How to get size in bytes of a CLOB column in Oracle?

Question

How do I get the size in bytes of a CLOB column in Oracle?

LENGTH() and DBMS_LOB.getLength() both return number of characters used in the CLOB but I need to know how many bytes are used (I'm dealing with multibyte charactersets).

Answer 1

After some thinking i came up with this solution:

 LENGTHB(TO_CHAR(SUBSTR(<CLOB-Column>,1,4000)))

SUBSTR returns only the first 4000 characters (max string size)

TO_CHAR converts from CLOB to VARCHAR2

LENGTHB returns the length in Bytes used by the string.

Answer 2

I'm adding my comment as an answer because it solves the original problem for a wider range of cases than the accepted answer. Note: you must still know the maximum length and the approximate proportion of multi-byte characters that your data will have.

If you have a CLOB greater than 4000 bytes, you need to use DBMS_LOB.SUBSTR rather than SUBSTR. Note that the amount and offset parameters are reversed in DBMS_LOB.SUBSTR.

Next, you may need to substring an amount less than 4000, because this parameter is the number of characters, and if you have multi-byte characters then 4000 characters will be more than 4000 bytes long, and you'll get ORA-06502: PL/SQL: numeric or value error: character string buffer too small because the substring result needs to fit in a VARCHAR2 which has a 4000 byte limit. Exactly how many characters you can retrieve depends on the average number of bytes per character in your data.

So my answer is:

LENGTHB(TO_CHAR(DBMS_LOB.SUBSTR(<CLOB-Column>,3000,1)))
+NVL(LENGTHB(TO_CHAR(DBM‌​S_LOB.SUBSTR(<CLOB-Column>,3000,3001))),0)
+NVL(LENGTHB(TO_CHAR(DBM‌​S_LOB.SUBSTR(<CLOB-Column>,6000,6001))),0)
+...

where you add as many chunks as you need to cover your longest CLOB, and adjust the chunk size according to average bytes-per-character of your data.

Answer 3

Try this one for CLOB sizes bigger than VARCHAR2:

We have to split the CLOB in parts of "VARCHAR2 compatible" sizes, run lengthb through every part of the CLOB data, and summarize all results.

declare
   my_sum int;
begin
   for x in ( select COLUMN, ceil(DBMS_LOB.getlength(COLUMN) / 2000) steps from TABLE ) 
   loop
       my_sum := 0;
       for y in 1 .. x.steps
       loop
          my_sum := my_sum + lengthb(dbms_lob.substr( x.COLUMN, 2000, (y-1)*2000+1 ));
          -- some additional output
          dbms_output.put_line('step:' || y );
          dbms_output.put_line('char length:' || DBMS_LOB.getlength(dbms_lob.substr( x.COLUMN, 2000 , (y-1)*2000+1 )));
          dbms_output.put_line('byte length:' || lengthb(dbms_lob.substr( x.COLUMN, 2000, (y-1)*2000+1 )));
          continue;
        end loop;
        dbms_output.put_line('char summary:' || DBMS_LOB.getlength(x.COLUMN));
        dbms_output.put_line('byte summary:' || my_sum);
        continue;
    end loop;
end;
/

Answer 4

The simple solution is to cast CLOB to BLOB and then request length of BLOB !

The problem is that Oracle doesn't have a function that cast CLOB to BLOB, but we can simply define a function to do that

create or replace
FUNCTION clob2blob (p_in clob) RETURN blob IS 
    v_blob        blob;
    v_desc_offset PLS_INTEGER := 1;
    v_src_offset  PLS_INTEGER := 1;
    v_lang        PLS_INTEGER := 0;
    v_warning     PLS_INTEGER := 0;  
BEGIN
    dbms_lob.createtemporary(v_blob,TRUE);
    dbms_lob.converttoblob
        ( v_blob
        , p_in
        , dbms_lob.getlength(p_in)
        , v_desc_offset
        , v_src_offset
        , dbms_lob.default_csid
        , v_lang
        , v_warning
        );
    RETURN v_blob;
END;

The SQL command to use to obtain number of bytes is

SELECT length(clob2blob(fieldname)) as nr_bytes 

or

SELECT dbms_lob.getlength(clob2blob(fieldname)) as nr_bytes

I have tested this on Oracle 10g without using Unicode(UTF-8). But I think that this solution must be correct using Unicode(UTF-8) Oracle instance :-)

I want render thanks to Nashev that has posted a solution to convert clob to blob How convert CLOB to BLOB in Oracle? and to this post written in german (the code is in PL/SQL) 13ter.info.blog that give additionally a function to convert blob to clob !

Can somebody test the 2 commands in Unicode(UTF-8) CLOB so I'm sure that this works with Unicode ?

Answer 5

NVL(length(clob_col_name),0) works for me.

Answer 6

Check the LOB segment name from dba_lobs using the table name.

select TABLE_NAME,OWNER,COLUMN_NAME,SEGMENT_NAME from dba_lobs where TABLE_NAME='<<TABLE NAME>>';

Now use the segment name to find the bytes used in dba_segments.

select s.segment_name, s.partition_name, bytes/1048576 "Size (MB)"
from dba_segments s, dba_lobs l
where s.segment_name = l.segment_name
and s.owner = '<< OWNER >> ' order by s.segment_name, s.partition_name;