Обсуждение: Copying Blobs between two tables using Insert stmt

Em 17/10/2008, às 18:18, John Skillings escreveu:

> Hi all,
>
> I am trying to copy blobs between two tables and need help on the
> best way to get this done.  My requirement is that the both the
> tables maintain their own copy of the large object, instead of
> sharing the OID.
>
> I created two tables:
>
> create table table_a
> (id bigserial not null,
>  filename oid);
>
> create table table_b
>  (id bigserial not null,
>   filename oid);
>
> In one of the tables, I uploaded a file from the filesystem.
>
> INSERT INTO table_a (id, filename)
>        VALUES ( nextval('table_a_id_seq'), lo_import('C:/
> applications/largeobj.zip'));
>
> The record is inserted, and I verified the record's integrity by:
>
> SELECT lo_export(filename, 'C:/applications/largeobj.zip_copy.zip')
> FROM table_a;
>
> Question
> ----------------
> Now to make a copy of the object from table to table_a to table_a.
> Currently I am exporting the file from table_a to the file system,
> and again doing an import into table_b.   However, in a large
> application, I find this workaround not practical because of the
> volume of the records, and also the size of the file (binary
> object).  My ideal solution to do an insert of the values from
> table_a into table_b directly.
>
> So,  what is best way to create a copy of this LOB from table_a to
> table_b?

You can copy only the oid, You don't need to have another copy of the
same file in the database, if you copy only the oid you'll have
another reference to the same file.

So a simple
INSERT INTO table_a SELECT * FROM table_b;
  will do the trick in your example.

--
Diogo Biazus
diogob@gmail.com
http://www.softa.com.br
http://www.postgresql.org.br

On Fri, 2008-10-17 at 13:40 -0700, John Skillings wrote:
> Hello Csaba,
>
> Back in the month of April, I noticed that you posted a similar
> request on copying blobs between two tables, having separate OID.
>
> Can you let me know your final solution please.

The final version I'm using is this one:

CREATE OR REPLACE FUNCTION copy_blob(p_blobId oid)
RETURNS oid
AS '
DECLARE
    v_NewOID OID;
    v_LODesc INTEGER;
BEGIN
    SELECT lo_create(0) INTO v_NewOID;

    SELECT lo_open(v_NewOID, -1) INTO v_LODesc;

    PERFORM lowrite(v_LODesc, data) FROM pg_largeobject WHERE loid =
p_blobId ORDER BY pageno;

    PERFORM lo_close(v_LODesc);

    RETURN v_NewOID;
END;
' LANGUAGE 'plpgsql';

The only drawback is that you must enable reading of pg_largeobject for
the application user, as by default only the postgres super user can
access it. You would do that with something like (as postgres super
user):

GRANT SELECT ON pg_largeobject TO my_app_user;

Cheers,
Csaba.