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How to read a BLOB

Oracle Tips by Burleson Consulting
April 22, 2008

 

Question:  I want to read a BLOB into my PL/SQL but I don't understand how to modify the select statement to accept a large result value.  Is there a general stored procedure for reading a BLOB in Oracle?

Answer:  Oracle supports over 20 data types, and you can even define your own custom data types.  For large objects, Oracle offers the BLOB, CLOB and LONG RAW (obsolete).

There are several documented methods to unload blob from the database, mostly using PL/SQL with utl_file.put_raw (see MOSC note 330146.1) and with java with FileOutputStream (see MOSC note 247546.1)

Note:  When defining a BLOB column, the LOB storage clause is not needed if the maximum size of the BLOB doesn't exceed 4,000 bytes. Up to 4,000 bytes can be stored in-line with the other data in the tablespace. If the length of the BLOB exceeds 4000 bytes it must be stored in either a system defaulted storage (the same as the default for the table it resides in) or in an explicitly defined LOB storage area.

Here are the details for defining, reading and writing BLOB columns:

Dr. Hall demonstrates the procedure for reading BLOB data types in his book "Oracle PL/SQL Tuning: Expert Secrets for High Performance Programming".  Here is a sample from Dr. Hall, reading a BLOB with PL/SQL:

CREATE OR REPLACE PROCEDURE extract_file(product_id in number) IS

vblob BLOB;
vstart NUMBER := 1;
bytelen NUMBER := 32000;
len NUMBER;
my_vr RAW(32000);
x NUMBER;

l_output utl_file.file_type;

BEGIN

-- define output directory
l_output := utl_file.fopen('DIR_TEMP', 'filename','wb', 32760);

vstart := 1;
bytelen := 32000;

-- get length of blob
SELECT dbms_lob.getlength(productblob)
INTO len
FROM products
WHERE id = product_id;

-- save blob length
x := len;

-- select blob into variable
SELECT product_blob
INTO vblob
FROM products
WHERE id = product_id;

-- if small enough for a single write
IF len < 32760 THEN
utl_file.put_raw(l_output,vblob);
utl_file.fflush(l_output);
ELSE -- write in pieces
vstart := 1;
WHILE vstart < len and bytelen > 0
LOOP
   dbms_lob.read(vblob,bytelen,vstart,my_vr);

   utl_file.put_raw(l_output,my_vr);
   utl_file.fflush(l_output);

   -- set the start position for the next cut
   vstart := vstart + bytelen;

   -- set the end position if less than 32000 bytes
   x := x - bytelen;
   IF x < 32000 THEN
      bytelen := x;
   END IF;
END IF;
utl_file.fclose(l_output);
end loop;

Also see these notes on reading and writing BLOBs:

If you like Oracle tuning, you might enjoy my book "Oracle Tuning: The Definitive Reference", with 950 pages of tuning tips and scripts. 

You can buy it direct from the publisher for 30%-off and get instant access to the code depot of Oracle tuning scripts.


 

 

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