index full scan和index fast full scan是指同樣的東西嗎？答案是no。兩者雖然從字面上看起來差不多，但是實現的機制完全不同。我們一起來看看兩者的區別在哪裡?

首先來看一下IFS,FFS能用在哪裡：在一句sql中，如果我們想搜尋的列都包含在索引裡面的話，那麼index full scan 和 index fast full scan 都可以被採用代替full table scan。比如以下語句：

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 ptimizer=CHOOSE (Cost=68 Card=17837 Bytes=71348)
   1    0   TABLE ACCESS (FULL) OF 'TEST' (Cost=68 Card=17837 Bytes=71348)

這時候 Oracle會選擇全表掃描，因為 object_id 列預設是可以為null的，來修改成 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 ptimizer=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)

當然我們也可以使用index full scan:

 
SQL> select/*+ index(test ind_TEST_ID)*/ object_id from test;

17837 rows selected.

Execution Plan
----------------------------------------------------------
   0      SELECT STATEMENT ptimizer=CHOOSE (Cost=41 Card=17837 Bytes=71348)
   1    0   INDEX (FULL SCAN) OF 'IND_TEST_ID' (NON-UNIQUE) (Cost=101 Card=17837 Bytes=71348)

我們看到了兩者都可以在這種情況下使用，那麼他們有什麼區別呢？有個地方可以看出兩者的區別，來看一下兩者的輸出結果，為了讓大家看清楚一點，我們只取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.

可以看到兩者的結果完全不一樣，這是為什麼呢？這是因為當進行index full scan的時候 oracle定位到索引的root block，然後到branch block（如果有的話），再定位到第一個leaf block, 然後根據leaf block的雙向連結串列順序讀取。它所讀取的塊都是有順序的，也是經過排序的。

而index fast full scan則不同，它是從段頭開始，讀取包含點陣圖塊，root block,所有的branch block, leaf block，讀取的順序完全有物理儲存位置決定，並採取多塊讀，沒次讀取db_file_multiblock_read_count個塊。

這就是為什麼兩者的結果區別如此之大的原因，我們再仔細跟蹤一下這兩條語句。首先來看一下索引的結構

 
SQL> select object_id from dba_objects where object_name='IND_TEST_ID';

 OBJECT_ID
----------
     70591

索引的object_id為70591，使用tree dump可以看到索引樹的結構

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讀取的是0x6c07ea0 這個塊，而index fast full scan讀取的是 0x6809b9a這個塊也就是包含資料的物理儲存位置最前的塊。分別看一下這兩個塊的內容

0x6c07ea0 =十進位制的113278624
0x6809b9a =十進位制的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

對照一下前面的結果集

block 32416的第一行為10616,資料內的儲存格式應該為

SQL> select dump(10616,16) from dual;

DUMP(10616,16)
----------------------
Typ=2 Len=4: c3,2,7,11

確實等於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 的結果也一樣

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

這就證明了上面所說的index full scan和index fast full scan的不同。
我們也可以用10046事件去跟蹤兩者走的路徑。

SQL> ALTER SESSION SET EVENTS 'immediate trace name flush_cache';

（清空buffer cache,以便觀看'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'是由於讀段頭等操作，我們來關注'db file scattered read'事件，因為index fast full scan是採用多塊讀，從39821開始讀取db_file_multiblock_read_count個塊（本例裡設定為16）。我們關心的39834塊正位於其中。

再來看index full scan的10046 trace

SQL> ALTER SESSION SET EVENTS 'immediate trace name flush_cache';

（清空buffer cache,以便觀看'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,正是先前索引樹dump裡面的 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走的路徑正是文章開始所提到的定位到root block，然後根據leaf block連結串列一路讀取塊。看到這裡大家應該比較瞭解index full scan 和index fast full scan的區別了，最後補充一下 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 ptimizer=CHOOSE (Cost=41 Card=17837 Bytes=71348)
   1    0   INDEX (FULL SCAN) OF 'IND_TEST_ID' (NON-UNIQUE) (Cost=101 Card=17837 Bytes=71348)

由於有排序所以oracle自動選擇了index full scan避免了排序。那麼強制用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 ptimizer=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會多一步sort order by，相信仔細看過這篇文章的人能知道其中結果了吧，還不知道的人請在文章中自己找答案吧。

Index Full Scan vs Index Fast Full Scan

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