Strip strings from LOB

Question

I am trying to strip out a string that appears multiple times in a lob field. I think I have the code, but it seems to be skipping every other occurrence, and I must be missing something silly, but need some help - this is on a 10g XE database.
create table t (num number, xml clob);
declare
l_xml1 clob;
l_xml2 clob;
l_sql varchar2(4000);
l_ctx sys.dbms_xmlgen.ctxHandle ;
l_pattern varchar2(4000) := '<?xml version="1.0"?>';
l_start_loc integer := 1;
l_offset integer := 1;
l_substr varchar2(4000);
l_nth integer := 1;
l_len number;
begin
dbms_output.put_line('Begin');
l_sql := 'select sysdate, 1 from dual';
l_ctx := sys.dbms_xmlgen.newContext(queryString => l_sql);
l_xml1 := sys.dbms_xmlgen.getXML(ctx => l_ctx);
sys.dbms_xmlgen.closeContext(l_ctx);
l_sql := 'select sysdate, 2 from dual';
l_ctx := sys.dbms_xmlgen.newContext(queryString => l_sql);
l_xml2 := sys.dbms_xmlgen.getXML(ctx => l_ctx);
sys.dbms_xmlgen.closeContext(l_ctx);
dbms_lob.append(src_lob => l_xml2, dest_lob =>l_xml1);
l_sql := 'select sysdate, 3 from dual';
l_ctx := sys.dbms_xmlgen.newContext(queryString => l_sql);
l_xml2 := sys.dbms_xmlgen.getXML(ctx => l_ctx);
sys.dbms_xmlgen.closeContext(l_ctx);
dbms_lob.append(src_lob => l_xml2, dest_lob =>l_xml1);
l_sql := 'select sysdate, 4 from dual';
l_ctx := sys.dbms_xmlgen.newContext(queryString => l_sql);
l_xml2 := sys.dbms_xmlgen.getXML(ctx => l_ctx);
sys.dbms_xmlgen.closeContext(l_ctx);
dbms_lob.append(src_lob => l_xml2, dest_lob =>l_xml1);
l_sql := 'select sysdate, 5 from dual';
l_ctx := sys.dbms_xmlgen.newContext(queryString => l_sql);
l_xml2 := sys.dbms_xmlgen.getXML(ctx => l_ctx);
sys.dbms_xmlgen.closeContext(l_ctx);
dbms_lob.append(src_lob => l_xml2, dest_lob =>l_xml1);
insert into t (num, xml) values (1, l_xml1);
l_len := length(l_pattern);
dbms_output.put_line('l_len: '||l_len);
while l_start_loc <> 0
loop
dbms_output.put_line('l_nth: '||l_nth);
l_start_loc := dbms_lob.instr(lob_loc => l_xml1,
pattern => l_pattern,
offset => l_offset,
nth => l_nth);
dbms_output.put_line('Start location: '||l_start_loc);
if l_start_loc <> 0 then
dbms_lob.erase(lob_loc => l_xml1, amount => l_len, offset => l_start_loc);
dbms_output.put_line('String deleted');
end if;
l_nth := l_nth + 1;
end loop;
insert into t (num, xml) values (2, l_xml1);
commit;
dbms_output.put_line('End');
end;
If you look at the 2 rows inserted into the table, it appears only every other string was removed, and I can't figure out why.
Thanks in advance

Answer 1

Well, your code's a bit hard to read but it looks like you are:
- Reading the nth occurrence of the string, with n = 1
- Erasing this occurrence of the string
- Reading the nth occurrence of the string in the remaining lob, with n = 2
- Erasing this occurrence of the string
So you will be reading the 1st, 3rd, 6th, ..., n(n+1)/2 th occurrences.
cheers,
Anthony