探究oracle clob欄位是如何儲存的

    clob欄位儲存的是文本，如果儲存少於4000個位元組則clob欄位會和記錄存放在一起，如果儲存超過4000個位元組，則不會與記錄存放在一起。值得注意的是，這裡的4000個位元組並不是等同於varchar2(4000),那相當於什麼呢，請看下列實驗，資料庫版本是10.2.0.1.0：

drop table test_clob purge;
create table test_clob
(
  id number,
  clob1  clob
);

insert into test_clob values(1,Lpad('111',100,'1'));--236
insert into test_clob values(2,Lpad('222',500,'1'));--1036
insert into test_clob values(3,Lpad('333',1000,'1'));--2036
insert into test_clob values(4,Lpad('444',1982,'1'));--4000
insert into test_clob values(5,Lpad('555',1983,'1'));--4002
commit;

select rowid,
       dbms_rowid.rowid_object(rowid) object_id,
       dbms_rowid.rowid_relative_fno(rowid) file_id,
       dbms_rowid.rowid_block_number(rowid) block_id,
       dbms_rowid.rowid_row_number(rowid) num
  from test_clob;
ROWID               OBJECT_ID    FILE_ID   BLOCK_ID        NUM
------------------ ---------- ---------- ---------- ----------
AAAYQbAAFAADRpcAAA      99355          5     858716          0
AAAYQbAAFAADRpcAAB      99355          5     858716          1
AAAYQbAAFAADRpcAAC      99355          5     858716          2
AAAYQbAAFAADRpdAAA      99355          5     858717          0
AAAYQbAAFAADRpgAAA      99355          5     858720          0

alter system dump datafile 5 block 858716;
alter system dump datafile 5 block 858717;
alter system dump datafile 5 block 858720;

查看dump檔案可以看到：

col  0: [ 2]  c1 02  --記錄id=1  為啥是1 請看我以前的文章：http://blog.csdn.net/stevendbaguo/article/details/8010105
col  1: [236]
 00 54 00 01 02 0c 80 00 00 02 00 00 00 01 00 00 00 02 1a 85 00 d8 09 00 00
 00 00 00 00 c8 00 00 00 00 00 01 00 31 00 31 00 31 00 31 00 31 00 31 00 31
 ...........................................................................
 ...........................................................................
 
col  0: [ 2]  c1 03   --記錄id=2
col  1: [1036]
 00 54 00 01 02 0c 80 00 00 02 00 00 00 01 00 00 00 02 1a 86 03 f8 09 00 00
 00 00 00 03 e8 00 00 00 00 00 01 00 31 00 31 00 31 00 31 00 31 00 31 00 31
 ...........................................................................
 ...........................................................................


col  0: [ 2]  c1 04   --記錄id=3
col  1: [2036]
 00 54 00 01 02 0c 80 00 00 02 00 00 00 01 00 00 00 02 1a 87 07 e0 09 00 00
 00 00 00 07 d0 00 00 00 00 00 01 00 31 00 31 00 31 00 31 00 31 00 31 00 31
 ...........................................................................
 ...........................................................................
  
col  0: [ 2]  c1 05   --記錄id=4
col  1: [4000]
 00 54 00 01 02 0c 80 00 00 02 00 00 00 01 00 00 00 02 1a 88 0f 8c 09 00 00
 00 00 00 0f 7c 00 00 00 00 00 01 00 31 00 31 00 31 00 31 00 31 00 31 00 31
 ...........................................................................
 ........................................................................... 


col  0: [ 2]  c1 06   --記錄id=5
col  1: [40]
 00 54 00 01 02 0c 80 00 00 02 00 00 00 01 00 00 00 02 1a 89 00 14 05 00 00
 00 00 00 0f 7e 00 00 00 00 00 02 01 4d 98 ac

記錄 Lpad('111',100,'1'));--236
記錄 Lpad('222',500,'1')--1036
記錄 Lpad('333',1000,'1')--2036
記錄 Lpad('444',1982,'1')--4000
記錄 Lpad('555',1983,'1')--4002

   看出規律來了，第一條記錄我們認為的長度是100，存在block中的長度是236，就是236=100*2+36，其他的規律都是這樣。同時也可以看到儲存超過4000個位元組，就是長度為1982時這個就是臨界值，大於這個之後就clob就存到其他地方了。