从下列实验可以看出全表扫描是根据extent/block顺序去取数据, 第一次实验有'db file scattered read'等待事件，第二次实验已经把数据加载到databuffer中，所以没有等待实验。
SQL> create table test as select * from dba_objects;
SQL> select extent_id, file_id, block_id, blocks
2 from dba_extents
3 where segment_name = 'TEST'
4 order by 1;
EXTENT_ID FILE_ID BLOCK_ID BLOCKS
---------- ---------- ---------- ----------
0 6 83849 8
1 6 83857 8
2 6 83865 8
3 6 83873 8
4 6 83881 8
5 6 83889 8
6 6 83897 8
7 6 83905 8
8 6 83913 8
9 6 83921 8
10 6 83929 8
11 6 83937 8
12 6 83945 8
13 6 100409 8
14 6 100417 8
15 6 117121 8
16 6 83977 128
17 6 84105 128
18 6 84233 128
19 6 84361 128
20 6 84489 128
SQL> show parameter db_file_multiblock_read_count
NAME TYPE VALUE
------------------------------------ ----------- ------------------------------
db_file_multiblock_read_count integer 16
SQL> alter session set events 'immediate trace name flush_cache level 1';
SQL> alter session set events '10046 trace name context forever,level 12';
第一次实验：
SQL> select count(1) from test;
COUNT(1)
----------
51255
第一次实验查看10046文件:
ARSING IN CURSOR #5 len=27 dep=0 uid=61 oct=3 lid=61 tim=3414138868 hv=1143379599 ad='302eb520'
select count(1) from test
END OF STMT
PARSE #5:c=46875,e=236549,p=330,cr=59,cu=0,mis=1,r=0,dep=0,og=1,tim=3414138867
BINDS #5:
EXEC #5:c=0,e=45,p=0,cr=0,cu=0,mis=0,r=0,dep=0,og=1,tim=3414139031
WAIT #5: nam='SQL*Net message to client' ela= 2 driver id=1413697536 #bytes=1 p3=0 obj#=95737 tim=3414139067
WAIT #5: nam='SQL*Net message from client' ela= 211 driver id=1413697536 #bytes=1 p3=0 obj#=95737 tim=3414139327
WAIT #5: nam='db file scattered read' ela= 4594 file#=6 block#=83852 blocks=5 obj#=95737 tim=3414143992
WAIT #5: nam='db file scattered read' ela= 424 file#=6 block#=83857blocks=4 obj#=95737 tim=3414144545
WAIT #5: nam='db file scattered read' ela= 161 file#=6 block#=83862 blocks=3 obj#=95737 tim=3414144820
WAIT #5: nam='db file scattered read' ela= 288 file#=6 block#=83866 blocks=7 obj#=95737 tim=3414145211
WAIT #5: nam='db file scattered read' ela= 314 file#=6 block#=83873 blocks=8 obj#=95737 tim=3414145670
WAIT #5: nam='db file scattered read' ela= 404 file#=6 block#=83882 blocks=7 obj#=95737 tim=3414146232
WAIT #5: nam='db file scattered read' ela= 156 file#=6 block#=83889 blocks=3 obj#=95737 tim=3414146536
WAIT #5: nam='db file scattered read' ela= 195 file#=6 block#=83893 blocks=4 obj#=95737 tim=3414146840
WAIT #5: nam='db file scattered read' ela= 280 file#=6 block#=83898 blocks=7 obj#=95737 tim=3414147226
WAIT #5: nam='db file scattered read' ela= 1438 file#=6 block#=83905 blocks=8 obj#=95737 tim=3414148811
WAIT #5: nam='db file scattered read' ela= 340 file#=6 block#=83914 blocks=7 obj#=95737 tim=3414149308
WAIT #5: nam='db file scattered read' ela= 365 file#=6 block#=83921 blocks=8 obj#=95737 tim=3414149813
WAIT #5: nam='db file scattered read' ela= 285 file#=6 block#=83930 blocks=6 obj#=95737 tim=3414150250
WAIT #5: nam='db file scattered read' ela= 128 file#=6 block#=83937 blocks=2 obj#=95737 tim=3414150515
WAIT #5: nam='db file scattered read' ela= 219 file#=6 block#=83940 blocks=5 obj#=95737 tim=3414150826
WAIT #5: nam='db file scattered read' ela= 343 file#=6 block#=83946 blocks=7 obj#=95737 tim=3414151317
WAIT #5: nam='db file scattered read' ela= 382 file#=6 block#=100409 blocks=5 obj#=95737 tim=3414151838
WAIT #5: nam='db file scattered read' ela= 187 file#=6 block#=100415 blocks=2 obj#=95737 tim=3414152147
WAIT #5: nam='db file scattered read' ela= 190 file#=6 block#=100418 blocks=2 obj#=95737 tim=3414152418
WAIT #5: nam='db file scattered read' ela= 249 file#=6 block#=100421 blocks=4 obj#=95737 tim=3414152745
。。。。。。。。。。。。。。。。。。。。。。。。。。。。。。。。。。。。。。。。。。。。。。。。。。。。。。。。
WAIT #5: nam='db file scattered read' ela= 2597 file#=6 block#=84562 blocks=10 obj#=95737 tim=3414213296
WAIT #5: nam='SQL*Net message to client' ela= 2 driver id=1413697536 #bytes=1 p3=0 obj#=95737 tim=3414213458