Convert a bytea column to OID while retaining values

Question

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?

Answer 1

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.

Answer 2

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.

SQL Fiddle.

More in this related answer:

• Understanding cast from bytea to oid