?Working with clob and blob - using Dbms_Lob

Question

I need to search through a blob and remove some of the data, but having problems working with dbms_lob.erase.
Reading the documentation, the procedure is supposed to work with either blobs or clobs, but I can't get it to work with the blob.
Here's what I've coded and it does not work correctly for blobs.
What have I've done wrong?
declare
v_start                   integer;
v_stop                    integer;
v_amount                  integer;
v_max_len                  integer:=32676;
v_offset                   integer:=1;
v_new_length               integer;
v_clob clob;
v_blob blob;
begin
update test_clob
set clob_id = clob_id
where clob_id = 1
returning clob_desc into v_clob;
v_start := 0;
v_stop  := 0;
v_amount := 0;
v_start := dbms_lob.instr(v_clob, '<property name="Name">SortMode', v_offset );
v_stop  := dbms_lob.instr(v_clob, '</property>',  v_start );
v_amount := ((v_stop - v_start)+11) ;
dbms_output.put_line('Clob: '||v_clob);
dbms_lob.erase(v_clob, v_amount, v_start);
dbms_output.put_line('Clob: '||v_clob);
rollback;
update test_clob
set clob_id = clob_id
where clob_id = 1
returning blob_desc into v_blob;
v_start := 0;
v_stop  := 0;
v_amount := 0;
v_start := dbms_lob.instr(v_blob, utl_raw.cast_to_raw('<property name="Name">SortMode'), v_offset );
v_stop  := dbms_lob.instr(v_blob, utl_raw.cast_to_raw('</property>'),  v_start );
v_amount := ((v_stop - v_start)+11) ;
dbms_output.put_line('Blob: '||utl_raw.cast_to_varchar2(v_blob) );
dbms_lob.erase(v_blob, v_amount, v_start);
dbms_output.put_line('Blob: '||utl_raw.cast_to_varchar2(v_blob) );
rollback;
end trg_bui_user_assoc_layout;
/This is the output
Clob: this is only a test <property name="Name">SortMode</property>  should leave this alone
Clob: this is only a test                                            should leave this alone
Blob: this is only a test <property name="Name">SortMode</property>  should leave this alone
Blob: this is only a test

Answer 1

Well, you left out the table DDL and your insert for sample data (would be nice to have) as well as your Oracle version (pretty much a necessity).
Since i had to make my own there could be a difference in how you populated your table, but i can't reproduce your findings.
ME_ORCL?drop table test_clob purge;
Table dropped.
Elapsed: 00:00:00.09
ME_ORCL?
ME_ORCL?create table test_clob
  2  (
  3     clob_id     number not null primary key,
  4     clob_desc   clob,
  5     blob_desc   blob
  6  );
Table created.
Elapsed: 00:00:00.03
ME_ORCL?
ME_ORCL?insert into test_clob values
  2  (
  3        1
  4     ,  'this is only a test <property name="Name">SortMode</property>  should leave this alone'
  5     ,  utl_raw.cast_to_raw('this is only a test <property name="Name">SortMode</property>  should leave this alone')
  6  );
1 row created.
Elapsed: 00:00:00.01
ME_ORCL?
ME_ORCL?commit;
Commit complete.
Elapsed: 00:00:00.01
ME_ORCL?
ME_ORCL?declare
  2  v_start                   integer;
  3  v_stop                    integer;
  4  v_amount                  integer;
  5  v_max_len                  integer:=32676;
  6  v_offset                   integer:=1;
  7  v_new_length               integer;
  8
  9  v_clob clob;
10  v_blob blob;
11
12  begin
13
14   update test_clob
15   set clob_id = clob_id
16   where clob_id = 1
17   returning clob_desc into v_clob;
18
19   v_start := 0;
20   v_stop  := 0;
21   v_amount := 0;
22
23   v_start := dbms_lob.instr(v_clob, '<property name="Name">SortMode', v_offset );
24   v_stop  := dbms_lob.instr(v_clob, '</property>',  v_start );
25   v_amount := ((v_stop - v_start)+11) ;
26
27   dbms_output.put_line('Clob: '||v_clob);
28
29   dbms_lob.erase(v_clob, v_amount, v_start);
30
31   dbms_output.put_line('Clob: '||v_clob);
32
33   rollback;
34
35   update test_clob
36   set clob_id = clob_id
37   where clob_id = 1
38   returning blob_desc into v_blob;
39
40   v_start := 0;
41   v_stop  := 0;
42   v_amount := 0;
43
44   v_start := dbms_lob.instr(v_blob, utl_raw.cast_to_raw('<property name="Name">SortMode'), v_offset );
45   v_stop  := dbms_lob.instr(v_blob, utl_raw.cast_to_raw('</property>'),  v_start );
46   v_amount := ((v_stop - v_start)+11) ;
47
48   dbms_output.put_line('Blob: '||utl_raw.cast_to_varchar2(v_blob) );
49
50   dbms_lob.erase(v_blob, v_amount, v_start);
51
52   dbms_output.put_line('Blob: '||utl_raw.cast_to_varchar2(v_blob) );
53
54   rollback;
55
56  end trg_bui_user_assoc_layout;
57  /
Clob: this is only a test <property name="Name">SortMode</property>  should leave this alone
Clob: this is only a test                                            should leave this alone
Blob: this is only a test <property name="Name">SortMode</property>  should leave this alone
Blob: this is only a test                                            should leave this alone
PL/SQL procedure successfully completed.
Elapsed: 00:00:00.03
ME_ORCL?select *
  2  from v$version;
BANNER
Oracle Database 11g Enterprise Edition Release 11.2.0.2.0 - Production
PL/SQL Release 11.2.0.2.0 - Production
CORE    11.2.0.2.0      Production
TNS for Linux: Version 11.2.0.2.0 - Production
NLSRTL Version 11.2.0.2.0 - Production
5 rows selected.
Elapsed: 00:00:00.03
ME_ORCL?