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Thread: pl/pgsql, cursors and C function

pl/pgsql, cursors and C function

From
Tomasz Myrta
Date:
Hi
I'm making my first steps in C functions. I want to avoid doing all the 
SQL job in them, pl/pgsql looks a better choice. I tried to do this by 
passing opened cursor from pl/pgsql function to C function.

Here is simple C function:
#include <server/postgres.h>
#include <server/executor/spi.h>
PG_FUNCTION_INFO_V1(test2);
Datum test2(PG_FUNCTION_ARGS)
{  Portal p;  int n;  p=SPI_cursor_find("xxx");  if(!p)    elog(ERROR,"Cursor error");  SPI_cursor_fetch(p,true,1);
n=SPI_processed; PG_RETURN_INT32(n);
 
}

And pl/pgsql one:
CREATE OR REPLACE FUNCTION test() returns integer AS '
DECLARE  _m CURSOR FOR select id from some_table limit 1;  n       integer;
BEGIN  _m=''xxx'';  open _m;  n=test2();  close _m;  return n;
END;
' language 'plpgsql';

select test();
I don't understand ERROR message at all: ERROR:  SPI_prepare() failed on "SELECT   $1 "

This error is raised when trying to execute SPI_cursor_fetch. What does 
it mean? What does the SPI_prepare have to already opened cursor?
Where can I find better SPI documentation than "Postgresql Server 
Programming" ?

Regards,
Tomasz Myrta

Re: pl/pgsql, cursors and C function

From
Tom Lane
Date:
Tomasz Myrta <jasiek@klaster.net> writes:
> I don't understand ERROR message at all:
>   ERROR:  SPI_prepare() failed on "SELECT   $1 "

This is a bug.  Or two bugs, actually: one of yours and one of PG's.

I have repaired the PG bug with the attached patch.  The bug in your
code is that your C function needs to call SPI_connect and SPI_finish
if it's going to use any SPI operations.
        regards, tom lane

Index: spi.c
===================================================================
RCS file: /cvsroot/pgsql-server/src/backend/executor/spi.c,v
retrieving revision 1.75.2.3
diff -c -r1.75.2.3 spi.c
*** spi.c    14 Feb 2003 21:12:54 -0000    1.75.2.3
--- spi.c    23 Sep 2003 15:09:39 -0000
***************
*** 1387,1393 ****         elog(ERROR, "invalid portal in SPI cursor operation");      /* Push the SPI stack */
!     _SPI_begin_call(true);      /* Reset the SPI result */     SPI_processed = 0;
--- 1387,1394 ----         elog(ERROR, "invalid portal in SPI cursor operation");      /* Push the SPI stack */
!     if (_SPI_begin_call(true) < 0)
!         elog(ERROR, "SPI cursor operation called while not connected");      /* Reset the SPI result */
SPI_processed= 0;

Re: pl/pgsql, cursors and C function

From
Tomasz Myrta
Date:
>>I don't understand ERROR message at all:
>>  ERROR:  SPI_prepare() failed on "SELECT   $1 "
> 
> 

>  > This is a bug.  Or two bugs, actually: one of yours and one of PG's.
> I have repaired the PG bug with the attached patch.  The bug in your
> code is that your C function needs to call SPI_connect and SPI_finish
> if it's going to use any SPI operations.
> 
>             regards, tom lane

Thanks a lot. The first answer helped me, the second one - the others.

Do you know anything about good source of C functions documentation and 
examples? Currently I'm walking over .h headers to find some useful 
functions and macros.


Regads,
Tomasz Myrta

Re: pl/pgsql, cursors and C function

From
Tom Lane
Date:
Tomasz Myrta <jasiek@klaster.net> writes:
> Do you know anything about good source of C functions documentation and 
> examples?

Look in the main sources (backend/utils/adt/, mostly) and/or contrib
modules for functions that do something like what you need.  The only
difference between a builtin function and a dynamically loaded one is
you have to add the PG_FUNCTION_INFO macro.
        regards, tom lane