Re: [PATCH 4/4] Add tests to dblink covering use of COPY TO FUNCTION - Mailing list pgsql-hackers

On Tue, Nov 24, 2009 at 4:37 AM, Pavel Stehule <pavel.stehule@gmail.com> wrote:
> 2009/11/24 Daniel Farina <drfarina@gmail.com>:
>> On Tue, Nov 24, 2009 at 2:10 AM, Pavel Stehule <pavel.stehule@gmail.com> wrote:
>>> Hello
>>>
>>> I thing, so this patch is maybe good idea. I am missing better
>>> function specification. Specification by name isn't enough - we can
>>> have a overloaded functions. This syntax doesn't allow to use explicit
>>> cast - from my personal view, the syntax is ugly - with type
>>> specification we don't need to keyword FUNCTION
>>
>> As long as things continue to support the INTERNAL-type behavior for
>> extremely low overhead bulk transfers I am open to suggestions about
>> how to enrich things...but how would I do so under this proposal?
>>
>
> using an INTERNAL type is wrong. It breaks design these functions for
> usual PL. I don't see any reason, why it's necessary.
>
>> I am especially fishing for suggestions in the direction of managing
>> state for the function between rows though...I don't like how the
>> current design seems to scream "use a global variable."
>>
>>> We have a fast copy statement - ok., we have a fast function ok, but
>>> inside a function we have to call "slow" sql query. Personally What is
>>> advantage?
>>
>> The implementation here uses a type 'internal' for performance.  It
>> doesn't even recompute the fcinfo because of the very particular
>> circumstances of how the function is called.  It doesn't do a memory
>> copy of the argument buffer either, to the best of my knowledge.  In
>> the dblink patches you basically stream directly from the disk, format
>> the COPY bytes, and shove it into a waiting COPY on another postgres
>> node...there's almost no additional work in-between.  All utilized
>> time would be some combination of the normal COPY byte stream
>> generation and libpq.
>>
>
> I understand and I dislike it. This design isn't general - or it is
> far from using a function. It doesn't use complete FUNCAPI interface.
> I thing so you need different semantic. You are not use a function.
> You are use some like "stream object". This stream object can have a
> input, output function, and parameters should be internal (I don't
> thing, so internal could to carry any significant performance here) or
> standard. Syntax should be similar to CREATE AGGREGATE.

I think you might be right about this.  At the time I was too shy to
add a DDL command for this hack, though.  But what I did want is a
form of currying, and that's not easily accomplished in SQL without
extension...

> then syntax should be:
>
> COPY table TO streamname(parameters)
>
> COPY table TO filestream('/tmp/foo.dta') ...
> COPY table TO dblinkstream(connectionstring) ...

I like this one quite a bit...it's a bit like an aggregate, except the
initial condition can be set in a rather function-callish way.

But that does seem to require making a DDL command, which leaves a
nice green field.  In particular, we could then make as many hooks,
flags, and options as we wanted, but sometimes there is a paradox of
choice...I just did not want to anticipate on Postgres being friendly
to a new DDL command when writing this the first time.

fdr