Determine length in bytes of CLOB/NCLOB with multibyte charset - oracle

Question

I'm working with an Oracle database and want to determine the length in bytes of a NCLOB using a multibyte charset (UTF-8).
LENGTHB() does not support CLOBs oder NCLOBS with multibyte charset. I could convert the NCLOB to a BLOB and get its length then. But isn't there a better way to do this?

Answer 1

Oracle stores CLOB in UTF-16 (or possibly your NCHARACTER_SET?). Each character is stored as two bytes.
Here's how you can see the raw data:
SQL> CREATE TABLE test_clob (c CLOB, v VARCHAR2(10 CHAR), nv NVARCHAR2(10));
Table created
SQL> INSERT INTO test_clob VALUES ('0123456789', '0123456789', '0123456789');
1 row inserted
SQL> SELECT dbms_rowid.rowid_relative_fno(ROWID),
2 dbms_rowid.rowid_block_number(ROWID)
3 FROM test_clob;
DBMS_ROWID.ROWID_RELATIVE_FNO( DBMS_ROWID.ROWID_BLOCK_NUMBER(
------------------------------ ------------------------------
13 94314
SQL> alter system dump datafile 13 block 94314;
System altered
Navigate to your USER_DUMP_DEST directory and open the trace file, you should see something like this:
col 0: [56]
00 54 00 01 02 0c 80 00 00 02 00 00 00 01 00 00 03 64 c6 a5 00 24 09 00 00
00 00 00 00 14 00 00 00 00 00 01 00 30 00 31 00 32 00 33 00 34 00 35 00 36
00 37 00 38 00 39
LOB
Locator:
Length: 84(56)
Version: 1
Byte Length: 2
LobID: 00.00.00.01.00.00.03.64.c6.a5
Flags[ 0x02 0x0c 0x80 0x00 ]:
Type: CLOB
[...]
Inline data[20]
[...]
col 1: [10] 30 31 32 33 34 35 36 37 38 39
col 2: [20] 00 30 00 31 00 32 00 33 00 34 00 35 00 36 00 37 00 38 00 39
As you can see the CLOB (column 0) is composed of a few header bytes and the same byte raw data as the UTF-16 NVARCHAR2 column.
As such I think you will have to convert your CLOB to UTF-8 to determine its length in this character set.
Here's an example I've used using DBMS_LOB.converttoblob:
SQL> DECLARE
2 l_clob CLOB := 'abcdéfghij'; -- the é will take two bytes!
3 l_blob BLOB;
4 l_dest_offset NUMBER := 1;
5 l_src_offset NUMBER := 1;
6 l_lang_context NUMBER := 0;
7 l_warning NUMBER;
8 BEGIN
9 dbms_lob.createtemporary(l_blob, FALSE, dbms_lob.call);
10 dbms_lob.converttoblob(dest_lob => l_blob,
11 src_clob => l_clob,
12 amount => dbms_lob.lobmaxsize,
13 dest_offset => l_dest_offset,
14 src_offset => l_src_offset,
15 blob_csid => nls_charset_id('AL32UTF8'),
16 lang_context => l_lang_context,
17 warning => l_warning);
18 dbms_output.put_line('byte length:'||dbms_lob.getlength(l_blob));
19 dbms_lob.freetemporary(l_blob);
20 END;
21 /
byte length:11
PL/SQL procedure successfully completed
You can convert to any character set using the function nls_charset_id.