Proper way of iterating over the column names in a trigger function.

Hi,

I am trying to interate over column names of a table on which a C trigger
function is called on UPDATE/DELETE and INSERT. SPI function
char * SPI_fname(TupleDesc rowdesc, int colnumber)
is being used. looks like the function is returning column names like
"........pg.dropped.2........"  for deleted colnumbers. My question is
what is the proper way for iterating over column names of a table using
SPI_* functions.

(sorry for bad english)


SQL transcript:
-----------------------

tradein_clients=> UPDATE public.test set x=10;
NOTICE:  no of column : 2
NOTICE:  colname: x
NOTICE:  colname: y
UPDATE 1
tradein_clients=> ALTER TABLE public.test add z int;
ALTER TABLE
tradein_clients=> UPDATE public.test set x=10;
NOTICE:  no of column : 3
NOTICE:  colname: x
NOTICE:  colname: y
NOTICE:  colname: z
UPDATE 1
tradein_clients=> ALTER TABLE public.test DROP y;
ALTER TABLE
tradein_clients=> UPDATE public.test set x=10;
NOTICE:  no of column : 2
NOTICE:  colname: x
NOTICE:  colname: ........pg.dropped.2........
UPDATE 1
tradein_clients=>

i am pasting the minial code  based on
http://www.postgresql.org/docs/8.1/static/trigger-example.html


--------------------------------------------------------------------------------------------------
#include "postgres.h"
#include "executor/spi.h"
#include "commands/trigger.h"

extern Datum trigf(PG_FUNCTION_ARGS);
static int get_no_columns(char *table_name, char *table_nspname);

PG_FUNCTION_INFO_V1(trigf);

Datum
trigf(PG_FUNCTION_ARGS)
{   TriggerData *trigdata = (TriggerData *) fcinfo->context;   TupleDesc   tupdesc;   HeapTuple   rettuple;
 char
 
*source_table,*source_table_nspname , *col_name ;   int         ret,ncolumns, i ;

   /* make sure it's called as a trigger at all */   if (!CALLED_AS_TRIGGER(fcinfo))       elog(ERROR, "trigf: not
calledby trigger manager");
 
   /* tuple to return to executor */   if (TRIGGER_FIRED_BY_UPDATE(trigdata->tg_event))       rettuple =
trigdata->tg_newtuple;  else       rettuple = trigdata->tg_trigtuple;
 
   tupdesc = trigdata->tg_relation->rd_att;

   /* connect to SPI manager */   if ((ret = SPI_connect()) < 0)       elog(INFO, "trigf : SPI_connect returned %d",
ret);

               source_table = SPI_getrelname(trigdata->tg_relation);               source_table_nspname =
SPI_getnspname(trigdata->tg_relation);
               ncolumns = get_no_columns( source_table,source_table_nspname );
               elog(NOTICE, "no of column : %d" , ncolumns);
               for (i = 1; i <= ncolumns  ; i++)               {                       col_name = SPI_fname(tupdesc,
i);                      elog (NOTICE , "colname: %s" , col_name);               }
 
   SPI_finish();   return PointerGetDatum(rettuple);
}

static int
get_no_columns(char *table_name,char *table_nspname )
{ char query[512]; int ret; int ncolumns = -1; snprintf(query, 511,                "SELECT COUNT(pg_attribute.attname)
ASa FROM
 
pg_class, pg_attribute , pg_namespace WHERE pg_class.relname='%s' and
pg_namespace.nspname='%s' AND pg_attribute.attnum > 0 AND
pg_attribute.attrelid=pg_class.oid and
pg_class.relnamespace=pg_namespace.oid and attisdropped is false" ,          table_name,          table_nspname
      ); if ((ret = SPI_exec(query, 0)) < 0) {   elog(ERROR, "get_no_columnss: could not get number of columns from
 
relation %s.%s ret: %d",        table_nspname , table_name , ret ); }
 if (SPI_processed > 0) {

/* this is a old code which is not using DatumGetInt64 , sorry for that :(if its a mistake.  */   ncolumns =
DatumGetInt32(DirectFunctionCall1                  (int4in,                    CStringGetDatum(SPI_getvalue
                      (SPI_tuptable->                                     vals[0], SPI_tuptable->tupdesc, 1))));   if
(ncolumns< 1)   {     elog(ERROR, "get_no_columns: relation %s.%s does not exist",
 
table_nspname, table_name);   } } else {   elog(ERROR,        "get_no_columns: could not get number columns in relation
%s.%s",       table_nspname , table_name); }
 
 return (ncolumns);
}
------------------------------  end of code
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