Re: How to get Relation tuples in C function

On Sun, Feb 14, 2021 at 5:23 AM Tom Lane <tgl(at)sss(dot)pgh(dot)pa(dot)us> wrote:

> Patrick Handja <patrick(dot)bungama(at)gmail(dot)com> writes:
> > I would like to know if there is a better way to pass a relation or if
> the
> > relation name (CString) as a parameter in a C function and thus be able
> to
> > manipulate its tuples. The documentation is available here:
> > https://www.postgresql.org/docs/13/xfunc-c.html#id-1.8.3.13.11. But it
> is
> > not quite clear enough on how to retrieve tuples.
>
> The thing I'd recommend you do is use SPI [1], which lets you execute
> SQL queries from inside a C function. If you don't want to do that
> for whatever reason, you need to open the relation, set up a scan,
> and fetch tuples from the scan, relying on low-level APIs that tend
> to change from version to version. contrib/pageinspect or
> contrib/pgstattuple might offer usable sample code, although with any
> prototype you might look at, it's going to be hard to see the forest
> for the trees.
>
> regards, tom lane
>
> [1] https://www.postgresql.org/docs/current/spi.html
>
>
> Thank you tom for the reply. What would be the difference between the
SPI and "write a pure SQL UDF" and call it with DirectFunctionCall1? I
just ran into a similar situation some days before. Currently I think
DirectFunctionCall1 doesn't need to maintain a connection but SPI has to
do that.

--
Best Regards
Andy Fan (https://www.aliyun.com/)