[20191218]降序索引疑問4.txt
[20191218]降序索引疑問4.txt
--//前幾天優化一個項目,我發現許多表裏面有有隱含字段,通常開發不多創建函數索引.我本身檢查發現裏面存在大量的降序索引.
--//我感受有點奇怪,爲何開發要創建大量降序索引有什麼好處呢?
--//我在連接http://www.itpub.net/thread-2122088-1-1.html裏面問這個問題,sqysl的解答給了我很好的提示,我經過例子說明使用降
--//序索引的一點點好處,經過例子說明:
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
--//不管那種倒序索引,索引都比正序索引大,特別是第1個字段選擇desc的狀況.由於我插入的字段是自增字段.
--//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個索引都沒法達到這樣的效果.
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.你們能夠自行測試.
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個字段正序,第2個字段倒序的狀況.
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.
--//能夠發現很好第使用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:
--//經過以上測試,僅僅一種特殊的狀況才須要創建倒序索引.存在id1 desc,id2 asc的狀況下,建議徹底可反過來創建(特別對於自增序列字段),
--//就是反過來創建索引id1 ,id2 desc,這樣的索引佔用磁盤空間更小.
--//其它狀況我看不到降序索引的優點.html
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