Function eval_windowfunction

src/backend/executor/nodeWindowAgg.c:1287–1329

* eval_windowfunction * * Arguments of window functions are not evaluated here, because a window * function can need random access to arbitrary rows in the partition. * The window function uses the special WinGetFuncArgInPartition and * WinGetFuncArgInFrame functions to evaluate the arguments for the rows * it wants. */

 * it wants.
 */
static void
eval_windowfunction(WindowAggState *winstate, WindowStatePerFunc perfuncstate,
					Datum *result, bool *isnull)
{
	LOCAL_FCINFO(fcinfo, FUNC_MAX_ARGS);
	MemoryContext oldContext;

	oldContext = MemoryContextSwitchTo(winstate->ss.ps.ps_ExprContext->ecxt_per_tuple_memory);

	/*
	 * We don't pass any normal arguments to a window function, but we do pass
	 * it the number of arguments, in order to permit window function
	 * implementations to support varying numbers of arguments.  The real info
	 * goes through the WindowObject, which is passed via fcinfo->context.
	 */
	InitFunctionCallInfoData(*fcinfo, &(perfuncstate->flinfo),
							 perfuncstate->numArguments,
							 perfuncstate->winCollation,
							 (void *) perfuncstate->winobj, NULL);
	/* Just in case, make all the regular argument slots be null */
	for (int argno = 0; argno < perfuncstate->numArguments; argno++)
		fcinfo->args[argno].isnull = true;
	/* Window functions don't have a current aggregate context, either */
	winstate->curaggcontext = NULL;

	*result = FunctionCallInvoke(fcinfo);
	*isnull = fcinfo->isnull;

	/*
	 * Make sure pass-by-ref data is allocated in the appropriate context. (We
	 * need this in case the function returns a pointer into some short-lived
	 * tuple, as is entirely possible.)
	 */
	if (!perfuncstate->resulttypeByVal && !fcinfo->isnull &&
		!MemoryContextContainsGenericAllocation(CurrentMemoryContext,
							   DatumGetPointer(*result))
		)
		*result = datumCopy(*result,
							perfuncstate->resulttypeByVal,
							perfuncstate->resulttypeLen);

	MemoryContextSwitchTo(oldContext);
}

/*
 * begin_partition