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創建一個測試表:如何在使用複合索引時避免排序?
create table customer (first_name varchar2(20), last_name varchar2(20) not null, address varchar(20));
insert into customer select dbms_random.string('U', 20), dbms_random.string('U', 20), dbms_random.string('U', 20) from dual connect by level <= 100000;
commit;
create index i_ln_fn_0 on customer(last_name, first_name,0); — just to be sure that all rows are indexed
現在解釋計劃:
explain plan for
select /*+ FIRST_ROWS(20) */ *
from CUSTOMER
where first_name like 'AB%'
and first_name is not null
order by last_name;
select * from table(dbms_xplan.display);
-------------------------------------------------------------------------------
| Id | Operation | Name | Rows | Bytes | Cost (%CPU)| Time |
-------------------------------------------------------------------------------
| 0 | SELECT STATEMENT | | 197 | 12411 | 275 (2)| 00:00:04 |
| 1 | SORT ORDER BY | | 197 | 12411 | 275 (2)| 00:00:04 |
|* 2 | TABLE ACCESS FULL| CUSTOMER | 197 | 12411 | 274 (1)| 00:00:04 |
-------------------------------------------------------------------------------
Predicate Information (identified by operation id):
---------------------------------------------------
2 - filter("FIRST_NAME" LIKE 'AB%' AND "FIRST_NAME" IS NOT NULL)
但因爲我想只有第一排,我想避免的排序整個表的。我想有這樣一個計劃:
SELECT STATEMENT
TABLE ACCESS BY ROWID (customer)
INDEX FULL SCAN (i_ln_fn_0)
如何說服數據庫,以避免排序?
問題更加嚴重。即使當我僅使用姓氏,無處不在:
explain plan for
select /*+ FIRST_ROWS(20) */ last_name
from CUSTOMER
where last_name like 'AB%'
and last_name is not null
order by last_name;
select * from table(dbms_xplan.display);
-------------------------------------------------------------------------------
| Id | Operation | Name | Rows | Bytes | Cost (%CPU)| Time |
-------------------------------------------------------------------------------
| 0 | SELECT STATEMENT | | 17 | 357 | 4 (25)| 00:00:01 |
| 1 | SORT ORDER BY | | 17 | 357 | 4 (25)| 00:00:01 |
|* 2 | INDEX RANGE SCAN| I_LN_FN_0 | 17 | 357 | 3 (0)| 00:00:01 |
-------------------------------------------------------------------------------
Predicate Information (identified by operation id):
---------------------------------------------------
2 - access("LAST_NAME" LIKE 'AB%')
filter("LAST_NAME" LIKE 'AB%')
這裏的排序真的沒有必要,但數據庫仍然使用它。爲什麼?
編輯:測試兩個
Oracle Database 11g Enterprise Edition Release 11.2.0.1.0 - 64bit Production
PL/SQL Release 11.2.0.1.0 - Production
"CORE 11.2.0.1.0 Production"
TNS for 64-bit Windows: Version 11.2.0.1.0 - Production
NLSRTL Version 11.2.0.1.0 - Production
和
Oracle Database 11g Enterprise Edition Release 11.2.0.2.0 - 64bit Production+
PL/SQL Release 11.2.0.2.0 - Production+
"CORE 11.2.0.2.0Production"+
TNS for IBM/AIX RISC System/6000: Version 11.2.0.2.0 - Production+
NLSRTL Version 11.2.0.2.0 - Production+
有同樣的計劃。
我無法重現你的結果。實際上,我使用'INDEX FULL SCAN'來得到你正在尋找的計劃。你使用的是什麼版本的Oracle?你沒有忽略任何統計收集?在「謂詞信息」部分沒有獲得用於此語句的動態採樣(level = 2),這很奇怪。您是否將參數optimizer_dynamic_sampling更改爲0? – 2013-03-07 06:32:40
奇怪,因爲我可以在我的個人(在我的筆記本上)和生產數據庫(我將db版本添加到原始問題)上重現問題。據我所知,數據庫不包含任何不尋常的調整。一切都很重要,默認值是V $ PARAMETER,optimizer_dynamic_sampling = 2。我用'exec dbms_stats.gather_table_stats('TEST','CUSTOMER',cascade => true)重新收集統計信息;'但沒有任何影響。 – xarx 2013-03-07 09:28:03