Function numeric_combine

src/backend/utils/adt/numeric.c:4849–4920

* Generic combine function for numeric aggregates which require sumX2 */

 * Generic combine function for numeric aggregates which require sumX2
 */
Datum
numeric_combine(PG_FUNCTION_ARGS)
{
	NumericAggState *state1;
	NumericAggState *state2;
	MemoryContext agg_context;
	MemoryContext old_context;

	if (!AggCheckCallContext(fcinfo, &agg_context))
		elog(ERROR, "aggregate function called in non-aggregate context");

	state1 = PG_ARGISNULL(0) ? NULL : (NumericAggState *) PG_GETARG_POINTER(0);
	state2 = PG_ARGISNULL(1) ? NULL : (NumericAggState *) PG_GETARG_POINTER(1);

	if (state2 == NULL)
	{
		if (state1 == NULL)
			PG_RETURN_NULL();
		PG_RETURN_POINTER(state1);
	}

	/* manually copy all fields from state2 to state1 */
	if (state1 == NULL)
	{
		old_context = MemoryContextSwitchTo(agg_context);

		state1 = makeNumericAggStateCurrentContext(true);
		state1->N = state2->N;
		state1->NaNcount = state2->NaNcount;
		state1->pInfcount = state2->pInfcount;
		state1->nInfcount = state2->nInfcount;
		state1->maxScale = state2->maxScale;
		state1->maxScaleCount = state2->maxScaleCount;

		accum_sum_copy(&state1->sumX, &state2->sumX);
		accum_sum_copy(&state1->sumX2, &state2->sumX2);

		MemoryContextSwitchTo(old_context);

		PG_RETURN_POINTER(state1);
	}

	state1->N += state2->N;
	state1->NaNcount += state2->NaNcount;
	state1->pInfcount += state2->pInfcount;
	state1->nInfcount += state2->nInfcount;

	if (state2->N > 0)
	{
		/*
		 * These are currently only needed for moving aggregates, but let's do
		 * the right thing anyway...
		 */
		if (state2->maxScale > state1->maxScale)
		{
			state1->maxScale = state2->maxScale;
			state1->maxScaleCount = state2->maxScaleCount;
		}