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$PostgreSQL: pgsql/contrib/spi/moddatetime.c,v 1.15 2009/01/07 13:44:36 tgl Exp $

What is this?
It is a function to be called from a trigger for the purpose of updating
a modification datetime stamp in a record when that record is UPDATEd.

This is 95%+ based on autoinc.c, which I used as a starting point as I do
not really know what I am doing.  I also had help from
Jan Wieck <jwieck@debis.com> who told me about the timestamp_in("now") function.
OH, me, I'm Terry Mackintosh <terry@terrym.com>
#include "postgres.h"

#include "catalog/pg_type.h"
#include "executor/spi.h"
#include "commands/trigger.h"


extern Datum moddatetime(PG_FUNCTION_ARGS);


      TriggerData *trigdata = (TriggerData *) fcinfo->context;
      Trigger    *trigger;          /* to get trigger name */
      int               nargs;                  /* # of arguments */
      int               attnum;                 /* positional number of field to change */
      Datum       newdt;                  /* The current datetime. */
      char    **args;               /* arguments */
      char     *relname;            /* triggered relation name */
      Relation    rel;              /* triggered relation */
      HeapTuple   rettuple = NULL;
      TupleDesc   tupdesc;          /* tuple description */

      if (!CALLED_AS_TRIGGER(fcinfo))
            /* internal error */
            elog(ERROR, "moddatetime: not fired by trigger manager");

      if (TRIGGER_FIRED_FOR_STATEMENT(trigdata->tg_event))
            /* internal error */
            elog(ERROR, "moddatetime: cannot process STATEMENT events");

      if (TRIGGER_FIRED_AFTER(trigdata->tg_event))
            /* internal error */
            elog(ERROR, "moddatetime: must be fired before event");

      if (TRIGGER_FIRED_BY_INSERT(trigdata->tg_event))
            /* internal error */
            elog(ERROR, "moddatetime: must be fired before event");
      else if (TRIGGER_FIRED_BY_UPDATE(trigdata->tg_event))
            rettuple = trigdata->tg_newtuple;
            /* internal error */
            elog(ERROR, "moddatetime: cannot process DELETE events");

      rel = trigdata->tg_relation;
      relname = SPI_getrelname(rel);

      trigger = trigdata->tg_trigger;

      nargs = trigger->tgnargs;

      if (nargs != 1)
            /* internal error */
            elog(ERROR, "moddatetime (%s): A single argument was expected", relname);

      args = trigger->tgargs;
      /* must be the field layout? */
      tupdesc = rel->rd_att;

      /* Get the current datetime. */
      newdt = DirectFunctionCall3(timestamp_in,

       * This gets the position in the tuple of the field we want. args[0] being
       * the name of the field to update, as passed in from the trigger.
      attnum = SPI_fnumber(tupdesc, args[0]);

       * This is were we check to see if the field we are supposed to update
       * even exits.    The above function must return -1 if name not found?
      if (attnum < 0)
                         errmsg("\"%s\" has no attribute \"%s\"",
                                    relname, args[0])));

       * OK, this is where we make sure the timestamp field that we are
       * modifying is really a timestamp field. Hay, error checking, what a
       * novel idea !-)
      if (SPI_gettypeid(tupdesc, attnum) != TIMESTAMPOID)
                         errmsg("attribute \"%s\" of \"%s\" must be type TIMESTAMP",
                                    args[0], relname)));

/* 1 is the number of items in the arrays attnum and newdt.
      attnum is the positional number of the field to be updated.
      newdt is the new datetime stamp.
      NOTE that attnum and newdt are not arrays, but then a 1 ellement array
      is not an array any more then they are.  Thus, they can be considered a
      one element array.
      rettuple = SPI_modifytuple(rel, rettuple, 1, &attnum, &newdt, NULL);

      if (rettuple == NULL)
            /* internal error */
            elog(ERROR, "moddatetime (%s): %d returned by SPI_modifytuple",
                   relname, SPI_result);

/* Clean up */

      return PointerGetDatum(rettuple);

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