Re: extensible external toast tuple support

On 2013-05-31 23:42:51 -0400, Robert Haas wrote:
> On Thu, May 30, 2013 at 7:42 AM, Andres Freund <andres@2ndquadrant.com> wrote:
> > In
> > http://archives.postgresql.org/message-id/20130216164231.GA15069%40awork2.anarazel.de
> > I presented the need for 'indirect' toast tuples which point into memory
> > instead of a toast table. In the comments to that proposal, off-list and
> > in-person talks the wish to make that a more general concept has
> > been voiced.
> >
> > The previous patch used varattrib_1b_e.va_len_1be to discern between
> > different types of external tuples. That obviously only works if the
> > data sizes of all possibly stored datum types are distinct which isn't
> > nice. So what the newer patch now does is to rename that field into
> > 'va_tag' and decide based on that what kind of Datum we have. To get the
> > actual length of that datum there now is a VARTAG_SIZE() macro which
> > maps the tags back to size.
> > To keep on-disk compatibility the size of an external toast tuple
> > containing a varatt_external is used as its tag value.
> >
> > This should allow for fairly easy development of a new compression
> > scheme for out-of-line toast tuples. It will *not* work for compressed
> > inline tuples (i.e. VARATT_4B_C). I am not convinced that that is a
> > problem or that if it is, that it cannot be solved separately.
> >
> > FWIW, in some quick microbenchmarks I couldn't find any performance
> > difference due to the slightly more complex size computation which I do
> > *not* find surprising.
> >
> > Opinions?
>
> Seems pretty sensible to me.  The patch is obviously WIP but the
> direction seems fine to me.

Here's the updated version. It shouldn't contain any obvious WIP pieces
anymore, although I think it needs some more documentation. I am just
not sure where to add it yet, postgres.h seems like a bad place :/

Greetings,

Andres Freund

--
 Andres Freund                       http://www.2ndQuadrant.com/
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