SQL最佳化中索引列使用函式之靈異事件

在SQL最佳化內容中有一種說法說的是避免在索引列上使用函式、運算等操作，否則Oracle最佳化器將不使用索引而使用全表掃描，但是也有一些例外的情況，今天我們就來看看該靈異事件。

一般而言，以下情況都會使Oracle的最佳化器走全表掃描，舉例:

1. substr(hbs_bh,1,4)=’5400’，最佳化處理：hbs_bh like ‘5400%’

2. trunc(sk_rq)=trunc(sysdate)，最佳化處理：sk_rq>=trunc(sysdate) and sk_rq

3. 進行了顯式或隱式的運算的欄位不能進行索引，如：

ss_df+20>50，最佳化處理：ss_df>30

'X' || hbs_bh>’X5400021452’，最佳化處理：hbs_bh>'5400021542'

sk_rq+5=sysdate，最佳化處理：sk_rq=sysdate-5

4. 條件內包括了多個本表的欄位運算時不能進行索引，如：ys_df>cx_df，無法進行最佳化

qc_bh || kh_bh='5400250000'，最佳化處理：qc_bh='5400' and kh_bh='250000'

5. 避免出現隱式型別轉化

hbs_bh=5401002554，最佳化處理：hbs_bh='5401002554'，注：此條件對hbs_bh 進行隱式的to_number轉換，因為hbs_bh欄位是字元型。

有一些其它的例外情況，如果select 後邊只有索引列且where查詢中的索引列含有非空約束的時候，以上規則不適用，如下示例：

先給出所有指令碼及結論：

drop table t purge;

Create Table t nologging As select * from dba_objects d ;

create index ind_objectname on t(object_name);

select t.object_name from t where t.object_name ='T'; --走索引

select t.object_name from t where UPPER(t.object_name) ='T'; --不走索引

select t.object_name from t where UPPER(t.object_name) ='T' and t.object_name IS NOT NULL ; --走索引 (INDEX FAST FULL SCAN)

select t.object_name from t where UPPER(t.object_name) ||'AAA' ='T'||'AAA' and t.object_name IS NOT NULL ; --走索引 (INDEX FAST FULL SCAN)

select t.object_name,t.owner from t where UPPER(t.object_name) ||'AAA' ='T'||'AAA' and t.object_name IS NOT NULL ; --不走索引

測試程式碼：

C:\Users\華榮>sqlplus lhr/lhr@orclasm

SQL*Plus: Release 11.2.0.1.0 Production on 星期三 11月 12 10:52:29 2014

連線到:

Oracle Database 11g Enterprise Edition Release 11.2.0.3.0 - 64bit Production

With the Partitioning, Automatic Storage Management, OLAP, Data Mining

and Real Application Testing options

SQL>

SQL> drop table t purge;

表已刪除。

SQL> Create Table t nologging As select * from dba_objects d ;

表已建立。

SQL> create index ind_objectname on t(object_name);

索引已建立。

---- t表所有列均可以為空

SQL> desc t

Name Null? Type

----------------------------------------- -------- ----------------------------

OWNER VARCHAR2(30)

OBJECT_NAME VARCHAR2(128)

SUBOBJECT_NAME VARCHAR2(30)

OBJECT_ID NUMBER

DATA_OBJECT_ID NUMBER

OBJECT_TYPE VARCHAR2(19)

CREATED DATE

LAST_DDL_TIME DATE

TIMESTAMP VARCHAR2(19)

STATUS VARCHAR2(7)

TEMPORARY VARCHAR2(1)

GENERATED VARCHAR2(1)

SECONDARY VARCHAR2(1)

NAMESPACE NUMBER

EDITION_NAME VARCHAR2(30)

SQL>

SQL> set autotrace traceonly;

SQL> select t.object_name from t where t.object_name ='T';

執行計劃

----------------------------------------------------------

Plan hash value: 4280870634

-----------------------------------------------------------------------------------

-----------------------------------------------------------------------------------

| 0 | SELECT STATEMENT | | 1 | 66 | 3 (0)| 00:00:01 |

|* 1 | INDEX RANGE SCAN| IND_OBJECTNAME | 1 | 66 | 3 (0)| 00:00:01 |

-----------------------------------------------------------------------------------

Predicate Information (identified by operation id):

---------------------------------------------------

1 - access("T"."OBJECT_NAME"='T')

Note

-----

- dynamic sampling used for this statement (level=2)

- SQL plan baseline "SQL_PLAN_503ygb00mbj6k165e82cd" used for this statement

統計資訊

----------------------------------------------------------

34 recursive calls

43 db block gets

127 consistent gets

398 physical reads

15476 redo size

349 bytes sent via SQL*Net to client

359 bytes received via SQL*Net from client

2 SQL*Net roundtrips to/from client

0 sorts (memory)

0 sorts (disk)

1 rows processed

SQL> select t.object_name from t where UPPER(t.object_name) ='T';

執行計劃

----------------------------------------------------------

Plan hash value: 1601196873

--------------------------------------------------------------------------

--------------------------------------------------------------------------

| 0 | SELECT STATEMENT | | 12 | 792 | 305 (1)| 00:00:04 |

|* 1 | TABLE ACCESS FULL| T | 12 | 792 | 305 (1)| 00:00:04 |

--------------------------------------------------------------------------

Predicate Information (identified by operation id):

---------------------------------------------------

1 - filter(UPPER("T"."OBJECT_NAME")='T')

Note

-----

- dynamic sampling used for this statement (level=2)

- SQL plan baseline "SQL_PLAN_9p76pys5gdb2b94ecae5c" used for this statement

統計資訊

----------------------------------------------------------

29 recursive calls

43 db block gets

1209 consistent gets

1092 physical reads

15484 redo size

349 bytes sent via SQL*Net to client

359 bytes received via SQL*Net from client

2 SQL*Net roundtrips to/from client

0 sorts (memory)

0 sorts (disk)

1 rows processed

SQL> select t.object_name from t where UPPER(t.object_name) ='T' and t.object_name IS NOT NULL ;

執行計劃

----------------------------------------------------------

Plan hash value: 3379870158

---------------------------------------------------------------------------------------

---------------------------------------------------------------------------------------

| 0 | SELECT STATEMENT | | 51 | 3366 | 110 (1)| 00:00:02 |

|* 1 | INDEX FAST FULL SCAN| IND_OBJECTNAME | 51 | 3366 | 110 (1)| 00:00:02 |

---------------------------------------------------------------------------------------

Predicate Information (identified by operation id):

---------------------------------------------------

1 - filter("T"."OBJECT_NAME" IS NOT NULL AND UPPER("T"."OBJECT_NAME")='T')

Note

-----

- dynamic sampling used for this statement (level=2)

- SQL plan baseline "SQL_PLAN_czkarb71kthws18b0c28f" used for this statement

統計資訊

----------------------------------------------------------

29 recursive calls

43 db block gets

505 consistent gets

384 physical reads

15612 redo size

349 bytes sent via SQL*Net to client

359 bytes received via SQL*Net from client

2 SQL*Net roundtrips to/from client

0 sorts (memory)

0 sorts (disk)

1 rows processed

SQL> select t.object_name,t.owner from t where UPPER(t.object_name) ||'AAA' ='T'||'AAA' and t.object_name IS NOT NULL ;

執行計劃

----------------------------------------------------------

Plan hash value: 1601196873

--------------------------------------------------------------------------

--------------------------------------------------------------------------

| 0 | SELECT STATEMENT | | 51 | 4233 | 304 (1)| 00:00:04 |

|* 1 | TABLE ACCESS FULL| T | 51 | 4233 | 304 (1)| 00:00:04 |

--------------------------------------------------------------------------

Predicate Information (identified by operation id):

---------------------------------------------------

1 - filter("T"."OBJECT_NAME" IS NOT NULL AND

UPPER("T"."OBJECT_NAME")||'AAA'='TAAA')

Note

-----

- dynamic sampling used for this statement (level=2)

- SQL plan baseline "SQL_PLAN_au9a1c4hwdtb894ecae5c" used for this statement

統計資訊

----------------------------------------------------------

30 recursive calls

44 db block gets

1210 consistent gets

1091 physical reads

15748 redo size

408 bytes sent via SQL*Net to client

359 bytes received via SQL*Net from client

2 SQL*Net roundtrips to/from client

0 sorts (memory)

0 sorts (disk)

1 rows processed

SQL> select t.object_name from t where UPPER(t.object_name) ||'AAA' ='T'||'AAA' and t.object_name IS NOT NULL ;

執行計劃

----------------------------------------------------------

Plan hash value: 3379870158

---------------------------------------------------------------------------------------

---------------------------------------------------------------------------------------

| 0 | SELECT STATEMENT | | 51 | 3366 | 110 (1)| 00:00:02 |

|* 1 | INDEX FAST FULL SCAN| IND_OBJECTNAME | 51 | 3366 | 110 (1)| 00:00:02 |

---------------------------------------------------------------------------------------

Predicate Information (identified by operation id):

---------------------------------------------------

1 - filter("T"."OBJECT_NAME" IS NOT NULL AND

UPPER("T"."OBJECT_NAME")||'AAA'='TAAA')

Note

-----

- dynamic sampling used for this statement (level=2)

- SQL plan baseline "SQL_PLAN_1gu36rnh3s2a318b0c28f" used for this statement

統計資訊

----------------------------------------------------------

28 recursive calls

44 db block gets

505 consistent gets

6 physical reads

15544 redo size

349 bytes sent via SQL*Net to client

359 bytes received via SQL*Net from client

2 SQL*Net roundtrips to/from client

0 sorts (memory)

0 sorts (disk)

1 rows processed

SQL>

其實很好理解的，索引可以看成是小表，一般而言索引總是比表本身要小得多，如果select 後需要檢索的專案在索引中就可以檢索的到那麼Oracle最佳化器為啥還去大表中尋找資料呢？

SQL最佳化中索引列使用函式之靈異事件

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