[20120228]IOT索引組織表相關資訊的學習.txt

IOT 是一種特殊的索引結構，使用它能夠解決特定場合的應用問題，但是在許多應用中很少使用，更多的是使用堆表。
我僅僅在生產系統中對3個表使用，我偏向使用靜態以及"只讀"的小表。

如果IOT表存在除主鍵外的第2索引，如果使用它存在物理猜"physical guess",我以前的理解一直是第2索引記錄了主鍵資訊，
透過這個資訊就可以定位IOT表中對應的資料，一直沒有很好的測試與理解。我最近也看了兩個連結,介紹了IOT的內容：

今天測試看看，看了以前的理解存在很大的偏差！

1.測試環境:

SQL> select * from v$version ;

BANNER

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

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 Linux: Version 11.2.0.1.0 - Production

NLSRTL Version 11.2.0.1.0 - Production


SQL> create table t_iot ( a varchar2(10),b varchar2(10),vc varchar2(1200), constraint t_iot_pk PRIMARY KEY(a)) ORGANIZATION INDEX;

SQL> create index i_t_iot_b on t_iot(b);

insert into t_iot values ('1'        ,'a','a');

insert into t_iot values ('22'       ,'b','a');

insert into t_iot values ('333'      ,'c','a');

insert into t_iot values ('4444'     ,'d','a');

insert into t_iot values ('55555'    ,'e','a');

insert into t_iot values ('666666'   ,'f','a');

insert into t_iot values ('7777777'  ,'g','a');

insert into t_iot values ('88888888' ,'h','a');

insert into t_iot values ('999999999','i','a');

commit ;


SQL> exec dbms_stats.gather_table_stats(ownname=>USER, tabname=>'T_iot');

2.轉儲IOT塊的資訊:

SQL> SELECT index_name, table_name, blevel, leaf_blocks FROM dba_indexes  WHERE table_name = 'T_IOT';

INDEX_NAME                     TABLE_NAME                         BLEVEL LEAF_BLOCKS

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

T_IOT_PK                       T_IOT                                   0           1

I_T_IOT_B                      T_IOT                                   0           1


--可以發現索引很小，blevel=0,leaf_blocks=1.


SQL> select header_file,header_block from dba_segments where segment_name='T_IOT_PK';

HEADER_FILE HEADER_BLOCK

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

          4          954


SQL> select object_id,data_object_id from dba_objects where object_name='T_IOT_PK';

 OBJECT_ID DATA_OBJECT_ID

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

     83453          83453

SQL> ALTER SESSION SET EVENTS 'immediate trace name treedump level 83453';


----- begin tree dump

leaf: 0x10003bb 16778171 (0: nrow: 9 rrow: 9)

----- end tree dump

僅僅佔用1個塊。HEADER_BLOCK=954,根節點=955.

SQL> alter system dump datafile 4 block 955 ;

Block header dump: 0x010003bb
Object id on Block? Y
seg/obj: 0x145fd csc: 0x00.63a965 itc: 2 flg: E typ: 2 - INDEX
     brn: 0 bdba: 0x10003b8 ver: 0x01 opc: 0
     inc: 0 exflg: 0

Itl           Xid                  Uba         Flag Lck        Scn/Fsc
0x01   0x0000.000.00000000 0x00000000.0000.00 ----    0 fsc 0x0000.00000000
0x02   0x0002.020.00000c9f 0x00c00b1b.066e.02 --U-    9 fsc 0x0000.0063a9ce
Leaf block dump
===============
header address 182924431972=0x2a97255264
kdxcolev 0
KDXCOLEV Flags = - - -
kdxcolok 0
kdxcoopc 0x90: pcode=0: iot flags=I-- is converted=Y
kdxconco 1
kdxcosdc 0
kdxconro 9
kdxcofbo 54=0x36
kdxcofeo 7897=0x1ed9
kdxcoavs 7843
kdxlespl 0
kdxlende 0
kdxlenxt 0=0x0
kdxleprv 0=0x0
kdxledsz 0
kdxlebksz 8032
row#0[8021] flag: K-----, lock: 2, len=11
col 0; len 1; (1): 31
tl: 7 fb: --H-FL-- lb: 0x0 cc: 2
col 0: [ 1] 61
col 1: [ 1] 61
row#1[8009] flag: K-----, lock: 2, len=12
col 0; len 2; (2): 32 32
tl: 7 fb: --H-FL-- lb: 0x0 cc: 2
col 0: [ 1] 62
col 1: [ 1] 61
row#2[7996] flag: K-----, lock: 2, len=13
col 0; len 3; (3): 33 33 33
tl: 7 fb: --H-FL-- lb: 0x0 cc: 2
col 0: [ 1] 63
col 1: [ 1] 61
row#3[7982] flag: K-----, lock: 2, len=14
col 0; len 4; (4): 34 34 34 34
tl: 7 fb: --H-FL-- lb: 0x0 cc: 2
col 0: [ 1] 64
col 1: [ 1] 61
row#4[7967] flag: K-----, lock: 2, len=15
col 0; len 5; (5): 35 35 35 35 35
tl: 7 fb: --H-FL-- lb: 0x0 cc: 2
col 0: [ 1] 65
col 1: [ 1] 61
row#5[7951] flag: K-----, lock: 2, len=16
col 0; len 6; (6): 36 36 36 36 36 36
tl: 7 fb: --H-FL-- lb: 0x0 cc: 2
col 0: [ 1] 66
col 1: [ 1] 61
row#6[7934] flag: K-----, lock: 2, len=17
col 0; len 7; (7): 37 37 37 37 37 37 37
tl: 7 fb: --H-FL-- lb: 0x0 cc: 2
col 0: [ 1] 67
col 1: [ 1] 61
row#7[7916] flag: K-----, lock: 2, len=18
col 0; len 8; (8): 38 38 38 38 38 38 38 38
tl: 7 fb: --H-FL-- lb: 0x0 cc: 2
col 0: [ 1] 68
col 1: [ 1] 61
row#8[7897] flag: K-----, lock: 2, len=19
col 0; len 9; (9): 39 39 39 39 39 39 39 39 39
tl: 7 fb: --H-FL-- lb: 0x0 cc: 2
col 0: [ 1] 69
col 1: [ 1] 61
----- end of leaf block dump -----
End dump data blocks tsn: 4 file#: 4 minblk 955 maxblk 955

--可以發現沒有記錄rowid資訊，
tl: 7 fb: --H-FL-- lb: 0x0 cc: 2
col 0: [ 1] 69
col 1: [ 1] 61
--記錄了主鍵外的資訊。

3.轉儲第2索引的資訊:

SQL> select header_file,header_block from dba_segments where segment_name='I_T_IOT_B';

HEADER_FILE HEADER_BLOCK

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

          4          962


SQL> select object_id,data_object_id from dba_objects where object_name='I_T_IOT_B';

 OBJECT_ID DATA_OBJECT_ID

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

     83454          83454


SQL> ALTER SESSION SET EVENTS 'immediate trace name treedump level 83454';

----- begin tree dump

leaf: 0x10003c3 16778179 (0: nrow: 9 rrow: 9)

----- end tree dump


僅僅佔用1個塊。HEADER_BLOCK=962,根節點=963.


SQL> alter system dump datafile 4 block 963 ;

Block header dump: 0x010003c3
Object id on Block? Y
seg/obj: 0x145fe csc: 0x00.63a9bd itc: 2 flg: E typ: 2 - INDEX
     brn: 0 bdba: 0x10003c0 ver: 0x01 opc: 0
     inc: 0 exflg: 0

Itl           Xid                  Uba         Flag Lck        Scn/Fsc
0x01   0x0000.000.00000000 0x00000000.0000.00 ----    0 fsc 0x0000.00000000
0x02   0x0002.020.00000c9f 0x00c00b1b.066e.03 --U-    9 fsc 0x0000.0063a9ce
Leaf block dump
===============
header address 182924431972=0x2a97255264
kdxcolev 0
KDXCOLEV Flags = - - -
kdxcolok 0
kdxcoopc 0x90: pcode=0: iot flags=I-- is converted=Y
kdxconco 2
kdxcosdc 0
kdxconro 9
kdxcofbo 54=0x36
kdxcofeo 7870=0x1ebe
kdxcoavs 7816
kdxlespl 0
kdxlende 0
kdxlenxt 0=0x0
kdxleprv 0=0x0
kdxledsz 0
kdxlebksz 8032
row#0[8018] flag: K-----, lock: 2, len=14
col 0; len 1; (1): 61
col 1; len 1; (1): 31
tl: 8 fb: --H-FL-- lb: 0x0 cc: 1
col 0: [ 4] 01 00 03 bb
row#1[8003] flag: K-----, lock: 2, len=15
col 0; len 1; (1): 62
col 1; len 2; (2): 32 32
tl: 8 fb: --H-FL-- lb: 0x0 cc: 1
col 0: [ 4] 01 00 03 bb
row#2[7987] flag: K-----, lock: 2, len=16
col 0; len 1; (1): 63
col 1; len 3; (3): 33 33 33
tl: 8 fb: --H-FL-- lb: 0x0 cc: 1
col 0: [ 4] 01 00 03 bb
row#3[7970] flag: K-----, lock: 2, len=17
col 0; len 1; (1): 64
col 1; len 4; (4): 34 34 34 34
tl: 8 fb: --H-FL-- lb: 0x0 cc: 1
col 0: [ 4] 01 00 03 bb
row#4[7952] flag: K-----, lock: 2, len=18
col 0; len 1; (1): 65
col 1; len 5; (5): 35 35 35 35 35
tl: 8 fb: --H-FL-- lb: 0x0 cc: 1
col 0: [ 4] 01 00 03 bb
row#5[7933] flag: K-----, lock: 2, len=19
col 0; len 1; (1): 66
col 1; len 6; (6): 36 36 36 36 36 36
tl: 8 fb: --H-FL-- lb: 0x0 cc: 1
col 0: [ 4] 01 00 03 bb
row#6[7913] flag: K-----, lock: 2, len=20
col 0; len 1; (1): 67
col 1; len 7; (7): 37 37 37 37 37 37 37
tl: 8 fb: --H-FL-- lb: 0x0 cc: 1
col 0: [ 4] 01 00 03 bb
row#7[7892] flag: K-----, lock: 2, len=21
col 0; len 1; (1): 68
col 1; len 8; (8): 38 38 38 38 38 38 38 38
tl: 8 fb: --H-FL-- lb: 0x0 cc: 1
col 0: [ 4] 01 00 03 bb
row#8[7870] flag: K-----, lock: 2, len=22
col 0; len 1; (1): 69
col 1; len 9; (9): 39 39 39 39 39 39 39 39 39
tl: 8 fb: --H-FL-- lb: 0x0 cc: 1
col 0: [ 4] 01 00 03 bb
----- end of leaf block dump -----
End dump data blocks tsn: 4 file#: 4 minblk 963 maxblk 963

-- 可以發現第2索引的記錄資訊如下：
col0 ＝》對應的是欄位b
col1 ＝》對應的是欄位a，也就是主鍵。
而後面還存在如下資訊：
tl: 8 fb: --H-FL-- lb: 0x0 cc: 1
col 0: [ 4] 01 00 03 bb
-- 可以發現記錄的資訊都是一樣的,col0: [ 4] 01 00 03 bb,應該跟上面的轉儲對上:

SQL> ALTER SESSION SET EVENTS 'immediate trace name treedump level 83453';
----- begin tree dump
leaf: 0x10003bb 16778171 (0: nrow: 9 rrow: 9)
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
----- end tree dump

也就是T_IOT_PK的根節點。
可以得到一些推測：
1.如果透過第2索引查詢主鍵，是不需要查詢IOT表的，因為第2索引裡面已經存在。
2.如果透過第2索引查詢資料資訊，IOT先透過索引節點中記錄的資料塊資訊查詢(這個就應該就是物理猜)，如果不行可以透過主鍵也就是對應例子的col1鍵值，來探查IOT索引組織表。

4.做一些測試：

SQL> set autotrace traceonly ;

SQL> select a from t_iot where b='a';

Execution Plan

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

Plan hash value: 3946844304

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

| Id  | Operation        | Name      | Rows  | Bytes | Cost (%CPU)| Time     |

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

|   0 | SELECT STATEMENT |           |     1 |     8 |     1   (0)| 00:00:01 |

|*  1 |  INDEX RANGE SCAN| I_T_IOT_B |     1 |     8 |     1   (0)| 00:00:01 |

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

Predicate Information (identified by operation id):

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

   1 - access("B"='a')

--可以發現並沒有查詢IOT表，而是直接訪問索引。

SQL> select vc from t_iot where b='a';

Execution Plan

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

Plan hash value: 1095339046

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

| Id  | Operation         | Name      | Rows  | Bytes | Cost (%CPU)| Time     |

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

|   0 | SELECT STATEMENT  |           |     1 |     4 |     1   (0)| 00:00:01 |

|*  1 |  INDEX UNIQUE SCAN| T_IOT_PK  |     1 |     4 |     1   (0)| 00:00:01 |

|*  2 |   INDEX RANGE SCAN| I_T_IOT_B |     1 |       |     1   (0)| 00:00:01 |

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

Predicate Information (identified by operation id):

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

   1 - access("B"='a')

   2 - access("B"='a')

Statistics

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

          1  recursive calls

          0  db block gets

          3  consistent gets

          0  physical reads

          0  redo size

        519  bytes sent via SQL*Net to client

        519  bytes received via SQL*Net from client

          2  SQL*Net roundtrips to/from client

          0  sorts (memory)

          0  sorts (disk)

          1  rows processed

--可以發現先查詢i_t_iot_b索引，然後在查詢T_IOT索引組織表。

5.再來了解一下邏輯rowid。

SQL> column vc format a30

SQL> select rowid,a.* from t_iot a;

ROWID                     A          B          VC

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

*BAEAA7sBMf4              1          a          a

*BAEAA7sCMjL+             22         b          a

*BAEAA7sDMzMz/g           333        c          a

*BAEAA7sENDQ0NP4          4444       d          a

*BAEAA7sFNTU1NTX+         55555      e          a

*BAEAA7sGNjY2NjY2/g       666666     f          a

*BAEAA7sHNzc3Nzc3N/4      7777777    g          a

*BAEAA7sIODg4ODg4ODj+     88888888   h          a

*BAEAA7sJOTk5OTk5OTk5/g   999999999  i          a


9 rows selected.

--可以發現一些特點，前面都是一樣的都是*BAEAA7s，而且主鍵鍵值的長度越長與rowid的長度也越長。再dump(rowid)

SQL> column x format a70

SQL> select dump(rowid) x,rowid,a,b from t_iot ;

X                                                                      ROWID                     A          B

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

Typ=208 Len=9: 2,4,1,0,3,187,1,49,254                                  *BAEAA7sBMf4              1          a

Typ=208 Len=10: 2,4,1,0,3,187,2,50,50,254                              *BAEAA7sCMjL+             22         b

Typ=208 Len=11: 2,4,1,0,3,187,3,51,51,51,254                           *BAEAA7sDMzMz/g           333        c

Typ=208 Len=12: 2,4,1,0,3,187,4,52,52,52,52,254                        *BAEAA7sENDQ0NP4          4444       d

Typ=208 Len=13: 2,4,1,0,3,187,5,53,53,53,53,53,254                     *BAEAA7sFNTU1NTX+         55555      e

Typ=208 Len=14: 2,4,1,0,3,187,6,54,54,54,54,54,54,254                  *BAEAA7sGNjY2NjY2/g       666666     f

Typ=208 Len=15: 2,4,1,0,3,187,7,55,55,55,55,55,55,55,254               *BAEAA7sHNzc3Nzc3N/4      7777777    g

Typ=208 Len=16: 2,4,1,0,3,187,8,56,56,56,56,56,56,56,56,254            *BAEAA7sIODg4ODg4ODj+     88888888   h

Typ=208 Len=17: 2,4,1,0,3,187,9,57,57,57,57,57,57,57,57,57,254         *BAEAA7sJOTk5OTk5OTk5/g   999999999  i


9 rows selected.


SQL> select dump(rowid,16) x,rowid,a,b from t_iot ;

X                                                                      ROWID                     A          B

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

Typ=208 Len=9: 2,4,1,0,3,bb,1,31,fe                                    *BAEAA7sBMf4              1          a

Typ=208 Len=10: 2,4,1,0,3,bb,2,32,32,fe                                *BAEAA7sCMjL+             22         b

Typ=208 Len=11: 2,4,1,0,3,bb,3,33,33,33,fe                             *BAEAA7sDMzMz/g           333        c

Typ=208 Len=12: 2,4,1,0,3,bb,4,34,34,34,34,fe                          *BAEAA7sENDQ0NP4          4444       d

Typ=208 Len=13: 2,4,1,0,3,bb,5,35,35,35,35,35,fe                       *BAEAA7sFNTU1NTX+         55555      e

Typ=208 Len=14: 2,4,1,0,3,bb,6,36,36,36,36,36,36,fe                    *BAEAA7sGNjY2NjY2/g       666666     f

Typ=208 Len=15: 2,4,1,0,3,bb,7,37,37,37,37,37,37,37,fe                 *BAEAA7sHNzc3Nzc3N/4      7777777    g

Typ=208 Len=16: 2,4,1,0,3,bb,8,38,38,38,38,38,38,38,38,fe              *BAEAA7sIODg4ODg4ODj+     88888888   h

Typ=208 Len=17: 2,4,1,0,3,bb,9,39,39,39,39,39,39,39,39,39,fe           *BAEAA7sJOTk5OTk5OTk5/g   999999999  i


9 rows selected.

--結合dump（rowid）,dump(rowid,16)的轉出資訊.很容易推匯出rowid的資訊。以下的例子來說明:
Typ=208 Len=17: 2,4,1,0,3,bb,9,39,39,39,39,39,39,39,39,39,fe *BAEAA7sJOTk5OTk5OTk5/g 999999999 i

1.開頭的2,4，以及結尾fe實在不好猜測，不過好像是固定不變的。
2.中間的1,0,3,bb正好對應的就是IOT的根節點，也就是對應資料IOT的資料塊。
3.9,39,39,39,39,39,39,39,39,39中9表示主鍵長度，39,39,39,39,39,39,39,39,39就是主鍵的資訊。

6.關於物理猜:

SQL> column name format a30

SQL> column value format 999999999

SQL> SELECT n.name, s.value FROM v$mystat s, v$statname n WHERE s.statistic# = n.statistic# AND lower(n.name) like '%leaf%';

NAME                                VALUE

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

leaf node splits                        0

leaf node 90-10 splits                  0


SQL> update t_iot set vc=lpad('a',1000,'a') ;

9 rows updated.


SQL> SELECT n.name, s.value FROM v$mystat s, v$statname n WHERE s.statistic# = n.statistic# AND lower(n.name) like '%leaf%';

NAME                                VALUE

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

leaf node splits                        1

leaf node 90-10 splits                  0


SQL> commit;

--vc長度加大到1000,原來的IOT已經無法存在這些資訊，可以發現IOT表發生了1次索引分裂。

SQL> select dump(rowid,16) x,rowid,a,b from t_iot ;

X                                                                      ROWID                     A          B

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

Typ=208 Len=9: 2,4,1,0,3,bd,1,31,fe                                    *BAEAA70BMf4              1          a

Typ=208 Len=10: 2,4,1,0,3,bd,2,32,32,fe                                *BAEAA70CMjL+             22         b

Typ=208 Len=11: 2,4,1,0,3,bd,3,33,33,33,fe                             *BAEAA70DMzMz/g           333        c

Typ=208 Len=12: 2,4,1,0,3,bd,4,34,34,34,34,fe                          *BAEAA70ENDQ0NP4          4444       d

Typ=208 Len=13: 2,4,1,0,3,bd,5,35,35,35,35,35,fe                       *BAEAA70FNTU1NTX+         55555      e

Typ=208 Len=14: 2,4,1,0,3,bd,6,36,36,36,36,36,36,fe                    *BAEAA70GNjY2NjY2/g       666666     f

Typ=208 Len=15: 2,4,1,0,3,be,7,37,37,37,37,37,37,37,fe                 *BAEAA74HNzc3Nzc3N/4      7777777    g

Typ=208 Len=16: 2,4,1,0,3,be,8,38,38,38,38,38,38,38,38,fe              *BAEAA74IODg4ODg4ODj+     88888888   h

Typ=208 Len=17: 2,4,1,0,3,be,9,39,39,39,39,39,39,39,39,39,fe           *BAEAA74JOTk5OTk5OTk5/g   999999999  i

9 rows selected.

--可以發現邏輯rowid記錄的塊資訊發現了變化。從1,0,3,bb變成了1,0,3,bd和1,0,3,be.

--再次轉儲第2索引的資訊。
SQL> alter system dump datafile 4 block 963 ;

Block header dump: 0x010003c3
Object id on Block? Y
seg/obj: 0x145fe csc: 0x00.63a9bd itc: 2 flg: E typ: 2 - INDEX
     brn: 0 bdba: 0x10003c0 ver: 0x01 opc: 0
     inc: 0 exflg: 0

Itl           Xid                  Uba         Flag Lck        Scn/Fsc
0x01   0x0000.000.00000000 0x00000000.0000.00 ----    0 fsc 0x0000.00000000
0x02   0x0002.020.00000c9f 0x00c00b1b.066e.03 --U-    9 fsc 0x0000.0063a9ce
Leaf block dump
===============
header address 182925401700=0x2a97341e64
kdxcolev 0
KDXCOLEV Flags = - - -
kdxcolok 0
kdxcoopc 0x90: pcode=0: iot flags=I-- is converted=Y
kdxconco 2
kdxcosdc 0
kdxconro 9
kdxcofbo 54=0x36
kdxcofeo 7870=0x1ebe
kdxcoavs 7816
kdxlespl 0
kdxlende 0
kdxlenxt 0=0x0
kdxleprv 0=0x0
kdxledsz 0
kdxlebksz 8032
row#0[8018] flag: K-----, lock: 2, len=14
col 0; len 1; (1): 61
col 1; len 1; (1): 31
tl: 8 fb: --H-FL-- lb: 0x0 cc: 1
col 0: [ 4] 01 00 03 bb
row#1[8003] flag: K-----, lock: 2, len=15
col 0; len 1; (1): 62
col 1; len 2; (2): 32 32
tl: 8 fb: --H-FL-- lb: 0x0 cc: 1
col 0: [ 4] 01 00 03 bb
row#2[7987] flag: K-----, lock: 2, len=16
col 0; len 1; (1): 63
col 1; len 3; (3): 33 33 33
tl: 8 fb: --H-FL-- lb: 0x0 cc: 1
col 0: [ 4] 01 00 03 bb
row#3[7970] flag: K-----, lock: 2, len=17
col 0; len 1; (1): 64
col 1; len 4; (4): 34 34 34 34
tl: 8 fb: --H-FL-- lb: 0x0 cc: 1
col 0: [ 4] 01 00 03 bb
row#4[7952] flag: K-----, lock: 2, len=18
col 0; len 1; (1): 65
col 1; len 5; (5): 35 35 35 35 35
tl: 8 fb: --H-FL-- lb: 0x0 cc: 1
col 0: [ 4] 01 00 03 bb
row#5[7933] flag: K-----, lock: 2, len=19
col 0; len 1; (1): 66
col 1; len 6; (6): 36 36 36 36 36 36
tl: 8 fb: --H-FL-- lb: 0x0 cc: 1
col 0: [ 4] 01 00 03 bb
row#6[7913] flag: K-----, lock: 2, len=20
col 0; len 1; (1): 67
col 1; len 7; (7): 37 37 37 37 37 37 37
tl: 8 fb: --H-FL-- lb: 0x0 cc: 1
col 0: [ 4] 01 00 03 bb
row#7[7892] flag: K-----, lock: 2, len=21
col 0; len 1; (1): 68
col 1; len 8; (8): 38 38 38 38 38 38 38 38
tl: 8 fb: --H-FL-- lb: 0x0 cc: 1
col 0: [ 4] 01 00 03 bb
row#8[7870] flag: K-----, lock: 2, len=22
col 0; len 1; (1): 69
col 1; len 9; (9): 39 39 39 39 39 39 39 39 39
tl: 8 fb: --H-FL-- lb: 0x0 cc: 1
col 0: [ 4] 01 00 03 bb
----- end of leaf block dump -----
End dump data blocks tsn: 4 file#: 4 minblk 963 maxblk 963

--可以發現如下資訊沒有發生變化，依舊是col 0: [ 4] 01 00 03 bb.
tl: 8 fb: --H-FL-- lb: 0x0 cc: 1
col 0: [ 4] 01 00 03 bb

這樣如果我們執行select substr(vc,1,20) from t_iot from where b='a';透過第2索引記錄的資料塊查詢應該找不到vc的值(可以理解物理猜失敗)，
必須在透過主鍵來查詢，這樣邏輯讀就會比原來加1.

SQL> set autotrace traceonly

SQL> select substr(vc,1,20) from t_iot where b='a';

Execution Plan

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

Plan hash value: 1095339046


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

| Id  | Operation         | Name      | Rows  | Bytes | Cost (%CPU)| Time     |

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

|   0 | SELECT STATEMENT  |           |     1 |     4 |     1   (0)| 00:00:01 |

|*  1 |  INDEX UNIQUE SCAN| T_IOT_PK  |     1 |     4 |     1   (0)| 00:00:01 |

|*  2 |   INDEX RANGE SCAN| I_T_IOT_B |     1 |       |     1   (0)| 00:00:01 |

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


Predicate Information (identified by operation id):

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


   1 - access("B"='a')

   2 - access("B"='a')


Statistics

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

          0  recursive calls

          0  db block gets

          4  consistent gets

          0  physical reads

          0  redo size

        551  bytes sent via SQL*Net to client

        519  bytes received via SQL*Net from client

          2  SQL*Net roundtrips to/from client

          0  sorts (memory)

          0  sorts (disk)

          1  rows processed

7.現在我們重建第2索引看看。

SQL> alter index i_t_iot_b rebuild ;

Index altered.


SQL> select header_file,header_block from dba_segments where segment_name='I_T_IOT_B';

HEADER_FILE HEADER_BLOCK

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

          4          970


SQL> select object_id,data_object_id from dba_objects where object_name='I_T_IOT_B';

 OBJECT_ID DATA_OBJECT_ID

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

     83454          83455


SQL> exec dbms_stats.gather_table_stats(ownname=>USER, tabname=>'T_iot');

PL/SQL procedure successfully completed.


SQL> SELECT index_name, table_name, blevel, leaf_blocks FROM dba_indexes  WHERE table_name = 'T_IOT';

INDEX_NAME                     TABLE_NAME                         BLEVEL LEAF_BLOCKS

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

T_IOT_PK                       T_IOT                                   1           2

I_T_IOT_B                      T_IOT                                   0           1


SQL> ALTER SESSION SET EVENTS 'immediate trace name treedump level 83454';

Session altered.

----- begin tree dump

leaf: 0x10003cb 16778187 (0: nrow: 9 rrow: 9)

----- end tree dump

I_T_IOT_B僅僅佔用1個塊。HEADER_BLOCK=970,根節點=971.

SQL> alter system dump datafile 4 block 971 ;

Block header dump: 0x010003cb
Object id on Block? Y
seg/obj: 0x145ff csc: 0x00.63b560 itc: 2 flg: E typ: 2 - INDEX
     brn: 0 bdba: 0x10003c8 ver: 0x01 opc: 0
     inc: 0 exflg: 0

Itl           Xid                  Uba         Flag Lck        Scn/Fsc
0x01   0x0000.000.00000000 0x00000000.0000.00 ----    0 fsc 0x0000.00000000
0x02   0xffff.000.00000000 0x00000000.0000.00 C---    0 scn 0x0000.0063b560
Leaf block dump
===============
header address 182925401700=0x2a97341e64
kdxcolev 0
KDXCOLEV Flags = - - -
kdxcolok 0
kdxcoopc 0x90: pcode=0: iot flags=I-- is converted=Y
kdxconco 2
kdxcosdc 0
kdxconro 9
kdxcofbo 54=0x36
kdxcofeo 7870=0x1ebe
kdxcoavs 7816
kdxlespl 0
kdxlende 0
kdxlenxt 0=0x0
kdxleprv 0=0x0
kdxledsz 0
kdxlebksz 8032
row#0[8018] flag: K-----, lock: 0, len=14
col 0; len 1; (1): 61
col 1; len 1; (1): 31
tl: 8 fb: --H-FL-- lb: 0x0 cc: 1
col 0: [ 4] 01 00 03 bd
row#1[8003] flag: K-----, lock: 0, len=15
col 0; len 1; (1): 62
col 1; len 2; (2): 32 32
tl: 8 fb: --H-FL-- lb: 0x0 cc: 1
col 0: [ 4] 01 00 03 bd
row#2[7987] flag: K-----, lock: 0, len=16
col 0; len 1; (1): 63
col 1; len 3; (3): 33 33 33
tl: 8 fb: --H-FL-- lb: 0x0 cc: 1
col 0: [ 4] 01 00 03 bd
row#3[7970] flag: K-----, lock: 0, len=17
col 0; len 1; (1): 64
col 1; len 4; (4): 34 34 34 34
tl: 8 fb: --H-FL-- lb: 0x0 cc: 1
col 0: [ 4] 01 00 03 bd
row#4[7952] flag: K-----, lock: 0, len=18
col 0; len 1; (1): 65
col 1; len 5; (5): 35 35 35 35 35
tl: 8 fb: --H-FL-- lb: 0x0 cc: 1
col 0: [ 4] 01 00 03 bd
row#5[7933] flag: K-----, lock: 0, len=19
col 0; len 1; (1): 66
col 1; len 6; (6): 36 36 36 36 36 36
tl: 8 fb: --H-FL-- lb: 0x0 cc: 1
col 0: [ 4] 01 00 03 bd
row#6[7913] flag: K-----, lock: 0, len=20
col 0; len 1; (1): 67
col 1; len 7; (7): 37 37 37 37 37 37 37
tl: 8 fb: --H-FL-- lb: 0x0 cc: 1
col 0: [ 4] 01 00 03 be
row#7[7892] flag: K-----, lock: 0, len=21
col 0; len 1; (1): 68
col 1; len 8; (8): 38 38 38 38 38 38 38 38
tl: 8 fb: --H-FL-- lb: 0x0 cc: 1
col 0: [ 4] 01 00 03 be
row#8[7870] flag: K-----, lock: 0, len=22
col 0; len 1; (1): 69
col 1; len 9; (9): 39 39 39 39 39 39 39 39 39
tl: 8 fb: --H-FL-- lb: 0x0 cc: 1
col 0: [ 4] 01 00 03 be
----- end of leaf block dump -----
End dump data blocks tsn: 4 file#: 4 minblk 971 maxblk 971

--可以發現索引中的資料塊資訊指向了正確的位置。
tl: 8 fb: --H-FL-- lb: 0x0 cc: 1
col 0: [ 4] 01 00 03 be

SQL> set autotrace traceonly

SQL> select substr(vc,1,20) from t_iot where b='a';

Execution Plan

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

Plan hash value: 1095339046


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

| Id  | Operation         | Name      | Rows  | Bytes | Cost (%CPU)| Time     |

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

|   0 | SELECT STATEMENT  |           |     1 |  1003 |     1   (0)| 00:00:01 |

|*  1 |  INDEX UNIQUE SCAN| T_IOT_PK  |     1 |  1003 |     1   (0)| 00:00:01 |

|*  2 |   INDEX RANGE SCAN| I_T_IOT_B |     1 |       |     1   (0)| 00:00:01 |

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

Predicate Information (identified by operation id):

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

   1 - access("B"='a')

   2 - access("B"='a')


Statistics

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

          0  recursive calls

          0  db block gets

          3  consistent gets

          0  physical reads

          0  redo size

        551  bytes sent via SQL*Net to client

        519  bytes received via SQL*Net from client

          2  SQL*Net roundtrips to/from client

          0  sorts (memory)

          0  sorts (disk)

          1  rows processed

--可以發現邏輯讀又變為3個.物理猜測是正確的。

8.總結：
1.IOT的第2索引包含主鍵資訊以及對應資訊的塊資訊。
2.當IOT索引分裂時，第2索引中記錄的塊資訊不會發生變化。這樣在使用第2索引探查IOT表時，物理猜就會失敗。
3.如果物理猜失敗很多，會導致邏輯讀增加，可以透過重建第2索引來解決這個問題。

[20120228]IOT索引組織表相關資訊的學習.txt

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