Re: Question about todo item

Tom Lane wrote:
>
> Bruce Momjian <pgman(at)candle(dot)pha(dot)pa(dot)us> writes:
> > I keep bugging Jan about it, since pre-7.1 and no one has come up with
> > an idea.
>
> Well, if you want an idea:
>
> BEGIN;
>
> SELECT open_toast_object(toastable_column) FROM tab WHERE ...;
>
> -- app checks that it got exactly one result back
>
> -- app lo_reads and/or lo_writes using ID returned by SELECT
>
> END;
>
> Implementation is left as an exercise for the reader ;-).
>
> Offhand this seems like it would be doable for a column-value that
> was actually moved out-of-line by TOAST, since the open_toast_object
> function could see and return the TOAST pointer, and then the read/
> write operations just hack on rows in pg_largeobject. The hard part
> is how to provide equivalent functionality (transparent to the client
> of course) when the particular value you select has *not* been moved
> out-of-line. Ideas anyone?

I'd propose the folllowing -

BEGIN;

DECLARE toastaccesscursor
CURSOR FOR
SELECT open_toast_object_handle(toastable_column) as
toast_object_handle FROM tab WHERE ...;

-- while you get any rows

FETCH 1 IN toastaccesscursor;

-- app lo_reads and/or lo_writes using toast_object_handle
returned by SELECT

END;

If we really wanted to have lo_xxx functionality on any toastable column
it should be doable by
creating a fake toast-handle and manipulating the column value directly,
preferrably automatically
moving the lo_written column to toast. Faking the handle should be easy
as it has to live only while
cursor is positioned on affected row .

But your another idea of creating special [B|C]LOB types that are
allways saved to toast seems nicer

CREATE TABLE breakfast (
main eggs_and_bacon WITH TOAST = 'always,nocompress'
);

and just raise an error or do a silent conversion if a section is
lo_written in a compressed
or non-toasted column.

As TOAST is a general purpose feature of postgres I think that providing
the WITH options is more
desirable than special types for only a few of them.

CLOB and BLOB could still be provided as shorthand names similar to
SERIAL.

---------------
Hannu