index fast full scan 、index full scan
[Oracle] Index Full Scan vs Index Fast Full Scan
作者：汪海 (Wanghai)
日期：14-Aug-2005
出处：http://spaces.msn.com/members/wzwanghai/
Index Full Scan vs Index Fast Full Scan
index full scan和index fast full scan是指同样的东西吗？
答案是no。
两者虽然从字面上看起来差不多，但是实现的机制完全不同。
我们一起来看看两者的区别在哪里?
首先来看一下IFS,FFS能用在哪里：在一句sql中，如果我们想搜索的列都包含在索引里面的话，那么index full scan 和 index fast full scan 都可以被采用代替full table scan。
比如以下语句：
1.创建测试表
SQL> CREATE TABLE TEST AS SELECT * FROM dba_objects WHERE 0=1;
2.创建测试索引
SQL> CREATE INDEX ind_test_id ON TEST(object_id);
3.插入测试数据
SQL> INSERT INTO TEST SELECT * FROM dba_objects WHERE object_id IS NOT NULL AND object_id > 10000 ORDER BY object_id DESC;
17837 rows created.
4.分析表 附带索引等等
SQL> analyze table test compute statistics for table for all columns for all indexes;
Table analyzed.
5.打开执行计划
SQL> set autotrace trace;
6.FFS示例
SQL> select object_id from test;
17837 rows selected.
Execution Plan
0 SELECT STATEMENT Optimizer=CHOOSE (Cost=68 Card=17837 Bytes=71348)
1 0 TABLE ACCESS (FULL) OF 'TEST' (Cost=68 Card=17837 Bytes=71348)
这时候 Oracle会选择全表扫描，因为 object_id 列默认是可以为null的，来修改成 not null
6.1修改字段属性 not null
SQL>alter table test modify(object_id not null);
6.2再次验证 FFS
SQL> select object_id from test;
17837 rows selected.
Execution Plan
0 SELECT STATEMENT Optimizer=CHOOSE (Cost=11 Card=17837 Bytes=71348)
1 0 INDEX (FAST FULL SCAN) OF 'IND_TEST_ID' (NON-UNIQUE) (Cost=11 Card=17837 Bytes=71348)
没有问题
7. IFS 示例
SQL> select/*+ index(test ind_TEST_ID)*/ object_id from test;
17837 rows selected.
Execution Plan
0 SELECT STATEMENT Optimizer=CHOOSE (Cost=41 Card=17837 Bytes=71348)
1 0 INDEX (FULL SCAN) OF 'IND_TEST_ID' (NON-UNIQUE) (Cost=101 Card=17837 Bytes=71348)
没有问题
我们看到了两者都可以在这种情况下使用，那么他们有什么区别呢？有个地方可以看出两者的区别， 来看一下两者的输出结果，为了让大家看清楚一点，我们只取10行。
8结果验证
8.1FFS(INDEX FAST FULL SCAN)
SQL> select object_id from test where rownum<11;
OBJECT_ID
66266
66267
66268
66269
66270
66271
66272
66273
66274
66275
10 rows selected.
8.2FS(INDEX FULL SCAN)
SQL> select/*+ index(test ind_TEST_ID)*/ object_id from test where rownum<11;
OBJECT_ID
10616
12177
12178
12179
12301
13495
13536
13539
13923
16503
10 rows selected.
结论：两者的结果完全不一样，这是为什么呢？
这是因为当进行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。
而进行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，读取的顺序完全由物理存储位置决定，并采取多块读，每次读取db_file_multiblock_read_count个块。
9.原因考证
这就是为什么两者的结果区别如此之大的原因，我们再仔细跟踪一下这两条语句。首先来看一下索引的结构
9.1 FFS 操作结果
SQL> select object_id from dba_objects where object_name='IND_TEST_ID';
OBJECT_ID
70591
索引的object_id为70591，使用tree dump可以看到索引树的结构
9.2 索引结构探究
SQL> ALTER SESSION SET EVENTS 'immediate trace name TREEDUMP level 70591';
- begin tree dump
branch: 0x6809b8d 109091725 (0: nrow: 100, level: 1)
leaf: 0x6809b96 109091734 (-1: nrow: 294 rrow: 0)
leaf: 0x6c07ec1 113278657 (0: nrow: 262 rrow: 0)
leaf: 0x6c07ebd 113278653 (1: nrow: 518 rrow: 0)
leaf: 0x6c07eb1 113278641 (2: nrow: 524 rrow: 0)
leaf: 0x6c07ead 113278637 (3: nrow: 524 rrow: 0)
leaf: 0x6c07ea9 113278633 (4: nrow: 524 rrow: 0)
leaf: 0x6c07ea5 113278629 (5: nrow: 524 rrow: 0)
leaf: 0x6c07ea1 113278625 (6: nrow: 524 rrow: 0)
leaf: 0x6c07e9d 113278621 (7: nrow: 524 rrow: 0)
leaf: 0x6c07e99 113278617 (8: nrow: 524 rrow: 0)
leaf: 0x6c07e95 113278613 (9: nrow: 532 rrow: 0)
leaf: 0x6c07e91 113278609 (10: nrow: 524 rrow: 0)
leaf: 0x6c07e8d 113278605 (11: nrow: 524 rrow: 0)
leaf: 0x6c07ec8 113278664 (12: nrow: 524 rrow: 0)
leaf: 0x6c07ec4 113278660 (13: nrow: 524 rrow: 0)
leaf: 0x6c07ec0 113278656 (14: nrow: 524 rrow: 0)
leaf: 0x6c07ebc 113278652 (15: nrow: 524 rrow: 0)
leaf: 0x6809bb2 109091762 (16: nrow: 524 rrow: 0)
leaf: 0x6c07eb8 113278648 (17: nrow: 524 rrow: 0)
leaf: 0x6c07eb4 113278644 (18: nrow: 524 rrow: 0)
leaf: 0x6c07eb0 113278640 (19: nrow: 524 rrow: 0)
leaf: 0x6c07eac 113278636 (20: nrow: 524 rrow: 0)
leaf: 0x6809bae 109091758 (21: nrow: 524 rrow: 0)
leaf: 0x6c07ea8 113278632 (22: nrow: 524 rrow: 0)
leaf: 0x6c07ea4 113278628 (23: nrow: 524 rrow: 0)
leaf: 0x6c07ea0 113278624 (24: nrow: 105 rrow: 105)
leaf: 0x6c07e9c 113278620 (25: nrow: 129 rrow: 129)
leaf: 0x6c07eb9 113278649 (26: nrow: 123 rrow: 123)
leaf: 0x6809baa 109091754 (27: nrow: 246 rrow: 246)
leaf: 0x6c07e98 113278616 (28: nrow: 246 rrow: 246)
leaf: 0x6c07e94 113278612 (29: nrow: 246 rrow: 246)
leaf: 0x6809ba6 109091750 (30: nrow: 246 rrow: 246)
leaf: 0x6809bce 109091790 (31: nrow: 246 rrow: 246)
leaf: 0x6809bca 109091786 (32: nrow: 246 rrow: 246)
leaf: 0x6809c05 109091845 (33: nrow: 248 rrow: 248)
leaf: 0x6809c01 109091841 (34: nrow: 246 rrow: 246)
leaf: 0x6809bfd 109091837 (35: nrow: 246 rrow: 246)
leaf: 0x6809bf9 109091833 (36: nrow: 246 rrow: 246)
leaf: 0x6809bf5 109091829 (37: nrow: 246 rrow: 246)
leaf: 0x6809bf1 109091825 (38: nrow: 246 rrow: 246)
leaf: 0x6809bed 109091821 (39: nrow: 246 rrow: 246)
leaf: 0x6809be9 109091817 (40: nrow: 246 rrow: 246)
leaf: 0x6809be5 109091813 (41: nrow: 246 rrow: 246)
leaf: 0x6809be1 109091809 (42: nrow: 246 rrow: 246)
leaf: 0x6809bdd 109091805 (43: nrow: 246 rrow: 246)
leaf: 0x6809bd9 109091801 (44: nrow: 246 rrow: 246)
leaf: 0x6809bd5 109091797 (45: nrow: 246 rrow: 246)
leaf: 0x6809bd1 109091793 (46: nrow: 248 rrow: 248)
leaf: 0x6809bcd 109091789 (47: nrow: 246 rrow: 246)
leaf: 0x6809bc9 109091785 (48: nrow: 246 rrow: 246)
leaf: 0x6809c08 109091848 (49: nrow: 246 rrow: 246)
leaf: 0x6809c04 109091844 (50: nrow: 246 rrow: 246)
leaf: 0x6809c00 109091840 (51: nrow: 246 rrow: 246)
leaf: 0x6809bfc 109091836 (52: nrow: 246 rrow: 246)
leaf: 0x6809bf8 109091832 (53: nrow: 246 rrow: 246)
leaf: 0x6809bf4 109091828 (54: nrow: 246 rrow: 246)
leaf: 0x6809bf0 109091824 (55: nrow: 246 rrow: 246)
leaf: 0x6809bec 109091820 (56: nrow: 246 rrow: 246)
leaf: 0x6809be8 109091816 (57: nrow: 246 rrow: 246)
leaf: 0x6809be4 109091812 (58: nrow: 246 rrow: 246)
leaf: 0x6809be0 109091808 (59: nrow: 248 rrow: 248)
leaf: 0x6809bdc 109091804 (60: nrow: 246 rrow: 246)
leaf: 0x6809bd8 109091800 (61: nrow: 246 rrow: 246)
leaf: 0x6809bd4 109091796 (62: nrow: 246 rrow: 246)
leaf: 0x6809bd0 109091792 (63: nrow: 246 rrow: 246)
leaf: 0x6809bcc 109091788 (64: nrow: 246 rrow: 246)
leaf: 0x6809c07 109091847 (65: nrow: 246 rrow: 246)
leaf: 0x6809c03 109091843 (66: nrow: 246 rrow: 246)
leaf: 0x6809bff 109091839 (67: nrow: 246 rrow: 246)
leaf: 0x6809bfb 109091835 (68: nrow: 246 rrow: 246)
leaf: 0x6809bf7 109091831 (69: nrow: 246 rrow: 246)
leaf: 0x6809bf3 109091827 (70: nrow: 246 rrow: 246)
leaf: 0x6809bef 109091823 (71: nrow: 246 rrow: 246)
leaf: 0x6809beb 109091819 (72: nrow: 248 rrow: 248)
leaf: 0x6809be7 109091815 (73: nrow: 246 rrow: 246)
leaf: 0x6809be3 109091811 (74: nrow: 246 rrow: 246)
leaf: 0x6809bdf 109091807 (75: nrow: 246 rrow: 246)
leaf: 0x6809bdb 109091803 (76: nrow: 246 rrow: 246)
leaf: 0x6809bd7 109091799 (77: nrow: 246 rrow: 246)
leaf: 0x6809bd3 109091795 (78: nrow: 246 rrow: 246)
leaf: 0x6809bcf 109091791 (79: nrow: 246 rrow: 246)
leaf: 0x6809bcb 109091787 (80: nrow: 246 rrow: 246)
leaf: 0x6809c06 109091846 (81: nrow: 246 rrow: 246)
leaf: 0x6809c02 109091842 (82: nrow: 246 rrow: 246)
leaf: 0x6809bfe 109091838 (83: nrow: 246 rrow: 246)
leaf: 0x6809bfa 109091834 (84: nrow: 246 rrow: 246)
leaf: 0x6809ba2 109091746 (85: nrow: 129 rrow: 129)
leaf: 0x6c07eb5 113278645 (86: nrow: 123 rrow: 123)
leaf: 0x6809bf6 109091830 (87: nrow: 246 rrow: 246)
leaf: 0x6809bf2 109091826 (88: nrow: 246 rrow: 246)
leaf: 0x6809bee 109091822 (89: nrow: 246 rrow: 246)
leaf: 0x6809bea 109091818 (90: nrow: 246 rrow: 246)
leaf: 0x6809b9e 109091742 (91: nrow: 246 rrow: 246)
leaf: 0x6809be6 109091814 (92: nrow: 246 rrow: 246)
leaf: 0x6809be2 109091810 (93: nrow: 246 rrow: 246)
leaf: 0x6809bde 109091806 (94: nrow: 246 rrow: 246)
leaf: 0x6809bda 109091802 (95: nrow: 246 rrow: 246)
leaf: 0x6809b9a 109091738 (96: nrow: 246 rrow: 246)
leaf: 0x6809bd6 109091798 (97: nrow: 246 rrow: 246)
leaf: 0x6809bd2 109091794 (98: nrow: 246 rrow: 246)
- end tree dump
index full scan 读取的是 0x6c07ea0 这个块
读取顺序：首先定位到索引的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。
结论：它所读取的块都是有顺序的，也是经过排序的。FS 数据应该是从头开始的
index fast full scan读取的是 0x6809b9a 这个块
读取顺序： 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，每次读取db_file_multiblock_read_count个块
结论：也就是包含数据的物理存储位置最前的块。 FFS数据应该不是从头开始的
9.3 分别看一下这两个块的内容
FS: 0x6c07ea0 =十进制的113278624
FFS:0x6809b9a =十进制的109091738
SQL> select dbms_utility.data_block_address_file(113278624) "file",dbms_utility.data_block_address_block(113278624) "block" from dual;
file block
27 32416
FS: 文件号 27 快号 32416
SQL> select dbms_utility.data_block_address_file(109091738) "file",dbms_utility.data_block_address_block(109091738)"block" from dual;
file block
26 39834
FFS: 文件号 26 块号 39834
9.4 dump该数据块
SQL> alter system dump datafile 26 block 39834;
block 39834的前10行
row#0[4591] flag: -, lock: 2
col 0; len 4; (4): c3 07 3f 43
col 1; len 6; (6): 02 81 71 f6 00 36
row#1[4605] flag: -, lock: 2
col 0; len 4; (4): c3 07 3f 44
col 1; len 6; (6): 02 81 71 f6 00 35
row#2[4619] flag: -, lock: 2
col 0; len 4; (4): c3 07 3f 45
col 1; len 6; (6): 02 81 71 f6 00 34
row#3[4633] flag: -, lock: 2
col 0; len 4; (4): c3 07 3f 46
col 1; len 6; (6): 02 81 71 f6 00 33
row#4[4647] flag: -, lock: 2
col 0; len 4; (4): c3 07 3f 47
col 1; len 6; (6): 02 81 71 f6 00 32
row#5[4661] flag: -, lock: 2
col 0; len 4; (4): c3 07 3f 48
col 1; len 6; (6): 02 81 71 f6 00 31
row#6[4675] flag: -, lock: 2
col 0; len 4; (4): c3 07 3f 49
col 1; len 6; (6): 02 81 71 f6 00 30
row#7[4689] flag: -, lock: 2
col 0; len 4; (4): c3 07 3f 4a
col 1; len 6; (6): 02 81 71 f6 00 2f
row#8[4703] flag: -, lock: 2
col 0; len 4; (4): c3 07 3f 4b
col 1; len 6; (6): 02 81 71 f6 00 2e
row#9[4717] flag: -, lock: 2
col 0; len 4; (4): c3 07 3f 4c
col 1; len 6; (6): 02 81 71 f6 00 2d
SQL> alter system dump datafile 27 block 32416;
block 32416的前10行
row#0[6564] flag: -, lock: 2
col 0; len 4; (4): c3 02 07 11
col 1; len 6; (6): 07 00 7c 20 00 2b
row#1[6578] flag: -, lock: 2
col 0; len 4; (4): c3 02 16 4e
col 1; len 6; (6): 07 00 7c 20 00 2a
row#2[6592] flag: -, lock: 2
col 0; len 4; (4): c3 02 16 4f
col 1; len 6; (6): 07 00 7c 20 00 29
row#3[6606] flag: -, lock: 2
col 0; len 4; (4): c3 02 16 50
col 1; len 6; (6): 07 00 7c 20 00 28
row#4[6620] flag: -, lock: 2
col 0; len 4; (4): c3 02 18 02
col 1; len 6; (6): 07 00 7c 20 00 27
row#5[6634] flag: -, lock: 2
col 0; len 4; (4): c3 02 23 60
col 1; len 6; (6): 07 00 7c 20 00 26
row#6[6648] flag: -, lock: 2
col 0; len 4; (4): c3 02 24 25
col 1; len 6; (6): 07 00 7c 20 00 25
row#7[6662] flag: -, lock: 2
col 0; len 4; (4): c3 02 24 28
col 1; len 6; (6): 07 00 7c 20 00 24
row#8[6676] flag: -, lock: 2
col 0; len 4; (4): c3 02 28 18
col 1; len 6; (6): 07 00 7c 20 00 23
row#9[6690] flag: -, lock: 2
col 0; len 4; (4): c3 02 42 04
col 1; len 6; (6): 07 00 7c 20 00 22
9.5 对照一下前面的结果集
block 32416的第一行为10616,数据内的存储格式应该为
SQL> select dump(10616,16) from dual;
DUMP(10616,16)
Typ=2 Len=4: c3,2,7,11
确实等于dump block所看到的
row#0[6564] flag: -, lock: 2
col 0; len 4; (4): c3 02 07 11
col 1; len 6; (6): 07 00 7c 20 00 2b
再看block 39834的第1行
SQL> select dump(66266,16) from dual;
DUMP(66266,16)
-
Typ=2 Len=4: c3,7,3f,43
跟dump 的结果也一样
row#0[4591] flag: -, lock: 2
col 0; len 4; (4): c3 07 3f 43
col 1; len 6; (6): 02 81 71 f6 00 36
9.6 结论：证明了上面所说的index full scan和index fast full scan的不同。
10.我们也可以用10046事件去跟踪两者走的路径
10.1.1 清空buffer_cache
SQL> ALTER SESSION SET EVENTS 'immediate trace name flush_cache';
（清空buffer cache,以便观看'db file sequential read','db file scattered read'事件）。
10.1.2 打开10046事件
SQL> alter session set events'10046 trace name context forever,level 12';
Session altered.
10.1.3 FFS 结果
SQL> select object_id from test where rownum<11;
OBJECT_ID
66266
66267
66268
66269
66270
66271
66272
66273
66274
66275
10 rows selected.
10.1.4 关闭10046事件
SQL> alter session set events'10046 trace name context off';
Session altered.
10.1.5 观察该 FFS 的 trc文件
[oracle@csdbc udump]$ grep read cs-dbc_ora_15596.trc
Redo thread mounted by this instance: 1
WAIT #1: nam='db file sequential read' ela= 33 p1=26 p2=39820 p3=1
WAIT #1: nam='db file sequential read' ela= 21 p1=26 p2=39817 p3=1
WAIT #1: nam='db file sequential read' ela= 17 p1=26 p2=39819 p3=1
WAIT #1: nam='db file parallel read' ela= 53 p1=2 p2=2 p3=2
WAIT #1: nam='db file scattered read' ela= 466 p1=26 p2=39821 p3=16
最前面的'db file sequential read'是由于读段头等操作，我们来关注'db file scattered read'事件，因为index fast full scan是采用多块读，从39821开始读取db_file_multiblock_read_count个块（本例里设置为16）。我们关心的39834块正位于其中。
10.2 再来看index full scan的10046 trace
10.2.1 清空buffer cache
SQL> ALTER SESSION SET EVENTS 'immediate trace name flush_cache';
（清空buffer cache,以便观看'db file sequential read','db file scattered read'事件）。
10.2.2 打开10046事件
SQL> alter session set events'10046 trace name context forever,level 12';
Session altered.
10.2.3 FS结果
SQL> select/*+ index(test ind_TEST_ID)*/ object_id from test where rownum<11;
OBJECT_ID
10616
12177
12178
12179
12301
13495
13536
13539
13923
16503
10 rows selected.
10.2.4 关闭10046事件
SQL> alter session set events'10046 trace name context off';
Session altered.
10.2.5 观察该 FS 的 trc文件
[oracle@csdbc udump]$ grep read cs-dbc_ora_15609.trc
Redo thread mounted by this instance: 1
WAIT #1: nam='db file sequential read' ela= 49 p1=26 p2=39821 p3=1
root block,正是先前索引树dump里面的 0x6809b8d
WAIT #1: nam='db file sequential read' ela= 32 p1=26 p2=39830 p3=1
WAIT #1: nam='db file sequential read' ela= 40 p1=27 p2=32449 p3=1
WAIT #1: nam='db file sequential read' ela= 35 p1=27 p2=32445 p3=1
WAIT #1: nam='db file sequential read' ela= 28 p1=27 p2=32433 p3=1
WAIT #1: nam='db file sequential read' ela= 19 p1=27 p2=32429 p3=1
WAIT #1: nam='db file sequential read' ela= 34 p1=27 p2=32425 p3=1
WAIT #1: nam='db file sequential read' ela= 32 p1=27 p2=32421 p3=1
WAIT #1: nam='db file sequential read' ela= 33 p1=27 p2=32417 p3=1
WAIT #1: nam='db file sequential read' ela= 29 p1=27 p2=32413 p3=1
WAIT #1: nam='db file sequential read' ela= 37 p1=27 p2=32409 p3=1
WAIT #1: nam='db file sequential read' ela= 32 p1=27 p2=32405 p3=1
WAIT #1: nam='db file sequential read' ela= 35 p1=27 p2=32401 p3=1
WAIT #1: nam='db file sequential read' ela= 34 p1=27 p2=32397 p3=1
WAIT #1: nam='db file sequential read' ela= 31 p1=27 p2=32456 p3=1
WAIT #1: nam='db file sequential read' ela= 29 p1=27 p2=32452 p3=1
WAIT #1: nam='db file sequential read' ela= 31 p1=27 p2=32448 p3=1
WAIT #1: nam='db file sequential read' ela= 30 p1=27 p2=32444 p3=1
WAIT #1: nam='db file sequential read' ela= 38 p1=26 p2=39858 p3=1
WAIT #1: nam='db file sequential read' ela= 31 p1=27 p2=32440 p3=1
WAIT #1: nam='db file sequential read' ela= 32 p1=27 p2=32436 p3=1
WAIT #1: nam='db file sequential read' ela= 35 p1=27 p2=32432 p3=1
WAIT #1: nam='db file sequential read' ela= 31 p1=27 p2=32428 p3=1
WAIT #1: nam='db file sequential read' ela= 29 p1=26 p2=39854 p3=1
WAIT #1: nam='db file sequential read' ela= 36 p1=27 p2=32424 p3=1
WAIT #1: nam='db file sequential read' ela= 32 p1=27 p2=32420 p3=1
WAIT #1: nam='db file sequential read' ela= 36 p1=27 p2=32416 p3=1
10.3 结论
index full scan走的路径正是文章开始所提到的定位到root block，然后根据leaf block链表一路读取块。
看到这里大家应该比较了解index full scan 和index fast full scan的区别了。
11. 最后补充一下 index full scan 和 index fast full scan 在排序上的不同。
11.1打开trace跟踪
SQL> set autotrace trace;
11.2 FS 结果验证
SQL> select object_id from test order by object_id;
17837 rows selected.
Execution Plan
0 SELECT STATEMENT Optimizer=CHOOSE (Cost=41 Card=17837 Bytes=71348)
1 0 INDEX (FULL SCAN) OF 'IND_TEST_ID' (NON-UNIQUE) (Cost=101 Card=17837 Bytes=71348)
由于有排序所以oracle自动选择了index full scan避免了排序。那么强制用index fast full scan呢？
11.3 FFS结果验证
SQL> select/*+ index_ffs(test ind_test_id)*/object_id from test order by object_id;
17837 rows selected.
Execution Plan
0 SELECT STATEMENT Optimizer=CHOOSE (Cost=59 Card=17837 Bytes=71348)
1 0 SORT (ORDER BY) (Cost=59 Card=17837 Bytes=71348)
2 1 INDEX (FAST FULL SCAN) OF 'IND_TEST_ID' (NON-UNIQUE) (Cost=11 Card=17837 Bytes=71348)
结论：
index full scan 会自动的执行 sort (order by)
index fast full scan会多一步 sort (order by)
本文原作者
WangHai(要了解他请访问他的Blog). 本文版权归原作者所有。转载请注明作者及出处！
