Connected to:
Oracle Database 11g Enterprise Edition Release 11.2.0.3.0 - 64bit Production
With the Partitioning and Data Mining options

SQL> @test
SQL> -- 1.1
SQL> declare
  2    l_stmt clob;
  3    cl clob;
  4    N pls_integer := (1 + 255) + 4000;
  5  begin
  6    l_stmt := 'select to_clob(''x'')';
  7    for i in 2 .. N loop
  8      l_stmt := l_stmt || '||''x''';
  9    end loop;
 10    l_stmt := l_stmt || 'from dual';
 11    execute immediate l_stmt into cl;
 12    dbms_output.put_line(length(cl));
 13  end;
 14  /

PL/SQL procedure successfully completed.

SQL> -- 1.2
SQL> declare
  2    l_stmt clob;
  3    cl clob;
  4    N pls_integer := (1 + 255) + (4000 + 1); -- =4257
  5  begin
  6    l_stmt := 'select to_clob(''x'')';
  7    for i in 2 .. N loop
  8      l_stmt := l_stmt || '||''x''';
  9    end loop;
 10    l_stmt := l_stmt || 'from dual';
 11    execute immediate l_stmt into cl;
 12    dbms_output.put_line(length(cl));
 13  end;
 14  /
declare
*
ERROR at line 1:
ORA-01489: result of string concatenation is too long
ORA-06512: at line 11


SQL> -- 1.3
SQL> declare
  2    l_stmt clob;
  3    cl clob;
  4    N pls_integer := (100 + 255) + 3902; -- =4257
  5  begin
  6    l_stmt := 'select (to_clob(''x'')';
  7    for i in 2 .. N loop
  8      l_stmt := l_stmt || '||''x'''||case i when 100 then ')' end; -- enclosed first 100 into brackets
  9    end loop;
 10    l_stmt := l_stmt || 'from dual';
 11    execute immediate l_stmt into cl;
 12    dbms_output.put_line(length(cl));
 13  end;
 14  /

PL/SQL procedure successfully completed.

SQL> -- 1.4
SQL> declare
  2    l_stmt clob;
  3    cl clob;
  4    N pls_integer := (100 + 255) + 4000;
  5  begin
  6    l_stmt := 'select (to_clob(''x'')';
  7    for i in 2 .. N loop
  8      l_stmt := l_stmt || '||''x'''||case i when 100 then ')' end; -- enclosed first 100 into brackets
  9    end loop;
 10    l_stmt := l_stmt || 'from dual';
 11    execute immediate l_stmt into cl;
 12    dbms_output.put_line(length(cl));
 13  end;
 14  /

PL/SQL procedure successfully completed.

SQL> -- 1.5 :
SQL> declare
  2    l_stmt clob;
  3    cl clob;
  4    N pls_integer := (100 + 255) + (4000 + 1);
  5  begin
  6    l_stmt := 'select (to_clob(''x'')';
  7    for i in 2 .. N loop
  8      l_stmt := l_stmt || '||''x'''||case i when 100 then ')' end; -- enclosed first 100 into brackets
  9    end loop;
 10    l_stmt := l_stmt || 'from dual';
 11    execute immediate l_stmt into cl;
 12    dbms_output.put_line(length(cl));
 13  end;
 14  /
declare
*
ERROR at line 1:
ORA-01489: result of string concatenation is too long
ORA-06512: at line 11

SQL> select FORCE_LOGGING from v$database;

FOR
---
YES

SQL>
SQL> select table_name, DEDUPLICATION,COMPRESSION, LOGGING,IN_ROW,CACHE  from user_lobs;

TABLE_NAME                     DEDUPLICATION   COMPRE LOGGING IN_ CACHE
------------------------------ --------------- ------ ------- --- ----------
LIZH_CLOB                      NONE            NONE   YES     YES NO

SQL>
SQL> col redosizeb new_value redosizeb
SQL>  SELECT value redosizeb
  2     FROM v$mystat
  3    WHERE statistic# = (  SELECT statistic# FROM v$statname WHERE name='redo size'   );

 REDOSIZEB
----------
     80980

SQL>
SQL> declare
  2  lc_clob clob;
  3  begin
  4   lc_clob := rpad('a',10000,'a');
  5  for i in 1..100 loop
  6    insert into lizh_clob values(lc_clob);
  7  end loop;
  8  commit;
  9  end;
 10  /

PL/SQL procedure successfully completed.

SQL>
SQL>  SELECT value - &redosizeb
  2     FROM v$mystat
  3    WHERE statistic# =  (  SELECT statistic# FROM v$statname WHERE name='redo size' );
old   1:  SELECT value - &redosizeb
new   1:  SELECT value -      80980

VALUE-80980
-----------
      75712

Acctually , I’m evaluating Clob and Long, and find Long is faster than Clob , not sure if there is any way to improve the performance of CLOB.

Thanks,
John

   DROP TABLE lizh_clob;
   create table lizh_clob(a clob);
  (not the first insertion)
   SQL> select LOGGING,IN_ROW,CACHE from user_lobs where table_name='LIZH_CLOB';

LOGGING IN_ CACHE                                                               
------- --- ----------                                                          
YES     YES NO                                                                  

SQL> col redosizeb new_value redosizeb
SQL>  SELECT value redosizeb
  2     FROM v$mystat
  3    WHERE statistic# = (  SELECT statistic# FROM v$statname WHERE name='redo size'   );

 REDOSIZEB                                                                      
----------                                                                      
  25173192                                                                      

SQL> 
SQL> declare
  2  lc_clob clob;
  3  begin
  4   lc_clob := rpad('a',10000,'a');
  5  for i in 1..100 loop
  6    insert into lizh_clob values(lc_clob);
  7  end loop;
  8  commit;
  9  end;
 10  /

PL/SQL procedure successfully completed.

SQL> 
SQL>  SELECT value - &redosizeb
  2     FROM v$mystat
  3    WHERE statistic# =  (  SELECT statistic# FROM v$statname WHERE name='redo size' );
old   1:  SELECT value - &redosizeb
new   1:  SELECT value -   25173192

VALUE-25173192                                                                  
--------------                                                                  
       2560392

Again you haven’t shown how you defined the CLOB. The obvious anomaly here is that the volume of redo (83KB) is less than the total volume of data you’ve inserted (1MB). Since your CLOB is set to logging I’d have to guess that you’ve enabled some securefiles deduplication or compression feature.

You haven’t told us how you’ve defined the LOB, but I’d guess that it’s defined as “disable storage in row”, cached, and logged.

The LOB data would be 1,000 bytes plus a little overhead. Then you have the LOB index entry, and the table entry. The LOB index and table entries would also generate some undo. As a starting point I’d expect about 250 – 300 bytes of redo for each of the three entries over and above their raw data – so 1,750 to 1,900 bytes. That’s still not enough to cover your 2,452 – but possibly this is the first insertion into the table, so there’s some space management stuff going on as well.

SQL> SELECT LOGGING FROM DBA_LOBS WHERE TABLE_NAME='LIZH_CLOB';

LOGGING
-------
YES

SQL>  SELECT value redosizeb
  2     FROM v$mystat
  3    WHERE statistic# = (  SELECT statistic# FROM v$statname WHERE name='redo size'   );

 REDOSIZEB
----------
    787760

SQL>
SQL> declare
  2  lc_clob clob;
  3  begin
  4   lc_clob := rpad('a',10000,'a');
  5  for i in 1..100 loop
  6    insert into lizh_clob values(lc_clob);
  7  end loop;
  8  commit;
  9  end;
 10  /

PL/SQL procedure successfully completed.

SQL>
SQL>  SELECT value - &redosizeb
  2     FROM v$mystat
  3    WHERE statistic# =  (  SELECT statistic# FROM v$statname WHERE name='redo size' );
old   1:  SELECT value - &redosizeb
new   1:  SELECT value -     787760

VALUE-787760
------------
       83308

  SQL> desc lizh_clob
 Name                                      Null?    Type
 ----------------------------------------- -------- ----------------------------
 A                                                  CLOB

SQL> col redosizeb new_value redosizeb
SQL>  SELECT value redosizeb
  2     FROM v$mystat
  3    WHERE statistic# = (  SELECT statistic# FROM v$statname WHERE name='redo size'   );

 REDOSIZEB
----------
     16108

SQL> insert into lizh_clob values(rpad('a',1000,'a'));

1 row created.

SQL>  SELECT value - &redosizeb
  2     FROM v$mystat
  3    WHERE statistic# =  (  SELECT statistic# FROM v$statname WHERE name='redo size' );
old   1:  SELECT value - &redosizeb
new   1:  SELECT value -      16108

VALUE-16108
-----------
       2452

Could you please give me any clue?

Thanks,
John