[20191218]降序索引疑問4.txt
[20191218]降序索引疑問4.txt
--//前幾天最佳化一個專案,我發現許多表裡面有有隱含欄位,一般開發很少建立函式索引.我自己檢查發現裡面存在大量的降序索引.
--//我感覺有點奇怪,為什麼開發要建立大量降序索引有什麼好處呢?
--//我在連結http://www.itpub.net/thread-2122088-1-1.html裡面問這個問題,sqysl的解答給了我很好的提示,我透過例子說明使用降
--//序索引的一點點好處,透過例子說明:
1.環境:
SCOTT@book> @ ver1
PORT_STRING VERSION BANNER
------------------- -------------- ----------------------------------------------------------------------------
x86_64/Linux 2.4.xx 11.2.0.4.0 Oracle Database 11g Enterprise Edition Release 11.2.0.4.0 - 64bit Production
SCOTT@book> alter system set pga_aggregate_target=4G;
System altered.
SCOTT@book> create table t (id1 number,id2 number,vc varchar2(50));
Table created.
create index i_t_id1_id2 on t (id1,id2);
create index i_t_id1desc_id2desc on t (id1 desc,id2 desc);
create index i_t_id1desc_id2 on t (id1 desc,id2 );
create index i_t_id1_id2desc on t (id1 ,id2 desc );
SCOTT@book> insert into t select rownum,rownum,lpad('a',50,'a') from dual connect by level<=1e6;
1000000 rows created.
SCOTT@book> commit ;
Commit complete.
--//分析略.
SCOTT@book> select index_name,index_type,blevel,leaf_blocks,distinct_keys from dba_indexes where owner=user and table_name='T';
INDEX_NAME INDEX_TYPE BLEVEL LEAF_BLOCKS DISTINCT_KEYS
------------------------------ --------------------------- ---------- ----------- -------------
I_T_ID1_ID2 NORMAL 2 2623 1000000
I_T_ID1DESC_ID2DESC FUNCTION-BASED NORMAL 2 5877 1000000
I_T_ID1DESC_ID2 FUNCTION-BASED NORMAL 2 5618 1000000
I_T_ID1_ID2DESC FUNCTION-BASED NORMAL 2 2753 1000000
--//無論那種倒序索引,索引都比正序索引大,特別是第1個欄位選擇desc的情況.因為我插入的欄位是自增欄位.
--//倒序索引分裂都是50-50分裂.
2.測試例子:
SCOTT@book> alter session set statistics_level = all;
Session altered.
SCOTT@book> select * from (select * from t where id1<=1e6 order by id1 desc,id2 ) where rownum<=10;
ID1 ID2 VC
---------- ---------- --------------------------------------------------
1000000 1000000 aaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaa
999999 999999 aaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaa
999998 999998 aaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaa
999997 999997 aaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaa
999996 999996 aaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaa
999995 999995 aaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaa
999994 999994 aaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaa
999993 999993 aaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaa
999992 999992 aaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaa
999991 999991 aaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaa
10 rows selected.
SCOTT@book> @ dpc '' ''
PLAN_TABLE_OUTPUT
-------------------------------------
SQL_ID 20hb1uyyqx4tf, child number 0
-------------------------------------
select * from (select * from t where id1<=1e6 order by id1 desc,id2 )
where rownum<=10
Plan hash value: 404407004
-------------------------------------------------------------------------------------------------------------------------------------------
| Id | Operation | Name | Starts | E-Rows |E-Bytes| Cost (%CPU)| E-Time | A-Rows | A-Time | Buffers |
-------------------------------------------------------------------------------------------------------------------------------------------
| 0 | SELECT STATEMENT | | 1 | | | 4 (100)| | 10 |00:00:00.01 | 6 |
|* 1 | COUNT STOPKEY | | 1 | | | | | 10 |00:00:00.01 | 6 |
| 2 | VIEW | | 1 | 10 | 530 | 4 (0)| 00:00:01 | 10 |00:00:00.01 | 6 |
| 3 | TABLE ACCESS BY INDEX ROWID| T | 1 | 1000K| 58M| 4 (0)| 00:00:01 | 10 |00:00:00.01 | 6 |
|* 4 | INDEX RANGE SCAN | I_T_ID1DESC_ID2 | 1 | 10 | | 3 (0)| 00:00:01 | 10 |00:00:00.01 | 4 |
-------------------------------------------------------------------------------------------------------------------------------------------
Query Block Name / Object Alias (identified by operation id):
-------------------------------------------------------------
1 - SEL$1
2 - SEL$2 / from$_subquery$_001@SEL$1
3 - SEL$2 / T@SEL$2
4 - SEL$2 / T@SEL$2
Predicate Information (identified by operation id):
---------------------------------------------------
1 - filter(ROWNUM<=10)
4 - access("T"."SYS_NC00004$">=HEXTORAW('3BFDFF') AND "T"."SYS_NC00004$" IS NOT NULL)
filter(SYS_OP_UNDESCEND("T"."SYS_NC00004$")<=1000000)
32 rows selected.
--//邏輯讀僅僅6個.而其它2個索引都無法達到這樣的效果.
select * from (select /*+ index(t I_T_ID1DESC_ID2DESC) */ * from t where id1<=1e6 order by id1 desc,id2 ) where rownum<=10;
select * from (select /*+ index_desc(t I_T_ID1DESC_ID2DESC) */ * from t where id1<=1e6 order by id1 desc,id2 ) where rownum<=10;
select * from (select /*+ index(t I_T_ID1_ID2) */ * from t where id1<=1e6 order by id1 desc,id2 ) where rownum<=10;
select * from (select /*+ index_desc(t I_T_ID1_ID2) */ * from t where id1<=1e6 order by id1 desc,id2 ) where rownum<=10;
--//結果不在貼出.大家可以自行測試.
3.繼續測試:
SCOTT@book> alter index I_T_ID1DESC_ID2 invisible;
Index altered.
SCOTT@book> select * from (select * from t where id1<=1e6 order by id1 desc,id2 ) where rownum<=1;
ID1 ID2 VC
---------- ---------- --------------------------------------------------
1000000 1000000 aaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaa
SCOTT@book> @ dpc '' ''
PLAN_TABLE_OUTPUT
-------------------------------------
SQL_ID 2r7j4a5gdhpnj, child number 0
-------------------------------------
select * from (select * from t where id1<=1e6 order by id1 desc,id2 )
where rownum<=1
Plan hash value: 3299198703
----------------------------------------------------------------------------------------------------------------------------------------------------------------------
| Id | Operation | Name | Starts | E-Rows |E-Bytes|E-Temp | Cost (%CPU)| E-Time | A-Rows | A-Time | Buffers | Reads | OMem | 1Mem | Used-Mem |
----------------------------------------------------------------------------------------------------------------------------------------------------------------------
| 0 | SELECT STATEMENT | | 1 | | | | 17500 (100)| | 1 |00:00:00.38 | 9285 | 74 | | | |
|* 1 | COUNT STOPKEY | | 1 | | | | | | 1 |00:00:00.38 | 9285 | 74 | | | |
| 2 | VIEW | | 1 | 1000K| 50M| | 17500 (1)| 00:03:30 | 1 |00:00:00.38 | 9285 | 74 | | | |
|* 3 | SORT ORDER BY STOPKEY| | 1 | 1000K| 58M| 69M| 17500 (1)| 00:03:30 | 1 |00:00:00.38 | 9285 | 74 | 65M| 2806K| |
|* 4 | TABLE ACCESS FULL | T | 1 | 1000K| 58M| | 2744 (1)| 00:00:33 | 1000K|00:00:00.12 | 9285 | 74 | | | |
----------------------------------------------------------------------------------------------------------------------------------------------------------------------
Query Block Name / Object Alias (identified by operation id):
-------------------------------------------------------------
1 - SEL$1
2 - SEL$2 / from$_subquery$_001@SEL$1
3 - SEL$2
4 - SEL$2 / T@SEL$2
Predicate Information (identified by operation id):
---------------------------------------------------
1 - filter(ROWNUM<=1)
3 - filter(ROWNUM<=1)
4 - filter("ID1"<=1000000)
32 rows selected.
--//oracle選擇全表掃描.
SCOTT@book> alter index I_T_ID1DESC_ID2 visible;
Index altered.
4.再繼續測試:
--//測試第1個欄位正序,第2個欄位倒序的情況.
SCOTT@book> select * from (select * from t where id1<=1e6 order by id1 ,id2 desc ) where rownum<=5;
ID1 ID2 VC
---------- ---------- --------------------------------------------------
1 1 aaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaa
2 2 aaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaa
3 3 aaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaa
4 4 aaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaa
5 5 aaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaa
SCOTT@book> @ dpc '' ''
PLAN_TABLE_OUTPUT
-------------------------------------
SQL_ID 8f5yj8c3frpnc, child number 0
-------------------------------------
select * from (select * from t where id1<=1e6 order by id1 ,id2 desc )
where rownum<=5
Plan hash value: 2787951352
-------------------------------------------------------------------------------------------------------------------------------------------
| Id | Operation | Name | Starts | E-Rows |E-Bytes| Cost (%CPU)| E-Time | A-Rows | A-Time | Buffers |
-------------------------------------------------------------------------------------------------------------------------------------------
| 0 | SELECT STATEMENT | | 1 | | | 4 (100)| | 5 |00:00:00.01 | 6 |
|* 1 | COUNT STOPKEY | | 1 | | | | | 5 |00:00:00.01 | 6 |
| 2 | VIEW | | 1 | 5 | 265 | 4 (0)| 00:00:01 | 5 |00:00:00.01 | 6 |
| 3 | TABLE ACCESS BY INDEX ROWID| T | 1 | 1000K| 58M| 4 (0)| 00:00:01 | 5 |00:00:00.01 | 6 |
|* 4 | INDEX RANGE SCAN | I_T_ID1_ID2DESC | 1 | 5 | | 3 (0)| 00:00:01 | 5 |00:00:00.01 | 4 |
-------------------------------------------------------------------------------------------------------------------------------------------
Query Block Name / Object Alias (identified by operation id):
-------------------------------------------------------------
1 - SEL$1
2 - SEL$2 / from$_subquery$_001@SEL$1
3 - SEL$2 / T@SEL$2
4 - SEL$2 / T@SEL$2
Predicate Information (identified by operation id):
---------------------------------------------------
1 - filter(ROWNUM<=5)
4 - access("ID1"<=1000000)
31 rows selected.
--//可以發現很好第使用I_T_ID1_ID2DESC索引.邏輯讀也很小,僅僅6.
SCOTT@book> alter index I_T_ID1_ID2DESC invisible;
Index altered.
select * from (select * from t where id1<=1e6 order by id1 ,id2 desc ) where rownum<=1;
SCOTT@book> select * from (select * from t where id1<=1e6 order by id1 ,id2 desc ) where rownum<=5;
ID1 ID2 VC
---------- ---------- --------------------------------------------------
1 1 aaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaa
2 2 aaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaa
3 3 aaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaa
4 4 aaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaa
5 5 aaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaa
SCOTT@book> @ dpc '' ''
PLAN_TABLE_OUTPUT
-------------------------------------
SQL_ID 8f5yj8c3frpnc, child number 0
-------------------------------------
select * from (select * from t where id1<=1e6 order by id1 ,id2 desc )
where rownum<=5
Plan hash value: 2145689175
-----------------------------------------------------------------------------------------------------------------------------------------------------
| Id | Operation | Name | Starts | E-Rows |E-Bytes| Cost (%CPU)| E-Time | A-Rows | A-Time | Buffers | Reads |
-----------------------------------------------------------------------------------------------------------------------------------------------------
| 0 | SELECT STATEMENT | | 1 | | | 4 (100)| | 5 |00:00:00.01 | 6 | 1 |
|* 1 | COUNT STOPKEY | | 1 | | | | | 5 |00:00:00.01 | 6 | 1 |
| 2 | VIEW | | 1 | 5 | 265 | 4 (0)| 00:00:01 | 5 |00:00:00.01 | 6 | 1 |
| 3 | TABLE ACCESS BY INDEX ROWID | T | 1 | 1000K| 58M| 4 (0)| 00:00:01 | 5 |00:00:00.01 | 6 | 1 |
|* 4 | INDEX RANGE SCAN DESCENDING| I_T_ID1DESC_ID2 | 1 | 5 | | 3 (0)| 00:00:01 | 5 |00:00:00.01 | 4 | 1 |
-----------------------------------------------------------------------------------------------------------------------------------------------------
Query Block Name / Object Alias (identified by operation id):
-------------------------------------------------------------
1 - SEL$1
2 - SEL$2 / from$_subquery$_001@SEL$1
3 - SEL$2 / T@SEL$2
4 - SEL$2 / T@SEL$2
Predicate Information (identified by operation id):
---------------------------------------------------
1 - filter(ROWNUM<=5)
4 - access("T"."SYS_NC00004$" IS NOT NULL AND "T"."SYS_NC00004$">=HEXTORAW('3BFDFF') )
filter(SYS_OP_UNDESCEND("T"."SYS_NC00004$")<=1000000)
32 rows selected.
--//你會發現一個奇特現象,oracle會使用"倒過來"的一個索引,也就是id1 desc,id2 asc的索引.
SCOTT@book> alter index I_T_ID1DESC_ID2 invisible;
Index altered.
SCOTT@book> select * from (select * from t where id1<=1e6 order by id1 ,id2 desc ) where rownum<=5;
ID1 ID2 VC
---------- ---------- --------------------------------------------------
1 1 aaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaa
2 2 aaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaa
3 3 aaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaa
4 4 aaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaa
5 5 aaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaa
SCOTT@book> @ dpc '' ''
PLAN_TABLE_OUTPUT
-------------------------------------
SQL_ID 8f5yj8c3frpnc, child number 0
-------------------------------------
select * from (select * from t where id1<=1e6 order by id1 ,id2 desc )
where rownum<=5
Plan hash value: 3299198703
----------------------------------------------------------------------------------------------------------------------------------------------------------------------
| Id | Operation | Name | Starts | E-Rows |E-Bytes|E-Temp | Cost (%CPU)| E-Time | A-Rows | A-Time | Buffers | Reads | OMem | 1Mem | Used-Mem |
----------------------------------------------------------------------------------------------------------------------------------------------------------------------
| 0 | SELECT STATEMENT | | 1 | | | | 17500 (100)| | 5 |00:00:00.41 | 9279 | 9269 | | | |
|* 1 | COUNT STOPKEY | | 1 | | | | | | 5 |00:00:00.41 | 9279 | 9269 | | | |
| 2 | VIEW | | 1 | 1000K| 50M| | 17500 (1)| 00:03:30 | 5 |00:00:00.41 | 9279 | 9269 | | | |
|* 3 | SORT ORDER BY STOPKEY| | 1 | 1000K| 58M| 69M| 17500 (1)| 00:03:30 | 5 |00:00:00.41 | 9279 | 9269 | 2048 | 2048 | 2048 (0)|
|* 4 | TABLE ACCESS FULL | T | 1 | 1000K| 58M| | 2744 (1)| 00:00:33 | 1000K|00:00:00.15 | 9279 | 9269 | | | |
----------------------------------------------------------------------------------------------------------------------------------------------------------------------
Query Block Name / Object Alias (identified by operation id):
-------------------------------------------------------------
1 - SEL$1
2 - SEL$2 / from$_subquery$_001@SEL$1
3 - SEL$2
4 - SEL$2 / T@SEL$2
Predicate Information (identified by operation id):
---------------------------------------------------
1 - filter(ROWNUM<=5)
3 - filter(ROWNUM<=5)
4 - filter("ID1"<=1000000)
32 rows selected.
--//選擇的是全表掃描.
SCOTT@book> alter index i_t_id1desc_id2desc invisible;
Index altered.
SCOTT@book> select * from (select * from t where id1<=1e6 order by id1 desc ,id2 desc ) where rownum<=1;
ID1 ID2 VC
---------- ---------- --------------------------------------------------
1000000 1000000 aaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaa
SCOTT@book> @ dpc '' ''
PLAN_TABLE_OUTPUT
-------------------------------------
SQL_ID 66z6w3qc77jwp, child number 0
-------------------------------------
select * from (select * from t where id1<=1e6 order by id1 desc ,id2
desc ) where rownum<=1
Plan hash value: 3873686303
----------------------------------------------------------------------------------------------------------------------------------------
| Id | Operation | Name | Starts | E-Rows |E-Bytes| Cost (%CPU)| E-Time | A-Rows | A-Time | Buffers |
----------------------------------------------------------------------------------------------------------------------------------------
| 0 | SELECT STATEMENT | | 1 | | | 4 (100)| | 1 |00:00:00.01 | 4 |
|* 1 | COUNT STOPKEY | | 1 | | | | | 1 |00:00:00.01 | 4 |
| 2 | VIEW | | 1 | 1 | 53 | 4 (0)| 00:00:01 | 1 |00:00:00.01 | 4 |
| 3 | TABLE ACCESS BY INDEX ROWID | T | 1 | 1000K| 58M| 4 (0)| 00:00:01 | 1 |00:00:00.01 | 4 |
|* 4 | INDEX RANGE SCAN DESCENDING| I_T_ID1_ID2 | 1 | 1 | | 3 (0)| 00:00:01 | 1 |00:00:00.01 | 3 |
----------------------------------------------------------------------------------------------------------------------------------------
--//也能很好的選擇正序索引.
5.總結:
--//透過以上測試,僅僅一種特殊的情況才需要建立倒序索引.存在id1 desc,id2 asc的情況下,建議完全可反過來建立(特別對於自增序列欄位),
--//就是反過來建立索引id1 ,id2 desc,這樣的索引佔用磁碟空間更小.
--//其它情況我看不到降序索引的優勢.
來自 “ ITPUB部落格 ” ，連結：http://blog.itpub.net/267265/viewspace-2669138/，如需轉載，請註明出處，否則將追究法律責任。
相關文章
- [20191209]降序索引疑問.txt索引
- [20181124]關於降序索引問題4.txt索引
- [20191209]降序索引疑問2.txt索引
- [20200303]降序索引疑問5.txt索引
- [20191210]降序索引疑問3.txt索引
- [20181123]關於降序索引問題.txt索引
- [20190910]關於降序索引問題5.txt索引
- 不能建立降序索引的問題的解決索引
- [20181124]關於降序索引問題3.txt索引
- [20181124]關於降序索引問題2.txt索引
- [20231116]降序索引取最大值.txt索引
- [20191219]降序索引與取最大值.txt索引
- 有些疑問
- 我的疑問
- pycharm 小疑問PyCharm
- 【疑問】記錄
- 做學問要在不疑之處有疑
- locust 使用的疑問
- 帶著疑問看故事
- 面試疑難問題面試
- Solidity學習疑問總結Solid
- 關於 dingo API 的疑問GoAPI
- [20190411]linux stat 命令疑問.txtLinux
- №20191218簡易操作1~7(今期：上=下=6)
- 安卓串列埠通訊疑問安卓串列埠
- Android 編譯打包的那些疑問Android編譯
- 反射型 XSS 疑問及延伸（CSRF）反射
- [20190401]那個更快的疑問.txt
- [20191218]各種編輯器的退出操作.txt
- 工作疑難問題解決4例
- [20191213]不完全恢復疑問.txt
- Vue學習遇到疑問的總結Vue
- RESTful設計中的常見疑問REST
- Android學習: 疑難問題總結Android
- java——ArrayList中contains（）方法中的疑問JavaAI
- [20231031]Index ITL Limit 4.txtIndexMIT
- 關於租用香港伺服器疑問解答伺服器
- 關於租用香港伺服器疑問解答。伺服器