Re: varlena beyond 1GB and matrix

2016-12-08 8:04 GMT+09:00 Robert Haas <robertmhaas(at)gmail(dot)com>:
> On Wed, Dec 7, 2016 at 8:50 AM, Kohei KaiGai <kaigai(at)kaigai(dot)gr(dot)jp> wrote:
>> I like to propose a new optional type handler 'typserialize' to
>> serialize an in-memory varlena structure (that can have indirect
>> references) to on-disk format.
>> If any, it shall be involced on the head of toast_insert_or_update()
>> than indirect references are transformed to something other which
>> is safe to save. (My expectation is, the 'typserialize' handler
>> preliminary saves the indirect chunks to the toast relation, then
>> put toast pointers instead.)
>
> This might not work. The reason is that we have important bits of
> code that expect that they can figure out how to do some operation on
> a datum (find its length, copy it, serialize it) based only on typlen
> and typbyval. See src/backend/utils/adt/datum.c for assorted
> examples. Note also the lengthy comment near the top of the file,
> which explains that typlen > 0 indicates a fixed-length type, typlen
> == -1 indicates a varlena, and typlen == -2 indicates a cstring. I
> think there's probably room for other typlen values; for example, we
> could have typlen == -3 indicate some new kind of thing -- a
> super-varlena that has a higher length limit, or some other kind of
> thing altogether.
>
> Now, you can imagine trying to treat what you're talking about as a
> new type of TOAST pointer, but I think that's not going to help,
> because at some point the TOAST pointer gets de-toasted into a varlena
> ... which is still limited to 1GB. So that's not really going to
> work. And it brings out another point, which is that if you define a
> new typlen code, like -3, for super-big things, they won't be
> varlenas, which means they won't work with the existing TOAST
> interfaces. Still, you might be able to fix that. You would probably
> have to do some significant surgery on the wire protocol, per the
> commit message for fa2fa995528023b2e6ba1108f2f47558c6b66dcd.
>
Hmm... The reason why I didn't introduce the idea of 64bit varlena format is
this approach seems too invasive for existing PostgreSQL core and extensions,
because I assumed this "long" variable length datum utilize/enhance existing
varlena infrastructure.
However, once we have completely independent infrastructure from the exiting
varlena, we may not need to have a risk of code mixture.
It seems to me an advantage.

My concern about ExpandedObject is its flat format still needs 32bit varlena
header that restrict the total length up to 1GB, so, it was not possible to
represent a large data chunk.
So, I didn't think the current ExpandedObject is a solution for us.

Regarding to the protocol stuff, I want to consider the support of a large
record next to the internal data format, because my expected primary
usage is an internal use for in-database analytics, then user will get
its results from a complicated logic described in PL function.

> I think it's probably a mistake to conflate objects with substructure
> with objects > 1GB. Those are two somewhat orthogonal needs. As Jim
> also points out, expanded objects serve the first need. Of course,
> those assume that we're dealing with a varlena, so if we made a
> super-varlena, we'd still need to create an equivalent. But perhaps
> it would be a fairly simple adaptation of what we've already got.
> Handling objects >1GB at all seems like the harder part of the
> problem.
>
I could get your point almost. Does the last line above mention about
amount of the data object >1GB? even if the "super-varlena" format
allows 64bit length?

Thanks,
--
KaiGai Kohei <kaigai(at)kaigai(dot)gr(dot)jp>