Oracle 11G INDEX FULL SCAN 和 INDEX FAST FULL SCAN 对比分析
SQL> drop table test;
表已删除。
SQL> create table test as select * from dba_objects where 1!=1;
表已创建。
SQL> create index idx_test_id on test(object_id);
索引已创建。
SQL> insert into test select * from dba_objects where object_id is not null and object_id >1000 order by object_id desc;
已创建71863行。
SQL> analyze table test compute statistics for table for all columns for all indexes;
表已分析。
SQL> select object_id from test where rownum<11;
OBJECT_ID
----------
75981
75980
75964
75963
75962
75960
75959
75958
75956
75955
已选择10行。
执行计划
----------------------------------------------------------
Plan hash value: 3931117773
---------------------------------------------------------------------------
| Id | Operation | Name | Rows | Bytes | Cost (%CPU)| Time |
---------------------------------------------------------------------------
| 0 | SELECT STATEMENT | | 10 | 40 | 2 (0)| 00:00:01 |
|* 1 | COUNT STOPKEY | | | | | |
| 2 | TABLE ACCESS FULL| TEST | 10 | 40 | 2 (0)| 00:00:01 |
---------------------------------------------------------------------------
Predicate Information (identified by operation id):
---------------------------------------------------
1 - filter(ROWNUM<11)
统计信息
----------------------------------------------------------
1 recursive calls
0 db block gets
5 consistent gets
0 physical reads
0 redo size
553 bytes sent via SQL*Net to client
419 bytes received via SQL*Net from client
2 SQL*Net roundtrips to/from client
0 sorts (memory)
0 sorts (disk)
10 rows processed
因为object_id 没有not null 约束,索引走了全部扫描.
SQL> alter table test modify object_id not null;
表已更改。
SQL> select object_id from test where rownum<11;
OBJECT_ID
----------
1001
1002
1003
1004
1005
1006
1007
1008
1009
1010
已选择10行。
执行计划
----------------------------------------------------------
Plan hash value: 1062221432
-------------------------------------------------------------------------------------
| Id | Operation | Name | Rows | Bytes | Cost (%CPU)| Time |
-------------------------------------------------------------------------------------
| 0 | SELECT STATEMENT | | 10 | 40 | 2 (0)| 00:00:01 |
|* 1 | COUNT STOPKEY | | | | | |
| 2 | INDEX FAST FULL SCAN| IDX_TEST_ID | 10 | 40 | 2 (0)| 00:00:01 |
-------------------------------------------------------------------------------------
Predicate Information (identified by operation id):
---------------------------------------------------
1 - filter(ROWNUM<11)
统计信息
----------------------------------------------------------
141 recursive calls
0 db block gets
26 consistent gets
0 physical reads
0 redo size
543 bytes sent via SQL*Net to client
419 bytes received via SQL*Net from client
2 SQL*Net roundtrips to/from client
5 sorts (memory)
0 sorts (disk)
10 rows processed
SQL> select /*+ index(test idx_test_id) */object_id from test where rownum<11;
OBJECT_ID
----------
1001
1002
1003
1004
1005
1006
1007
1008
1009
1010
已选择10行。
执行计划
----------------------------------------------------------
Plan hash value: 2708872424
--------------------------------------------------------------------------------
| Id | Operation | Name | Rows | Bytes | Cost (%CPU)| Time |
--------------------------------------------------------------------------------
| 0 | SELECT STATEMENT | | 10 | 40 | 2 (0)| 00:00:01 |
|* 1 | COUNT STOPKEY | | | | | |
| 2 | INDEX FULL SCAN| IDX_TEST_ID | 10 | 40 | 2 (0)| 00:00:01 |
--------------------------------------------------------------------------------
Predicate Information (identified by operation id):
---------------------------------------------------
1 - filter(ROWNUM<11)
统计信息
----------------------------------------------------------
1 recursive calls
0 db block gets
3 consistent gets
0 physical reads
0 redo size
543 bytes sent via SQL*Net to client
419 bytes received via SQL*Net from client
2 SQL*Net roundtrips to/from client
0 sorts (memory)
0 sorts (disk)
10 rows processed
这里走INDEX FULL SCAN 和 INDEX FAST FULL SCAN 找到的数据相同只是一个巧合. 这说明索引做最左边的leaf block也同时是索引存储的最前的一个block.
下面DUMP 出索引 来具体看看.
SQL> select object_id from test where object_name='IDX_TEST_ID';
OBJECT_ID
----------
75981
SQL> alter session set events 'immediate trace name treedump level 75981';
会话已更改。
下面是dump的全部信息.
----- begin tree dump
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leaf: 0x45cd50 4574544 (274: nrow: 246 rrow: 246)
leaf: 0x45cd2f 4574511 (275: nrow: 246 rrow: 246)
leaf: 0x45cd2e 4574510 (276: nrow: 246 rrow: 246)
leaf: 0x45cd2d 4574509 (277: nrow: 246 rrow: 246)
leaf: 0x45cd2c 4574508 (278: nrow: 246 rrow: 246)
leaf: 0x45cd2b 4574507 (279: nrow: 246 rrow: 246)
leaf: 0x45cd2a 4574506 (280: nrow: 246 rrow: 246)
leaf: 0x45cd29 4574505 (281: nrow: 246 rrow: 246)
leaf: 0x45cd28 4574504 (282: nrow: 246 rrow: 246)
leaf: 0x45cd0f 4574479 (283: nrow: 246 rrow: 246)
leaf: 0x45cd0e 4574478 (284: nrow: 246 rrow: 246)
leaf: 0x45cd0d 4574477 (285: nrow: 246 rrow: 246)
leaf: 0x45cd0c 4574476 (286: nrow: 246 rrow: 246)
leaf: 0x45cd0b 4574475 (287: nrow: 246 rrow: 246)
----- end tree dump
对于B*Tree Index的dump文件，稍微解释一下：
branch/leaf代表该行是分支节点还是页节点（第一行的branch其实是root节点）。
随后是该节点的rdba，前面是16进制的，后面是10进制的。
括号里的第一个数字是同一个level的节点位置计数，root节点从0开始，其他的level从-1开始。
nrow：节点中曾经有过的index entry数，
rrow：节点中当前的index entry数，从这两个值可以分析出index的空间使用效率。
对于branch节点，后面还有个level，叶节点隐含level为0，向上计数直到root。
分析发现 下面的leaf 是最左边的leaf，同时也是最小的RDBA.
leaf: 0x45cd0a 4574474 (-1: nrow: 416 rrow: 416) --RDBA 地址 4574474 是所有leaf中最小的，也就是物理存储在最前面的.
这个叶子含有416条记录，那么也就是说让我们只选前416条记录的时候，INDEX FULL SCAN 和 INDEX FAST FULL SCAN 的结果相同.
因为两者都只是读了同一个block.
如果读两个block，那么
INDEX FULL SCAN 这按照dump出来leaf从上到下的顺利读取前面两个leaf
leaf: 0x45cd0a 4574474 (-1: nrow: 416 rrow: 416)
leaf: 0x45d222 4575778 (0: nrow: 262 rrow: 262)
INDEX FAST FULL SCAN 则按照 RDBA的顺读取下面两个block
leaf: 0x45cd0a 4574474 (-1: nrow: 416 rrow: 416)
leaf: 0x45cd0b 4574475 (287: nrow: 246 rrow: 246)
我们参试查询前419条记录，它会读两个block,因为第一个block 4574474只含有 416条记录.
SQL>select /*+ index(test idx_test_id) */object_id from test where rownum<420;
OBJECT_ID
----------
1001
1002
1003
1004
1005
1006
1007
.... //中间的记录省略
1425
1426
1427
1428
1429
1430
已选择419行。
执行计划
----------------------------------------------------------
Plan hash value: 2708872424
--------------------------------------------------------------------------------
| Id | Operation | Name | Rows | Bytes | Cost (%CPU)| Time |
--------------------------------------------------------------------------------
| 0 | SELECT STATEMENT | | 419 | 1676 | 3 (0)| 00:00:01 |
|* 1 | COUNT STOPKEY | | | | | |
| 2 | INDEX FULL SCAN| IDX_TEST_ID | 419 | 1676 | 3 (0)| 00:00:01 |
--------------------------------------------------------------------------------
Predicate Information (identified by operation id):
---------------------------------------------------
1 - filter(ROWNUM<420)
统计信息
----------------------------------------------------------
1 recursive calls
0 db block gets
31 consistent gets
0 physical reads
0 redo size
6118 bytes sent via SQL*Net to client
716 bytes received via SQL*Net from client
29 SQL*Net roundtrips to/from client
0 sorts (memory)
0 sorts (disk)
419 rows processed
SQL> select object_id from test where rownum<420;
OBJECT_ID
----------
1001
1002
1003
1004
1005
1006
1007
.... //中间的记录省略
1425
1426
1427
74733
74734
74735
已选择419行。
执行计划
----------------------------------------------------------
Plan hash value: 1062221432
-------------------------------------------------------------------------------------
| Id | Operation | Name | Rows | Bytes | Cost (%CPU)| Time |
-------------------------------------------------------------------------------------
| 0 | SELECT STATEMENT | | 419 | 1676 | 2 (0)| 00:00:01 |
|* 1 | COUNT STOPKEY | | | | | |
| 2 | INDEX FAST FULL SCAN| IDX_TEST_ID | 419 | 1676 | 2 (0)| 00:00:01 |
-------------------------------------------------------------------------------------
Predicate Information (identified by operation id):
---------------------------------------------------
1 - filter(ROWNUM<420)
统计信息
----------------------------------------------------------
1 recursive calls
0 db block gets
37 consistent gets
0 physical reads
0 redo size
6121 bytes sent via SQL*Net to client
716 bytes received via SQL*Net from client
29 SQL*Net roundtrips to/from client
0 sorts (memory)
0 sorts (disk)
419 rows processed
果然，此时结果出现了不同,下面的记录object_id 非常大，显然是最后一个块4574475的记录.
74733
74734
74735
查看RDBA =4574474 的leaf 的 file# 和 block#
SQL>select dbms_utility.data_block_address_file('4574474') FILE_ID,dbms_utility.data_block_address_block('4574474') BLOCK_ID from dual;
FILE_ID BLOCK_ID
---------- ----------
1 380170
生成DUMP
SQL> alter system dump datafile 1 block 380170
Block dump from cache:
Dump of buffer cache at level 4 for tsn=0, rdba=4574474
BH (0x2BFE8C40) file#: 1 rdba: 0x0045cd0a (1/380170) class: 1 ba: 0x2BCF4000
.....//省略部分内容
col 0; len 3; (3): c2 0b 02 //object_id = 1001
col 1; len 6; (6): 00 45 d1 fc 00 00 //非扩展的rowid 又 file#+blcok#+row# 组成,前八位包括file#和block#
row#1[2645] flag: ------, lock: 0, len=13
col 0; len 3; (3): c2 0b 03
col 1; len 6; (6): 00 45 d1 fb 00 4d
row#2[2658] flag: ------, lock: 0, len=13
col 0; len 3; (3): c2 0b 04
col 1; len 6; (6): 00 45 d1 fb 00 4c
row#3[2671] flag: ------, lock: 0, len=13
col 0; len 3; (3): c2 0b 05
col 1; len 6; (6): 00 45 d1 fb 00 4b
row#4[2684] flag: ------, lock: 0, len=13
col 0; len 3; (3): c2 0b 06
col 1; len 6; (6): 00 45 d1 fb 00 4a
row#5[2697] flag: ------, lock: 0, len=13
col 0; len 3; (3): c2 0b 07
col 1; len 6; (6): 00 45 d1 fb 00 49
下面的数据证明 第一个索引行就是 object_id=1001 对应的索引记录
SQL> select dump(1001,16) from dual;
DUMP(1001,16)
--------------------------------------
Typ=2 Len=3: c2,b,2
下面查看 object_id=1001 对应的rowid 的file# 和block#
SQL> select
dbms_rowid.rowid_relative_fno(rowid) file_id,
dbms_rowid.rowid_block_number(rowid) block_id,
dbms_rowid.rowid_row_number(rowid) row#
from test
where object_id=1001;
FILE_ID BLOCK_ID ROW#
---------- ---------- ----------
1 381436 0
下面将object_id=1001对应的索引记录中存储的非扩展的16进制rowid "00 45 d1 fc 00 00" 转换得到file# blcok#
先把16进制的前8位转换成 10进制数
SQL> select to_number('0045d1fc','xxxxxxxx') RDBA from dual;
RDBA
----------
4575740
将10进制的RDBA中提取中 file# 和blcok#
SQL>select dbms_utility.data_block_address_file('4575740') FILE_ID,dbms_utility.data_block_address_block('4575740') BLOCK_ID from dual;
FILE_ID BLOCK_ID
---------- ----------
1 381436
结果显示和前面直接用table记录里的扩展类型的rowid 得到的信息一致.
SQL> select dbms_utility.data_block_address_file('4574475') FILE_ID,dbms_utility.data_block_address_block('4574475') BLOCK_ID from dual;
FILE_ID BLOCK_ID
---------- ----------
1 380171
SQL> alter system dump datafile 1 block 380171;
系统已更改。
Block dump from cache:
Dump of buffer cache at level 4 for tsn=0, rdba=4574475
BH (0x2BFE8B6C) file#: 1 rdba: 0x0045cd0b (1/380171) class: 1 ba: 0x2BCF2000
set: 5 pool 3 bsz: 8192 bsi: 0 sflg: 2 pwc: 351,28
dbwrid: 0 obj: 75981 objn: 75981 tsn: 0 afn: 1 hint: f
hash: [0x2C7F2FE0,0x48227468] lru: [0x2BFE8CEC,0x2BFE8B44]
ckptq: [NULL] fileq: [NULL] objq: [0x2BFE8D04,0x2BFE88E0]
st: XCURRENT md: NULL tch: 6
flags: block_written_once redo_since_read
LRBA: [0x0.0.0] LSCN: [0x0.0] HSCN: [0xffff.ffffffff] HSUB: [1]
cr pin refcnt: 0 sh pin refcnt: 0
.....//省略部分内容
row#0[4596] flag: ------, lock: 0, len=14
col 0; len 4; (4): c3 08 30 22 //object_id = 74733
col 1; len 6; (6): 00 45 cd 04 00 25
row#1[4610] flag: ------, lock: 0, len=14
col 0; len 4; (4): c3 08 30 23
col 1; len 6; (6): 00 45 cd 04 00 24
row#2[4624] flag: ------, lock: 0, len=14
col 0; len 4; (4): c3 08 30 24
col 1; len 6; (6): 00 45 cd 04 00 23
row#3[4638] flag: ------, lock: 0, len=14
col 0; len 4; (4): c3 08 30 25
col 1; len 6; (6): 00 45 cd 04 00 22
row#4[4652] flag: ------, lock: 0, len=14
下面的数据证明 第一个索引行就是 object_id=74733 对应的索引记录
SQL> select dump(74733,16) from dual;
DUMP(74733,16)
----------------------------------------------
Typ=2 Len=4: c3,8,30,22
我们也可以用下面的 PL/SQL 直接将dump出来的 16进制数据还原
SQL> declare n number;
2 begin
3 dbms_stats.convert_raw_value('c3083022',n);
4 dbms_output.put_line(n);
5 end;
6 /
74733
PL/SQL 过程已成功完成。
下面的SQL也能做到.
SQL> select utl_raw.cast_to_number('c3083022') from dual;
UTL_RAW.CAST_TO_NUMBER('C3083022')
----------------------------------
74733
结论：当进行index full scan的时候 oracle定位到索引的root block，然后到branch block（如果有的话），再定位到第一个leaf block, 然后根据leaf block的双向链表顺序读取。它所读取的块都是有顺序的，也是经过排序的。
而index fast full scan则不同，它是从段头开始，读取包含位图块，root block,所有的branch block, leaf block，读取的顺序完全有物理存储位置决定，并采取多块读，没次读取db_file_multiblock_read_count个块。
