操作blob字段是否会产生大量redo,答案是不会，下面来做一个实验,测试数据库版本是11.2.0.1.0:

--创建一张表做测试之用

create table test_blob
(
  id number,
  tupian blob
);

import java.io.FileInputStream;
import java.io.OutputStream;
import java.sql.Connection;
import java.sql.DriverManager;
import java.sql.ResultSet;
import java.sql.SQLException;
import java.sql.Statement;

import oracle.sql.BLOB;

public class BlobExample {
    static final String driver_class  = "oracle.jdbc.driver.OracleDriver";
    static final String connectionURL = "jdbc:oracle:thin:@10.10.15.25:1521:orcl";
    static final String userID        = "test";
    static final String userPassword  = "test";

    private void insertTestBlob() {
        Connection conn=null;
        Statement stm=null;
        ResultSet rs=null;
        BLOB blob = null;
        FileInputStream fin=null;
        OutputStream out=null;
        try{
            conn = DriverManager.getConnection(connectionURL, userID, userPassword);
            stm = conn.createStatement();
            conn.setAutoCommit(false);
            String sql = "insert into test_blob values(1,EMPTY_BLOB())";
            stm.executeUpdate(sql);
            rs = stm.executeQuery("SELECT tupian FROM test_blob WHERE id=1 FOR UPDATE ");
           fin = new FileInputStream("d://20130317.jpg");
           byte[] blobBuf = new byte[(int)fin.available()];
           fin.read(blobBuf);
           fin.close();

           if(rs.next()) {
            blob = (oracle.sql.BLOB)rs.getBlob(1);
            out = blob.getBinaryOutputStream();
            out.write(blobBuf);
            out.close();
            conn.commit();
           }
        }catch(Exception e){
            e.printStackTrace();
        }finally{
            try {
                rs.close();
                stm.close();
                conn.close();
            } catch (SQLException e) {
                e.printStackTrace();
            }
        }
    }
    public static void main(String args[]){
        BlobExample  blobClobExample = new BlobExample();
        blobClobExample.insertTestBlob();
    }
}

insert into test_blob  select * from test_blob;

insert into test_blob  select * from test_blob;

.......

commit;

--准备dump block

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_blob;

--update之前blob的状态

alter session set tracefile_identifier = ‘Look_For_Me‘;
alter system switch logfile;
alter system switch logfile;
alter system dump datafile 5 block 3274932;

--update之后blob的状态，同时测试一下，此时update产生了多少redo
select name, value
  from v$mystat, v$statname
 where v$mystat.statistic# = v$statname.statistic#
   and v$statname.name = ‘redo size‘
update test_blob set tupian = null;
commit;

select name, value
  from v$mystat, v$statname
 where v$mystat.statistic# = v$statname.statistic#
   and v$statname.name = ‘redo size‘

alter system switch logfile;
alter system switch logfile;
alter session set tracefile_identifier = ‘Look_For_Me1‘;
alter system dump datafile 5 block 3274932;

测试结果：我传的图片是5.1M，一共产生了350条数据，都把blob置为空以后，共产生了7.6M的redo，很显然是blob的内容是没有产生redo的。

分析原理，得借助分析dump block的内容,可以看到设置blob字段为null后产生的redo只是类似col  1: [84]这些信息。

blob设置为空以前:

Block header dump:  0x0171f8b4
 Object id on Block? Y
 seg/obj: 0x17f1d  csc: 0x9a8.7256c728  itc: 2  flg: E  typ: 1 - DATA
     brn: 0  bdba: 0x171f8b0 ver: 0x01 opc: 0
     inc: 0  exflg: 0
 
 Itl           Xid                  Uba         Flag  Lck        Scn/Fsc
0x01   0x0003.01b.000033cf  0x00c001f5.132c.39  --U-    1  fsc 0x0000.7256c775
0x02   0x0000.000.00000000  0x00000000.0000.00  ----    0  fsc 0x0000.00000000
bdba: 0x0171f8b4
data_block_dump,data header at 0x2b35c4c56064
===============
tsiz: 0x1f98
hsiz: 0x14
pbl: 0x2b35c4c56064
     76543210
flag=--------
ntab=1
nrow=1
frre=-1
fsbo=0x14
fseo=0x1f12
avsp=0x1f29
tosp=0x1f29
0xe:pti[0] nrow=1offs=0
0x12:pri[0] offs=0x1f12
block_row_dump:
tab 0, row 0, @0x1f12
tl: 91 fb: --H-FL-- lb: 0x1  cc: 2
col  0: [ 2]  c1 02
col  1: [84]
 00 54 00 01 01 0c 00 00 00 01 00 00 00 01 00 00 00 0e 48 41 00 40 05 00 00
 00 02 76 12 a0 00 00 00 00 00 02 01 71 f8 bc 01 71 f8 bd 01 71 f8 be 01 71
 f8 bf 01 71 f8 bb 01 ca 11 3d 01 ca 11 3e 01 ca 11 3f 01 ca 11 39 01 ca 11
 3a 01 ca 11 3b 01 ca 11 3c
LOB
Locator:
  Length:        84(84)
  Version:        1
  Byte Length:    1
  LobID: 00.00.00.01.00.00.00.0e.48.41
  Flags[ 0x01 0x0c 0x00 0x00 ]:
    Type: BLOB 
    Storage: BasicFile
    Enable Storage in Row 
    Characterset Format: IMPLICIT
    Partitioned Table: No
    Options: ReadWrite 
  Inode: 
    Size:     64
    Flag:     0x05 [ Valid InodeInRow(ESIR) ]
    Future:   0x00 (should be ‘0x00‘)
    Blocks:   630
    Bytes:    4768
    Version:  00000.0000000002
    DBA Array[12]:
      0x0171f8bc 0x0171f8bd 0x0171f8be 0x0171f8bf
      0x0171f8bb 0x01ca113d 0x01ca113e 0x01ca113f
      0x01ca1139 0x01ca113a 0x01ca113b 0x01ca113c

............................................................

............................................................

End dump data blocks tsn: 6 file#: 5 minblk 3274932 maxblk 3274932

blob设置为空之后:

Block header dump:  0x0171f8b4
 Object id on Block? Y
 seg/obj: 0x17f1d  csc: 0x9a8.7256c99f  itc: 2  flg: E  typ: 1 - DATA
     brn: 0  bdba: 0x171f8b0 ver: 0x01 opc: 0
     inc: 0  exflg: 0
 
 Itl           Xid                  Uba         Flag  Lck        Scn/Fsc
0x01   0x0003.01b.000033cf  0x00c001f5.132c.39  C---    0  scn 0x09a8.7256c775
0x02   0x0001.003.0000315e  0x00c0218e.1348.3a  --U-    1  fsc 0x0052.7256c9a7
bdba: 0x0171f8b4
data_block_dump,data header at 0x2b7ad6cce464
===============
tsiz: 0x1f98
hsiz: 0x14
pbl: 0x2b7ad6cce464
     76543210
flag=--------
ntab=1
nrow=1
frre=-1
fsbo=0x14
fseo=0x1f0c
avsp=0x1f29
tosp=0x1f7b
0xe:pti[0] nrow=1offs=0
0x12:pri[0] offs=0x1f0c
block_row_dump:
tab 0, row 0, @0x1f0c
tl: 6 fb: --H-FL-- lb: 0x2  cc: 1
col  0: [ 2]  c1 02

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