Re: libpq and Datums management with embedded C function

On Fri, Feb 18, 2011 at 2:04 AM, Alessandro Candini <candini(at)meeo(dot)it> wrote:
>> On Thu, Feb 17, 2011 at 4:16 AM, Alessandro Candini<candini(at)meeo(dot)it>
>>  wrote:
>>>
>>> I'm dealing witch a C function embedded into postgresql-9.0.2.
>>>
>>> I'm returning a set of rows and following the example here
>>> http://www.postgresql.org/docs/9.0/static/xfunc-c.html everything works
>>> fine, but only if I use the C string approach.
>>>
>>> I'm using libpq to retrieve data and something like this works fine:
>>> snprintf(values[0], 16, "%s", PQgetvalue(funcctx->user_fctx, call_cntr,
>>> 0));
>>> because PQgetvalue returns an array of char.
>>>
>>> But if I want to get the original data as output, I have to deal with
>>> Datums
>>> and I don't know how to do this with lippq.
>>> Something like the following it doesn't work:
>>>
>>> Datum values[1];
>>> values[0] = (Datum) PQgetvalue(funcctx->user_fctx, call_cntr, 1);
>>> snprintf(values[0], 16, "%s", PQgetvalue(funcctx->user_fctx, call_cntr,
>>> 0));
>>>
>>> Have you got any idea of how to solve this...do you know another way to
>>> perform a query indide the db without involve libpq?
>>> I would prefer to use them because I can connect to several postgresql
>>> instances.
>>
>> Have you ruled out dblink?  Also for connecting to self it would be
>> preferable to use SPI
>> (http://www.postgresql.org/docs/9.0/static/spi.html) vs libpq.
>>
>> merlin
>
> I see that dblink is 2500 lines more or less...

well, dblink is a library that manages some of the tedious work of
implementing libpq inside of a C procedure. A lot of the code deals
with connection management and the tedious work of creating the result
set.

> Is there anyone how to set up correctly the 2 lines I posted before?

Well, for starters, you can't sprintf into a Datum. A Datum is a
void* more or less to an internal postgresql stucture which has
nothing at all whatsoever to do with what libpq returns. As data comes
or goes out through libpq, it is translated to/from text or binary,
depending on if libpq is using the text or binary protocol. When
dealing with data in the backend in this way, text is definitely
easier. You can translate data to/from Datums using type conversion
functions that you can look up by type oid and execute. I am
skeptical though that you really need to do that.

> I will explore SPI anyway hoping it will gett my life easier...

SPI is a way of executing queries *against the database the procedure
is executing from*, and doesn't use libpq. If you are doing cross
database queries, I would strongly advise to simply load dblink and
start using it -- it's very mature and works well.

merlin