How to get Relation tuples in C function
Greetings!
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 handling of these is
quite clear. The only function I'm currently using (but not working) is the
TupleDesc TypeGetTupleDesc (Oid typeoid, List * colaliases) function. Do we
have a function like TupleDesc RelationNameGetTupleDesc (const char *
relname) (Old and deprecated)?
regards,
*Andjasubu Bungama, Patrick *
Patrick Handja <patrick.bungama@gmail.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]https://www.postgresql.org/docs/current/spi.html, 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
On Sun, Feb 14, 2021 at 5:23 AM Tom Lane <tgl@sss.pgh.pa.us> wrote:
Patrick Handja <patrick.bungama@gmail.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 itis
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/)
On Sun, Feb 14, 2021 at 09:29:08AM +0800, Andy Fan wrote:
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.
Hard to say without knowing your use case. A PL function is more
simple to maintain than a C function, though usually less performant
from the pure point of view of its operations. A SQL function could
finish by being inlined, allowing the planner to apply optimizations
as it would know the function body. Going with SPI has the advantage
to have code able to work without any changes across major versions,
which is a no-brainer when it comes to long-term maintenance.
--
Michael
On Sun, Feb 14, 2021 at 7:56 PM Michael Paquier <michael@paquier.xyz> wrote:
On Sun, Feb 14, 2021 at 09:29:08AM +0800, Andy Fan wrote:
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.Hard to say without knowing your use case. A PL function is more
simple to maintain than a C function, though usually less performant
from the pure point of view of its operations. A SQL function could
finish by being inlined, allowing the planner to apply optimizations
as it would know the function body. Going with SPI has the advantage
to have code able to work without any changes across major versions,
which is a no-brainer when it comes to long-term maintenance.
--
Michael
Thank you Michael for the response.
--
Best Regards
Andy Fan (https://www.aliyun.com/)