| E-Time | A-Rows | A-Time | Buffers | --------------------------------------------------------------------------------------------------------------------------------------- | 0 | SELECT STATEMENT | | 1 | | | 3 (100)| | 2 |00:00:00.01 | 4 | | 1 | TABLE ACCESS BY INDEX ROWID | EMP | 1 | 2 | 76 | 3 (0)| 00:00:01 | 2 |00:00:00.01 | 4 | | 2 | BITMAP CONVERSION TO ROWIDS | | 1 | | | | | 2 |00:00:00.01 | 2 | | 3 | BITMAP OR | | 1 | | | | | 1 |00:00:00.01 | 2 | | 4 | BITMAP CONVERSION FROM ROWIDS| | 1 | | | | | 1 |00:00:00.01 | 1 | |* 5 | INDEX RANGE SCAN | PK_EMP | 1 | | | 0 (0)| | 1 |00:00:00.01 | 1 | | 6 | BITMAP CONVERSION FROM ROWIDS| | 1 | | | | | 1 |00:00:00.01 | 1 | |* 7 | INDEX RANGE SCAN | NAME_IND | 1 | | | 1 (0)| 00:00:01 | 1 |00:00:00.01 | 1 | --------------------------------------------------------------------------------------------------------------------------------------- Predicate Information (identified by operation id): --------------------------------------------------- 5 - access("EMPNO"=7782) 7 - access("ENAME"='FORD')
引數預設不調整的情況下
alter system set "_no_or_expansion"=false; set linesize 500 set termout off alter session set statistics_level=all; select * from scott.emp where EMPNO=7782 OR ename='FORD'; select * from table(dbms_xplan.display_cursor(null,null,'ADVANCED ALLSTATS LAST PEEKED_BINDS')); --------------------------------------------------------------------------------------------------------------------------------------- | Id | Operation | Name | Starts | E-Rows |E-Bytes| Cost (%CPU)| E-Time | A-Rows | A-Time | Buffers | --------------------------------------------------------------------------------------------------------------------------------------- | 0 | SELECT STATEMENT | | 1 | | | 3 (100)| | 2 |00:00:00.01 | 4 | | 1 | TABLE ACCESS BY INDEX ROWID | EMP | 1 | 2 | 76 | 3 (0)| 00:00:01 | 2 |00:00:00.01 | 4 | | 2 | BITMAP CONVERSION TO ROWIDS | | 1 | | | | | 2 |00:00:00.01 | 2 | | 3 | BITMAP OR | | 1 | | | | | 1 |00:00:00.01 | 2 | | 4 | BITMAP CONVERSION FROM ROWIDS| | 1 | | | | | 1 |00:00:00.01 | 1 | |* 5 | INDEX RANGE SCAN | PK_EMP | 1 | | | 0 (0)| | 1 |00:00:00.01 | 1 | | 6 | BITMAP CONVERSION FROM ROWIDS| | 1 | | | | | 1 |00:00:00.01 | 1 | |* 7 | INDEX RANGE SCAN | NAME_IND | 1 | | | 1 (0)| 00:00:01 | 1 |00:00:00.01 | 1 | ---------------------------------------------------------------------------------------------------------------------------------------
沒啥子區別，可以說明這個引數到了Oracle 11.2.0.4 版本已經被廢棄了，引數沒有起到效果！！！
2.3 追溯為什麼要設定這個引數？
--客戶歷史老的DBA不可能無緣無故調整隱含引數，一般是出現了問題，隨後的DB都統一調整！
那麼與這個引數相關的問題，有哪些？ 請閱讀如下MOS擷取的資訊 如果出現ORA-932 並且符合這個版本，可以考慮重點關注一下！！！
Bug 9592003 : ALL_SDO_GEOM_METADATA QUERIES RESULTS IN ORA-00932 UDT GOT MDSYS.SDO_DIM_ARRAY Product Version 10.2.0.4 Bug 9592003 - ORA-932 error on query with join on UNION ALL view (Doc ID 9592003.8) Oracle Database - Enterprise Edition - Version 10.1.0.2 and later Description ORA-932 error on query with join on union view. Rediscovery Notes: If a query with a join on a union view gets ORA-932 error and disabling or-Expansion allows the query to execute then you may have encountered this bug. This can have a major impact in Oracle Spatial Systems as it was discovered in a very common recursive query against the core ALL_SDO_GEOM_METADATA view. Workaround Use NO_EXPAND hint or set "_no_or_expansion" to TRUE. Bug 12642813 : WRONG RESULT WITH OR-EXPANSION CAUSED BY FIX TO BUG 399198 Hdr: 12642813 11.2.0.2 RDBMS 11.2.0.2 QRY OPTIMIZER PRODID-5 PORTID-23 Abstract: WRONG RESULT WITH OR-EXPANSION CAUSED BY FIX TO BUG 399198 Product Version 11.2.0.2 Fixed in Product Version 12.1 Platform 23 - Oracle Solaris on SPARC (64-bit)
客戶新的機器DB版本11.2.0.4，結論是可以不設定這個引數，如果DB <=11.2.0.2的情況下，如果有補丁也行，否則這個引數設定可以不用取消，放著就行！