Function postquel_get_single_result

src/backend/executor/functions.c:1112–1150

* Extract the SQL function's value from a single result row. This is used * both for scalar (non-set) functions and for each row of a lazy-eval set * result. */

 * result.
 */
static Datum
postquel_get_single_result(TupleTableSlot *slot,
						   FunctionCallInfo fcinfo,
						   SQLFunctionCachePtr fcache,
						   MemoryContext resultcontext)
{
	Datum		value;
	MemoryContext oldcontext;

	/*
	 * Set up to return the function value.  For pass-by-reference datatypes,
	 * be sure to allocate the result in resultcontext, not the current memory
	 * context (which has query lifespan).  We can't leave the data in the
	 * TupleTableSlot because we intend to clear the slot before returning.
	 */
	oldcontext = MemoryContextSwitchTo(resultcontext);

	if (fcache->returnsTuple)
	{
		/* We must return the whole tuple as a Datum. */
		fcinfo->isnull = false;
		value = ExecFetchSlotHeapTupleDatum(slot);
	}
	else
	{
		/*
		 * Returning a scalar, which we have to extract from the first column
		 * of the SELECT result, and then copy into result context if needed.
		 */
		value = slot_getattr(slot, 1, &(fcinfo->isnull));

		if (!fcinfo->isnull)
			value = datumCopy(value, fcache->typbyval, fcache->typlen);
	}

	MemoryContextSwitchTo(oldcontext);

	return value;
}

/*
 * fmgr_sql: function call manager for SQL functions