dump浅析B树索引
oracle中默认的索引类型是B树索引。还有位图索引，反向键索引，hash索引，基于函数的索引。
本篇主要介绍B树索引，通过转储分析。对于索引的扫描类型，索引的基本操作不做详细的介绍。
系统信息：
[oracle@localhost ~]$ cat /etc/issue
Enterprise Linux Enterprise Linux Server release 5.5 (Carthage)
Kernel \r on an \m
数据库版本：
SQL> select * from v$version where rownum =1 ;
BANNER
--------------------------------------------------------------------------------
Oracle Database 11g Enterprise Edition Release 11.2.0.1.0 - Production
SQL> show user;
USER 为 "HR"
SQL> desc tt;
名称 是否为空? 类型
----------------------------------------- -------- ----------------------------
ID NUMBER
NAME VARCHAR2(10)
COUNT(ROWNUM)
-------------
3670016
leaf: 0x10038ac 16791724 (-1: nrow: 512 rrow: 512) 我们选取这一列：
把十六进制，和十进制数相互转换：
SQL> select to_number('10038ac','xxxxxxxxxxxxxxxx') from dual;
TO_NUMBER('10038AC','XXXXXXXXXXXXXXXX')
---------------------------------------
16791724
SQL> select to_char('16791724','xxxxxxxxxxxxxxxx') from dual;
TO_CHAR('16791724','XXXXXXXXXXXXXX
----------------------------------
10038ac
我们利用oracle中提供的一个包可以求得索引所在的文件号，块号：
SQL> select dbms_utility.data_block_address_file(16791724) from dual;
DBMS_UTILITY.DATA_BLOCK_ADDRESS_FILE(16791724)
----------------------------------------------
4
SQL> select dbms_utility.data_block_address_block(16791724) from dual;
DBMS_UTILITY.DATA_BLOCK_ADDRESS_BLOCK(16791724)
-----------------------------------------------
14508
通过查视图dba_extends,索引存储在数据文件4,块14508在起始块范围内。
此时我们dump数据文件4，块14508：
SQL> alter system dump datafile 4 block 14508;
系统已更改。
row#0[8020] flag: ------, lock: 0, len=12
col 0; len 2; (2): c1 02
col 1; len 6; (6): 01 00 1b ab 00 01
row#1[8008] flag: ------, lock: 0, len=12
col 0; len 2; (2): c1 02
col 1; len 6; (6): 01 00 1b ab 00 03
row#2[7996] flag: ------, lock: 0, len=12
col 0; len 2; (2): c1 02
col 1; len 6; (6): 01 00 1b ab 00 08
row#3[7984] flag: ------, lock: 0, len=12
col 0; len 2; (2): c1 02
col 1; len 6; (6): 01 00 1b ab 00 0a
row#4[7972] flag: ------, lock: 0, len=12
col 0; len 2; (2): c1 02
col 1; len 6; (6): 01 00 1b ab 00 0f
row#5[7960] flag: ------, lock: 0, len=12
col 0; len 2; (2): c1 02
col 1; len 6; (6): 01 00 1b ab 00 11
row#6[7948] flag: ------, lock: 0, len=12
col 0; len 2; (2): c1 02
col 1; len 6; (6): 01 00 1b ab 00 16
row#7[7936] flag: ------, lock: 0, len=12
col 0; len 2; (2): c1 02
col 1; len 6; (6): 01 00 1b ab 00 18
row#8[7924] flag: ------, lock: 0, len=12
col 0; len 2; (2): c1 02
col 1; len 6; (6): 01 00 1b ab 00 1d
row#9[7912] flag: ------, lock: 0, len=12
col 0; len 2; (2): c1 02
col 1; len 6; (6): 01 00 1b ab 00 1f
row#10[7900] flag: ------, lock: 0, len=12
col 0; len 2; (2): c1 02
col 1; len 6; (6): 01 00 1b ab 00 24
row#11[7888] flag: ------, lock: 0, len=12
col 0; len 2; (2): c1 02
col 1; len 6; (6): 01 00 1b ab 00 26
row#12[7876] flag: ------, lock: 0, len=12
col 0; len 2; (2): c1 02
col 1; len 6; (6): 01 00 1b ab 00 2b
row#13[7864] flag: ------, lock: 0, len=12
col 0; len 2; (2): c1 02
col 1; len 6; (6): 01 00 1b ab 00 2d
row#14[7852] flag: ------, lock: 0, len=12
col 0; len 2; (2): c1 02
col 1; len 6; (6): 01 00 1b ab 00 32
row#15[7840] flag: ------, lock: 0, len=12
col 0; len 2; (2): c1 02
col 1; len 6; (6): 01 00 1b ab 00 34
row#16[7828] flag: ------, lock: 0, len=12
col 0; len 2; (2): c1 02
col 1; len 6; (6): 01 00 1b ab 00 39
row#17[7816] flag: ------, lock: 0, len=12
col 0; len 2; (2): c1 02
col 1; len 6; (6): 01 00 1b ab 00 3b
row#18[7804] flag: ------, lock: 0, len=12
col 0; len 2; (2): c1 02
col 1; len 6; (6): 01 00 1b ab 00 40
row#19[7792] flag: ------, lock: 0, len=12
col 0; len 2; (2): c1 02
col 1; len 6; (6): 01 00 1b ab 00 42
row#20[7780] flag: ------, lock: 0, len=12
...............................................................
.......
选取这一行为例：
row#0[8020] flag: ------, lock: 0, len=12
col 0; len 2; (2): c1 02
col 1; len 6; (6): 01 00 1b ab 00 01
col 0 表示第一列，长度为2，c1 02表示是多少呢？
SQL> select * from tt where rownum<4 order by id;
ID NAME
---------- --------------------
1 wO
2 wang
6 hong
SQL> select dump(1,16) from dual;
DUMP(1,16)
----------------------------------
Typ=2 Len=2: c1,2 ==>这里是c1 02,0省略了
这下清楚了吧，第一行第一列存储的是1，orale存储数据的方法很复杂。
col 1表示的是第二列，长度是6， 01 00 1b ab 00 01就是索引的值，是十六进制数，我们可以转化为二进制数：
00000001 00000000 00011011 10101011 00000000 00000001
-----------------------------------------------------------------------------------------------------------------------------------------------------------
00000001 00 ==>1x2x2=4 前10位表示了数据文件号
000000 00011011 10101011 ==>4096+2048+512+256+128+32+8+2+1=7083 这里的22位表示了块号
00000000 00000001==>1 16位代表着行号
此时排序并查rowid：
此时用第一列的rowid,
通过oracle提供的一个包，可以求出对象编号，文件号，块号：
执行如下图：
上面从索引存储的段以及数据段进行分析。
我们知道索引不一定会提高查询效率，往往乱建索引会严重影响查询效率，系统用不用索引，我们不能干预（但是dba可以手动改变），是oracle CBO选择的结果。
下面我们可以做一个小实验：
SQL> select count(rowid) from t;
COUNT(ROWID)
------------
4718644
SQL> select count(rowid) from t where id=1;
COUNT(ROWID)
------------
4718592
SQL> select count(rowid) from t where id=2;
COUNT(ROWID)
------------
26
SQL> select count(rowid) from t where id=3;
COUNT(ROWID)
------------
26
SQL> set autotrace traceonly;
SQL> select * from tt;
执行计划
----------------------------------------------------------
Plan hash value: 264906180
--------------------------------------------------------------------------
| Id | Operation | Name | Rows | Bytes | Cost (%CPU)| Time |
--------------------------------------------------------------------------
| 0 | SELECT STATEMENT | | 3556K| 67M| 1742 (2)| 00:00:21 |
| 1 | TABLE ACCESS FULL| TT | 3556K| 67M| 1742 (2)| 00:00:21 |
--------------------------------------------------------------------------
Note
-----
- dynamic sampling used for this statement (level=2)
此时是全表扫描读取，是多块读取，，这样读取比较快，如果此时用索引，则效率会低。
SQL> select * from tt where id=5;
执行计划
----------------------------------------------------------
Plan hash value: 3103123359
--------------------------------------------------------------------------------
-------
| Id | Operation | Name | Rows | Bytes | Cost (%CPU)| Tim
e |
--------------------------------------------------------------------------------
-------
| 0 | SELECT STATEMENT | | 5 | 100 | 4 (0)| 00:
00:01 |
| 1 | TABLE ACCESS BY INDEX ROWID| TT | 5 | 100 | 4 (0)| 00:
00:01 |
|* 2 | INDEX RANGE SCAN | INDEX_T | 5 | | 3 (0)| 00:
00:01 |
--------------------------------------------------------------------------------
-------
Predicate Information (identified by operation id):
---------------------------------------------------
2 - access("ID"=5)
Note
-----
- dynamic sampling used for this statement (level=2)
此时是索引读取。
SQL> select * from tt where id=1;
执行计划
----------------------------------------------------------
Plan hash value: 264906180
--------------------------------------------------------------------------
| Id | Operation | Name | Rows | Bytes | Cost (%CPU)| Time |
--------------------------------------------------------------------------
| 0 | SELECT STATEMENT | | 2121K| 40M| 3644 (2)| 00:00:44 |
|* 1 | TABLE ACCESS FULL| TT | 2121K| 40M| 3644 (2)| 00:00:44 |
--------------------------------------------------------------------------
Predicate Information (identified by operation id):
---------------------------------------------------
1 - filter("ID"=1)
Note
-----
- dynamic sampling used for this statement (level=2)
看到了吧，此时是全表扫描读取，数据库是很聪明的吧！
本篇主要介绍B树索引，通过转储分析。对于索引的扫描类型，索引的基本操作不做详细的介绍。
系统信息：
[oracle@localhost ~]$ cat /etc/issue
Enterprise Linux Enterprise Linux Server release 5.5 (Carthage)
Kernel \r on an \m
数据库版本：
SQL> select * from v$version where rownum =1 ;
BANNER
--------------------------------------------------------------------------------
Oracle Database 11g Enterprise Edition Release 11.2.0.1.0 - Production
USER 为 "HR"
SQL> desc tt;
名称 是否为空? 类型
----------------------------------------- -------- ----------------------------
ID NUMBER
NAME VARCHAR2(10)
SQL> select count(rownum) from tt;
COUNT(ROWNUM)
-------------
3670016
基于ID创建索引index_t
SQL> create index index_t on tt(id) tablespace users;
索引已创建。
SQL> select object_id from dba_objects where object_name=
2 'INDEX_T';
数据库中segment有数据段，索引段，undo段，它们和表名，索引名不是同一概念，但是名字是相同的。
OBJECT_ID
----------
76332
转储索引：
SQL> alter session set events 'immediate trace name treedump level 76332';
会话已更改。
....
....
----- end tree dump
这是一棵平衡树，因为平衡树的查找效率很高，根节点到所有的叶子节点的高度相同。
branch表示的是根节点。以上选取了一部分，已经按从左向右拍好序了。SQL> create index index_t on tt(id) tablespace users;
索引已创建。
SQL> select object_id from dba_objects where object_name=
2 'INDEX_T';
数据库中segment有数据段，索引段，undo段，它们和表名，索引名不是同一概念，但是名字是相同的。
OBJECT_ID
----------
76332
转储索引：
SQL> alter session set events 'immediate trace name treedump level 76332';
会话已更改。
----- begin tree dump
branch: 0x10038ab 16791723 (0: nrow: 15, level: 2)
branch: 0x100540b 16798731 (-1: nrow: 503, level: 1)
leaf: 0x10038ac 16791724 (-1: nrow: 512 rrow: 512)
leaf: 0x10038ad 16791725 (0: nrow: 512 rrow: 512)
leaf: 0x10038ae 16791726 (1: nrow: 512 rrow: 512)
leaf: 0x10038af 16791727 (2: nrow: 512 rrow: 512)
leaf: 0x10038b0 16791728 (3: nrow: 512 rrow: 512)
leaf: 0x10038b1 16791729 (4: nrow: 512 rrow: 512)
leaf: 0x10038b2 16791730 (5: nrow: 512 rrow: 512)
leaf: 0x10038b3 16791731 (6: nrow: 512 rrow: 512)
leaf: 0x10038b4 16791732 (7: nrow: 512 rrow: 512)
leaf: 0x10038b5 16791733 (8: nrow: 512 rrow: 512)
leaf: 0x10038b6 16791734 (9: nrow: 512 rrow: 512)
leaf: 0x10038b7 16791735 (10: nrow: 512 rrow: 512)
leaf: 0x10038b9 16791737 (11: nrow: 512 rrow: 512)
leaf: 0x10038ba 16791738 (12: nrow: 512 rrow: 512)
leaf: 0x10038bb 16791739 (13: nrow: 512 rrow: 512)
leaf: 0x10038bc 16791740 (14: nrow: 512 rrow: 512)
leaf: 0x10038bd 16791741 (15: nrow: 512 rrow: 512)
leaf: 0x10038be 16791742 (16: nrow: 512 rrow: 512)
leaf: 0x10038bf 16791743 (17: nrow: 512 rrow: 512)
leaf: 0x10038c0 16791744 (18: nrow: 512 rrow: 512)
leaf: 0x10038c1 16791745 (19: nrow: 512 rrow: 512)
leaf: 0x10038c2 16791746 (20: nrow: 512 rrow: 512)
leaf: 0x10038c3 16791747 (21: nrow: 512 rrow: 512)
leaf: 0x10038c4 16791748 (22: nrow: 512 rrow: 512)
leaf: 0x10038c5 16791749 (23: nrow: 512 rrow: 512)
leaf: 0x10038c6 16791750 (24: nrow: 512 rrow: 512)
leaf: 0x10038c7 16791751 (25: nrow: 512 rrow: 512)
leaf: 0x10038c9 16791753 (26: nrow: 512 rrow: 512)
leaf: 0x10038ca 16791754 (27: nrow: 512 rrow: 512)
leaf: 0x10038cb 16791755 (28: nrow: 512 rrow: 512)
leaf: 0x10038cc 16791756 (29: nrow: 512 rrow: 512)
leaf: 0x10038cd 16791757 (30: nrow: 512 rrow: 512)
leaf: 0x10038ce 16791758 (31: nrow: 512 rrow: 512)
leaf: 0x10038cf 16791759 (32: nrow: 512 rrow: 512)
leaf: 0x10038d0 16791760 (33: nrow: 512 rrow: 512)
leaf: 0x10038d1 16791761 (34: nrow: 512 rrow: 512)
leaf: 0x10038d2 16791762 (35: nrow: 512 rrow: 512)
leaf: 0x10038d3 16791763 (36: nrow: 512 rrow: 512)
leaf: 0x10038d4 16791764 (37: nrow: 512 rrow: 512)
leaf: 0x10038d5 16791765 (38: nrow: 512 rrow: 512)
leaf: 0x10038d6 16791766 (39: nrow: 512 rrow: 512)
leaf: 0x10038d7 16791767 (40: nrow: 512 rrow: 512)
leaf: 0x10038d9 16791769 (41: nrow: 512 rrow: 512)
leaf: 0x10038da 16791770 (42: nrow: 512 rrow: 512)
leaf: 0x10038db 16791771 (43: nrow: 512 rrow: 512)
leaf: 0x10038dc 16791772 (44: nrow: 512 rrow: 512)
leaf: 0x10038dd 16791773 (45: nrow: 512 rrow: 512)
leaf: 0x10038de 16791774 (46: nrow: 512 rrow: 512)
...........................................................................
....branch: 0x10038ab 16791723 (0: nrow: 15, level: 2)
branch: 0x100540b 16798731 (-1: nrow: 503, level: 1)
leaf: 0x10038ac 16791724 (-1: nrow: 512 rrow: 512)
leaf: 0x10038ad 16791725 (0: nrow: 512 rrow: 512)
leaf: 0x10038ae 16791726 (1: nrow: 512 rrow: 512)
leaf: 0x10038af 16791727 (2: nrow: 512 rrow: 512)
leaf: 0x10038b0 16791728 (3: nrow: 512 rrow: 512)
leaf: 0x10038b1 16791729 (4: nrow: 512 rrow: 512)
leaf: 0x10038b2 16791730 (5: nrow: 512 rrow: 512)
leaf: 0x10038b3 16791731 (6: nrow: 512 rrow: 512)
leaf: 0x10038b4 16791732 (7: nrow: 512 rrow: 512)
leaf: 0x10038b5 16791733 (8: nrow: 512 rrow: 512)
leaf: 0x10038b6 16791734 (9: nrow: 512 rrow: 512)
leaf: 0x10038b7 16791735 (10: nrow: 512 rrow: 512)
leaf: 0x10038b9 16791737 (11: nrow: 512 rrow: 512)
leaf: 0x10038ba 16791738 (12: nrow: 512 rrow: 512)
leaf: 0x10038bb 16791739 (13: nrow: 512 rrow: 512)
leaf: 0x10038bc 16791740 (14: nrow: 512 rrow: 512)
leaf: 0x10038bd 16791741 (15: nrow: 512 rrow: 512)
leaf: 0x10038be 16791742 (16: nrow: 512 rrow: 512)
leaf: 0x10038bf 16791743 (17: nrow: 512 rrow: 512)
leaf: 0x10038c0 16791744 (18: nrow: 512 rrow: 512)
leaf: 0x10038c1 16791745 (19: nrow: 512 rrow: 512)
leaf: 0x10038c2 16791746 (20: nrow: 512 rrow: 512)
leaf: 0x10038c3 16791747 (21: nrow: 512 rrow: 512)
leaf: 0x10038c4 16791748 (22: nrow: 512 rrow: 512)
leaf: 0x10038c5 16791749 (23: nrow: 512 rrow: 512)
leaf: 0x10038c6 16791750 (24: nrow: 512 rrow: 512)
leaf: 0x10038c7 16791751 (25: nrow: 512 rrow: 512)
leaf: 0x10038c9 16791753 (26: nrow: 512 rrow: 512)
leaf: 0x10038ca 16791754 (27: nrow: 512 rrow: 512)
leaf: 0x10038cb 16791755 (28: nrow: 512 rrow: 512)
leaf: 0x10038cc 16791756 (29: nrow: 512 rrow: 512)
leaf: 0x10038cd 16791757 (30: nrow: 512 rrow: 512)
leaf: 0x10038ce 16791758 (31: nrow: 512 rrow: 512)
leaf: 0x10038cf 16791759 (32: nrow: 512 rrow: 512)
leaf: 0x10038d0 16791760 (33: nrow: 512 rrow: 512)
leaf: 0x10038d1 16791761 (34: nrow: 512 rrow: 512)
leaf: 0x10038d2 16791762 (35: nrow: 512 rrow: 512)
leaf: 0x10038d3 16791763 (36: nrow: 512 rrow: 512)
leaf: 0x10038d4 16791764 (37: nrow: 512 rrow: 512)
leaf: 0x10038d5 16791765 (38: nrow: 512 rrow: 512)
leaf: 0x10038d6 16791766 (39: nrow: 512 rrow: 512)
leaf: 0x10038d7 16791767 (40: nrow: 512 rrow: 512)
leaf: 0x10038d9 16791769 (41: nrow: 512 rrow: 512)
leaf: 0x10038da 16791770 (42: nrow: 512 rrow: 512)
leaf: 0x10038db 16791771 (43: nrow: 512 rrow: 512)
leaf: 0x10038dc 16791772 (44: nrow: 512 rrow: 512)
leaf: 0x10038dd 16791773 (45: nrow: 512 rrow: 512)
leaf: 0x10038de 16791774 (46: nrow: 512 rrow: 512)
...........................................................................
....
....
----- end tree dump
这是一棵平衡树，因为平衡树的查找效率很高，根节点到所有的叶子节点的高度相同。
leaf: 0x10038ac 16791724 (-1: nrow: 512 rrow: 512) 我们选取这一列：
把十六进制，和十进制数相互转换：
SQL> select to_number('10038ac','xxxxxxxxxxxxxxxx') from dual;
TO_NUMBER('10038AC','XXXXXXXXXXXXXXXX')
---------------------------------------
16791724
SQL> select to_char('16791724','xxxxxxxxxxxxxxxx') from dual;
TO_CHAR('16791724','XXXXXXXXXXXXXX
----------------------------------
10038ac
我们利用oracle中提供的一个包可以求得索引所在的文件号，块号：
SQL> select dbms_utility.data_block_address_file(16791724) from dual;
DBMS_UTILITY.DATA_BLOCK_ADDRESS_FILE(16791724)
----------------------------------------------
4
SQL> select dbms_utility.data_block_address_block(16791724) from dual;
DBMS_UTILITY.DATA_BLOCK_ADDRESS_BLOCK(16791724)
-----------------------------------------------
14508
通过查视图dba_extends,索引存储在数据文件4,块14508在起始块范围内。
SQL> alter system dump datafile 4 block 14508;
系统已更改。
row#0[8020] flag: ------, lock: 0, len=12
col 0; len 2; (2): c1 02
col 1; len 6; (6): 01 00 1b ab 00 01
row#1[8008] flag: ------, lock: 0, len=12
col 0; len 2; (2): c1 02
col 1; len 6; (6): 01 00 1b ab 00 03
row#2[7996] flag: ------, lock: 0, len=12
col 0; len 2; (2): c1 02
col 1; len 6; (6): 01 00 1b ab 00 08
row#3[7984] flag: ------, lock: 0, len=12
col 0; len 2; (2): c1 02
col 1; len 6; (6): 01 00 1b ab 00 0a
row#4[7972] flag: ------, lock: 0, len=12
col 0; len 2; (2): c1 02
col 1; len 6; (6): 01 00 1b ab 00 0f
row#5[7960] flag: ------, lock: 0, len=12
col 0; len 2; (2): c1 02
col 1; len 6; (6): 01 00 1b ab 00 11
row#6[7948] flag: ------, lock: 0, len=12
col 0; len 2; (2): c1 02
col 1; len 6; (6): 01 00 1b ab 00 16
row#7[7936] flag: ------, lock: 0, len=12
col 0; len 2; (2): c1 02
col 1; len 6; (6): 01 00 1b ab 00 18
row#8[7924] flag: ------, lock: 0, len=12
col 0; len 2; (2): c1 02
col 1; len 6; (6): 01 00 1b ab 00 1d
row#9[7912] flag: ------, lock: 0, len=12
col 0; len 2; (2): c1 02
col 1; len 6; (6): 01 00 1b ab 00 1f
row#10[7900] flag: ------, lock: 0, len=12
col 0; len 2; (2): c1 02
col 1; len 6; (6): 01 00 1b ab 00 24
row#11[7888] flag: ------, lock: 0, len=12
col 0; len 2; (2): c1 02
col 1; len 6; (6): 01 00 1b ab 00 26
row#12[7876] flag: ------, lock: 0, len=12
col 0; len 2; (2): c1 02
col 1; len 6; (6): 01 00 1b ab 00 2b
row#13[7864] flag: ------, lock: 0, len=12
col 0; len 2; (2): c1 02
col 1; len 6; (6): 01 00 1b ab 00 2d
row#14[7852] flag: ------, lock: 0, len=12
col 0; len 2; (2): c1 02
col 1; len 6; (6): 01 00 1b ab 00 32
row#15[7840] flag: ------, lock: 0, len=12
col 0; len 2; (2): c1 02
col 1; len 6; (6): 01 00 1b ab 00 34
row#16[7828] flag: ------, lock: 0, len=12
col 0; len 2; (2): c1 02
col 1; len 6; (6): 01 00 1b ab 00 39
row#17[7816] flag: ------, lock: 0, len=12
col 0; len 2; (2): c1 02
col 1; len 6; (6): 01 00 1b ab 00 3b
row#18[7804] flag: ------, lock: 0, len=12
col 0; len 2; (2): c1 02
col 1; len 6; (6): 01 00 1b ab 00 40
row#19[7792] flag: ------, lock: 0, len=12
col 0; len 2; (2): c1 02
col 1; len 6; (6): 01 00 1b ab 00 42
row#20[7780] flag: ------, lock: 0, len=12
...............................................................
.......
选取这一行为例：
row#0[8020] flag: ------, lock: 0, len=12
col 0; len 2; (2): c1 02
col 1; len 6; (6): 01 00 1b ab 00 01
col 0 表示第一列，长度为2，c1 02表示是多少呢？
SQL> select * from tt where rownum<4 order by id;
ID NAME
---------- --------------------
1 wO
2 wang
6 hong
SQL> select dump(1,16) from dual;
DUMP(1,16)
----------------------------------
Typ=2 Len=2: c1,2 ==>这里是c1 02,0省略了
这下清楚了吧，第一行第一列存储的是1，orale存储数据的方法很复杂。
col 1表示的是第二列，长度是6， 01 00 1b ab 00 01就是索引的值，是十六进制数，我们可以转化为二进制数：
00000001 00000000 00011011 10101011 00000000 00000001
-----------------------------------------------------------------------------------------------------------------------------------------------------------
00000001 00 ==>1x2x2=4 前10位表示了数据文件号
000000 00011011 10101011 ==>4096+2048+512+256+128+32+8+2+1=7083 这里的22位表示了块号
00000000 00000001==>1 16位代表着行号
此时排序并查rowid：
此时用第一列的rowid,
通过oracle提供的一个包，可以求出对象编号，文件号，块号：
执行如下图：
上面从索引存储的段以及数据段进行分析。
我们知道索引不一定会提高查询效率，往往乱建索引会严重影响查询效率，系统用不用索引，我们不能干预（但是dba可以手动改变），是oracle CBO选择的结果。
下面我们可以做一个小实验：
SQL> select count(rowid) from t;
COUNT(ROWID)
------------
4718644
SQL> select count(rowid) from t where id=1;
COUNT(ROWID)
------------
4718592
SQL> select count(rowid) from t where id=2;
COUNT(ROWID)
------------
26
SQL> select count(rowid) from t where id=3;
COUNT(ROWID)
------------
26
SQL> set autotrace traceonly;
SQL> select * from tt;
执行计划
----------------------------------------------------------
Plan hash value: 264906180
--------------------------------------------------------------------------
| Id | Operation | Name | Rows | Bytes | Cost (%CPU)| Time |
--------------------------------------------------------------------------
| 0 | SELECT STATEMENT | | 3556K| 67M| 1742 (2)| 00:00:21 |
| 1 | TABLE ACCESS FULL| TT | 3556K| 67M| 1742 (2)| 00:00:21 |
--------------------------------------------------------------------------
Note
-----
- dynamic sampling used for this statement (level=2)
此时是全表扫描读取，是多块读取，，这样读取比较快，如果此时用索引，则效率会低。
SQL> select * from tt where id=5;
执行计划
----------------------------------------------------------
Plan hash value: 3103123359
--------------------------------------------------------------------------------
-------
| Id | Operation | Name | Rows | Bytes | Cost (%CPU)| Tim
e |
--------------------------------------------------------------------------------
-------
| 0 | SELECT STATEMENT | | 5 | 100 | 4 (0)| 00:
00:01 |
| 1 | TABLE ACCESS BY INDEX ROWID| TT | 5 | 100 | 4 (0)| 00:
00:01 |
|* 2 | INDEX RANGE SCAN | INDEX_T | 5 | | 3 (0)| 00:
00:01 |
--------------------------------------------------------------------------------
-------
Predicate Information (identified by operation id):
---------------------------------------------------
2 - access("ID"=5)
Note
-----
- dynamic sampling used for this statement (level=2)
此时是索引读取。
SQL> select * from tt where id=1;
执行计划
----------------------------------------------------------
Plan hash value: 264906180
--------------------------------------------------------------------------
| Id | Operation | Name | Rows | Bytes | Cost (%CPU)| Time |
--------------------------------------------------------------------------
| 0 | SELECT STATEMENT | | 2121K| 40M| 3644 (2)| 00:00:44 |
|* 1 | TABLE ACCESS FULL| TT | 2121K| 40M| 3644 (2)| 00:00:44 |
--------------------------------------------------------------------------
Predicate Information (identified by operation id):
---------------------------------------------------
1 - filter("ID"=1)
Note
-----
- dynamic sampling used for this statement (level=2)
Stay Hungry. Stay Foolish.
