I'm trying to alter a bytea column to have type oid and still retain the values.
I have tried using queries like:
ALTER TABLE mytable ADD COLUMN mycol_tmp oid; UPDATE mytable SET mycol_tmp = CAST(mycol as oid); ALTER TABLE mytable DROP COLUMN mycol; ALTER TABLE mytable RENAME mycol_tmp TO mycol; But that just gives me the error:
ERROR: cannot cast type bytea to oid Is there any way to achieve what I want?
A column of type Oid is just a reference to the binary contents which are actually stored in the system's pg_largeobject table. In terms of storage, an Oid a 4 byte integer. On the other hand, a column of type bytea is the actual contents.
To transfer a bytea into a large object, a new large object should be created with the file-like API of large objects: lo_create() to get a new OID, then lo_open() in write mode, then writes with lo_write() or lowrite(), and then lo_close().
This can't reasonably be done with just a cast.
Basically, you would need to write a ~10 lines piece of code in the language of your choice (at least one that supports the large object API, including plpgsql) to do this conversion.
I think the best answer can be found at Grace Batumbya's Blog, in verbis:
The algorithm is pretty simple, get the binary data, if it is null, return null. Else create a large object and in the lowrite function, pass it the binary value, instead of a path to a file.
The code for the procedure is below. Note that the lo_manage package should be installed for this to work.
create or replace function blob_write(lbytea bytea) returns oid volatile language plpgsql as $f$ declare loid oid; lfd integer; lsize integer; begin if(lbytea is null) then return null; end if; loid := lo_create(0); lfd := lo_open(loid,131072); lsize := lowrite(lfd,lbytea); perform lo_close(lfd); return loid; end; $f$; CREATE CAST (bytea AS oid) WITH FUNCTION blob_write(bytea) AS ASSIGNMENT; So now the following code works: CREATE TABLE bytea_to_lo ( lo largeObj );
INSERT INTO bytea_to_lo VALUES ( DECODE('00AB','hex')); I've tried it and works like a charm.
Postgres 9.4 adds a built-in function for this:
lo_from_bytea(loid oid, string bytea) From the release notes:
- Add SQL functions to allow [large object reads/writes][12] at arbitrary offsets (Pavel Stehule)
For older versions, this is more efficient than what has been posted before:
CREATE OR REPLACE FUNCTION blob_write(bytea) RETURNS oid AS $func$ DECLARE loid oid := lo_create(0); lfd int := lo_open(loid, 131072); -- = 2^17 = x2000 -- symbolic constant defined in the header file libpq/libpq-fs.h -- #define INV_WRITE 0x00020000 BEGIN PERFORM lowrite(lfd, $1); PERFORM lo_close(lfd); RETURN loid; END $func$ LANGUAGE plpgsql VOLATILE STRICT; The STRICT modifier is smarter than handling NULL manually.
More in this related answer:
I am sure its late, but for anybody having the same problem in future.
I also faced a similar issue where I had old data in the columns of text directly in the columns not as OIDs. And when I was trying to use that data with upgraded application I too was getting
I used the knowledge of this thread to solve this issue. I strongly feel that whoever stumbles upon this question would surely like to have a look at this here
To solve the problem, I successfully used blob_write procedure from Grace Batumbya's Blog: http://gbatumbya.wordpress.com/2011/06/.