Re: proposal: lob conversion functionality - Mailing list pgsql-hackers

+1

badly need the large object and bytea convert function.

  

Once I have to use the ugly pg_read_file() to put some text to pg,I tried to use large object but find it is useless without function to convert large object to bytea. 

Jov

blog: http:amutu.com/blog

2013/8/10 Pavel Stehule <pavel.stehule@gmail.com>

Hello

I had to enhance my older project, where XML documents are parsed and
created on server side - in PLpgSQL and PLPerl procedures. We would to
use a LO API for client server communication, but we have to
parse/serialize LO on server side.

I found so there are no simple API for working with LO from PL without
access to file system. I had to use a ugly hacks:

CREATE OR REPLACE FUNCTION parser.save_as_lob(bytea)
RETURNS oid AS $$
DECLARE
  _loid oid;
  _substr bytea;
BEGIN
  _loid := lo_creat(-1);
  FOR i IN 0..length($1)/2048
  LOOP
    _substr := substring($1 FROM i * 2048 + 1 FOR 2048);
    IF _substr <> '' THEN
      INSERT INTO pg_largeobject(loid, pageno, data)
        VALUES(_loid, i, _substr);
    END IF;
  END LOOP;

  EXECUTE format('GRANT SELECT ON LARGE OBJECT %s TO ohs', _loid);
  RETURN _loid;
END;
$$ LANGUAGE plpgsql SECURITY DEFINER STRICT SET search_path = 'pg_catalog';

and

CREATE OR REPLACE FUNCTION fbuilder.attachment_to_xml(attachment oid)
RETURNS xml AS $$
DECLARE
  b_cum bytea = '';
  b bytea;
BEGIN
  FOR b IN SELECT l.data
              FROM pg_largeobject l
             WHERE l.loid = attachment_to_xml.attachment
             ORDER BY l.pageno
  LOOP
    b_cum := b_cum || b;
  END LOOP;
  IF NOT FOUND THEN
    RETURN NULL;
  ELSE
    RETURN xmlelement(NAME "attachment",
                                       encode(b_cum, 'base64'));
  END IF;
END;
$$ LANGUAGE plpgsql STRICT SECURITY DEFINER SET search_path = 'pg_catalog';

These functions can be simplified if we supports some functions like
encode, decode for LO

So my proposal is creating functions:

* lo_encode(loid oid) .. returns bytea
* lo_encode(loid oid, encoding text) .. returns text
* lo_make(loid oid, data bytea)
* lo_make(loid oid, data text, encoding text)

This can simplify all transformation between LO and VARLENA. Known
limit is 1G for varlena, but it is still relative enough high.

Notes. comments?

Regards

Pavel


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