Extracting BLOB from ORDImage

Question

Hi
I need to extract the BLOB from an ORDImage object and store it in the same table (for use in an APEX report).
I've managed to accomplish this but can't understand why another simpler (to me anyway) approach fails.
CREATE TABLE ORDImage_images
(image_id   NUMBER PRIMARY KEY,
filename   VARCHAR2(20),
image      ORDSYS.ORDImage,
thumbnail  ORDSYS.ORDIMAGE,
blob_thumb BLOB);
-- load the image and confirm the image is loaded and thumbnail and blob_thumb columns are null
-- try to extract the BLOB
DECLARE
  l_image_id INTEGER:= 1;
  l_orig     ORDSYS.ORDImage;
  l_thumb    ORDSYS.ORDImage;
BEGIN
  -- lock row
  SELECT image
  INTO l_orig
  FROM ORDImage_images
  WHERE image_id = l_image_id FOR UPDATE;
  -- clear thumbnail and blob_thumb columns just in case
  UPDATE ORDImage_images
  SET thumbnail = null, blob_thumb = null
  WHERE image_id = l_image_id;
  -- this doesn't work
  l_thumb := ORDSYS.ORDImage.Init();
  -- but this does
  UPDATE ORDImage_images
  SET thumbnail = ORDSYS.ORDImage.Init()
  WHERE image_id = l_image_id;
  SELECT thumbnail
  INTO l_thumb
  FROM ORDImage_images
  WHERE image_id = l_image_id;
  l_orig.processCopy('maxScale=128 128',l_thumb);
  UPDATE ORDImage_images
  SET blob_thumb = l_thumb.source.localdata
  WHERE image_id = l_image_id;
  COMMIT;
END;I get the following error:
ORA-29400: data cartridge error
IMG-00710: unable to write to destination image
ORA-22275: invalid LOB locator specified
ORA-06512: at "ORDSYS.ORDIMG_PKG", line 1074
ORA-06512: at "ORDSYS.ORDIMAGE", line 175
ORA-06512: at line 32
29400. 00000 -  "data cartridge error\n%s"
*Cause:    An error has occurred in a data cartridge external procedure.
           This message will be followed by a second message giving
           more details about the data cartridge error.
*Action:   See the data cartridge documentation
           for an explanation of the second error message.I can't see why I need to use the thumbnail column (which I don't have any other use for) to initialise the l_thumb local variable. Why doesn't
l_thumb := ORDSYS.ORDImage.Init();accomplish the same thing? Or am I missing something really obvious?
Thanks for any help
Brian
Oracle Database 11g Enterprise Edition Release 11.2.0.3.0 - 64bit Production
PL/SQL Release 11.2.0.3.0 - Production
CORE     11.2.0.3.0     Production
TNS for 64-bit Windows: Version 11.2.0.3.0 - Production
NLSRTL Version 11.2.0.3.0 - Production

Answer 1

ORDSYS.ORDImage.Init() initializes the image object and provides a LOB locator to the BLOB field of the object.
When a LOB that is being initialized is a persistent LOB (a column in the table), the LOB locator points to a location where LOB content can be written to. So processCopy can write to the BLOB. This is what happens in your example that works.
When a LOB that is being initialized is not persistent (for example, is a variable in a PL/SQL program), then the LOB locator is not pointing to any location. A temporary LOB has to be created for this LOB locator.
For example,
l_thumb := ORDSYS.ORDImage.Init();
dbms_lob.createTemporary(l_thumb.source.localData, true);
-- processCopy
-- don't forget to free the space when done
dbms_lob.freeTemporary(l_thumb.source.localData);
Now you would not have to create a column just to be a destination for the processCopy.
You could also directly processCopy to the destination BLOB (blob_thumb in your example), using the relational interface for processCopy. This will avoid a copy from the local variable to the destination BLOB, and the overhead of creating and deleting the temporary lob.
blob_thumb := empty_blob(); -- to get the LOB locator
processCopy(l_orig.source.localData, 'maxscale=128x128',blob_thumb);
-- see http://docs.oracle.com/cd/E11882_01/appdev.112/e10776/ch_relatref.htm#g1116554 (processCopy for BLOBs) for details.
Finally, if you want to keep the thumbnail image in ORDImage and still use APEX, you could do that - specify to APEX the location of the BLOB in the ORDImage object (by referring to thumbnail.source.localData). Then you could work with image and thumbnail columns in your table, initialize both with ORDSYS.ORDImage.init(), and use processCopy for ORDImage objects.
Edited by: mannamal on Oct 17, 2012 2:32 PM