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：
SQL> CREATE TABLE TEST AS SELECT * FROM dba_objects WHERE 0=1;
SQL> CREATE INDEX ind_test_id ON TEST(object_id);
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.
SQL> analyze table test compute statistics for table for all columns for all indexes;
Table analyzed.
SQL> set autotrace trace;
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:
SQL>alter table test modify(object_id not null);
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)
当然我们也可以使用index full scan:
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)
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。
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.
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.
可以看到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。
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
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
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，而index fast full scan读取的是 0x6809b9a这个块也就是包含数据的物理存储位置最前的块。分别看一下这两个块的内容
0x6c07ea0 =十进制的113278624
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
SQL> select dbms_utility.data_block_address_file(109091738) "file",dbms_utility.data_block_address_block(109091738)"block" from dual;
file block
---------- ----------
26 39834
SQL> alter system dump datafile 27 block 32416;
SQL> alter system dump datafile 26 block 39834;
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
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
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
这就证明了上面所说的index full scan和index fast full scan的不同。
我们也可以用10046事件去跟踪两者走的路径。
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）。
SQL> alter session set events'10046 trace name context forever,level 12';
Session altered.
SQL> select object_id from test where rownum<11;
OBJECT_ID
----------
66266
66267
66268
66269
66270
66271
66272
66273
66274
66275
10 rows selected.
SQL> alter session set events'10046 trace name context off';
Session altered.
[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。
再来看index full scan的10046 trace
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）。
SQL> alter session set events'10046 trace name context forever,level 12';
Session altered.
SQL>
OBJECT_ID
----------
10616
12177
12178
12179
12301
13495
13536
13539
13923
16503
10 rows selected.
SQL> alter session set events'10046 trace name context off';
Session altered.
[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
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，最后补充一下 index full scan 和 index fast full scan 在排序上的不同。
SQL> set autotrace trace;
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？
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)
index fast full scan会多一步sort order by，相信仔细看过这篇文章的人能知道其中结果了吧，还不知道的人请在文章中自己找答案吧。
