Re: proposal: lob conversion functionality

2013/10/21 Noah Misch <noah(at)leadboat(dot)com>

> On Mon, Sep 30, 2013 at 01:06:15PM +0300, Heikki Linnakangas wrote:
> > On 12.08.2013 21:08, Pavel Stehule wrote:
> >> 2013/8/10 Tom Lane<tgl(at)sss(dot)pgh(dot)pa(dot)us>:
> >>> Pavel Stehule<pavel(dot)stehule(at)gmail(dot)com> writes:
> >>>> I found so there are no simple API for working with LO from PL without
> >>>> access to file system.
> >>>
> >>> What? See lo_open(), loread(), lowrite(), etc.
> >>
> >> yes, so there are three problems with these functions:
> >>
> >> a) probably (I didn't find) undocumented
> >
> > It's there, although it's a bit difficult to find by searching. See:
> > http://www.postgresql.org/docs/devel/static/lo-funcs.html.
> >
> > I don't actually agree with this phrase on that page:
> >
> >> The ones that are actually useful to call via SQL commands are
> >> lo_creat, lo_create, lo_unlink, lo_import, and lo_export
> >
> > Calling lo_open, loread and lowrite seems equally useful to me.
> >
> >> b) design with lo handler is little bit PL/pgSQL unfriendly.
> >
> > It's a bit awkward, I agree.
>
> All the other large object functions are named like lo*, so I think new
> ones
> should also be lo* rather than *_lo. One of the key benefits of large
> objects, compared to a bytea column in some table, is their efficiency when
> reading or writing only a subset of the object. However, the proposed
> functions only deal with the large object as a whole. We could easily fix
> that. Consider this list of new functions in their place:
>
> lo_create(oid, bytea) RETURNS oid -- new LO with content (similar to
> make_lo)
> lo_get(oid) RETURNS bytea -- read entire LO (same
> as load_lo)
> lo_get(oid, bigint, int) RETURNS bytea -- read from offset for length
> lo_put(oid, bigint, bytea) RETURNS void -- write data at offset
>
> Anything we do here effectively provides wrappers around the existing
> functions tailored toward the needs of libpq. A key outstanding question
> is
> whether doing so provides a compelling increment in usability. On the plus
> side, adding such functions resolves the weirdness of having a variety of
> database object that is easy to access from libpq but awkward to access
> from
> plain SQL. On the minus side, this could easily live as an extension
> module.
> I have not used the large object facility to any significant degree, but I
> generally feel this is helpful enough to justify core inclusion. Any other
> opinions on the general suitability or on the specifics of the API offered?
>

fast reply - I reply again later in my office.

I don't think so new functions (for bytea type) has any sense in libpq.
From C is usually better to use a native C interface than ensure conversion
to bytea. Probably the interface libpq should be modernized, but it
complete and enough now. I don't have a motivation to enhance a API now.
And a fact, so proposed bytea functions are not in libpq is a reason why I
used different naming convention. A main motivation was a access from PL to
LO without obscure patterns - mainly for PL/pgSQL. For other languages it
is available - but maybe better direction is direct mapping to related
primitives based on host environment possibilities.

Today evening I'll look on your proposal with some more time.

Regards

Pavel

>
> Thanks,
> nm
>
> --
> Noah Misch
> EnterpriseDB http://www.enterprisedb.com
>