Oracle是如何进行全表扫描实验

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

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