Function array_agg_combine

src/backend/utils/adt/array_userfuncs.c:555–649

}

Datum
array_agg_combine(PG_FUNCTION_ARGS)
{
	ArrayBuildState *state1;
	ArrayBuildState *state2;
	MemoryContext agg_context;
	MemoryContext old_context;
	int			i;

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

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

	if (state2 == NULL)
	{
		/*
		 * NULL state2 is easy, just return state1, which we know is already
		 * in the agg_context
		 */
		if (state1 == NULL)
			PG_RETURN_NULL();
		PG_RETURN_POINTER(state1);
	}

	if (state1 == NULL)
	{
		/* We must copy state2's data into the agg_context */
		state1 = initArrayResultWithSize(state2->element_type, agg_context,
										 false, state2->alen);

		old_context = MemoryContextSwitchTo(agg_context);

		for (i = 0; i < state2->nelems; i++)
		{
			if (!state2->dnulls[i])
				state1->dvalues[i] = datumCopy(state2->dvalues[i],
											   state1->typbyval,
											   state1->typlen);
			else
				state1->dvalues[i] = (Datum) 0;
		}

		MemoryContextSwitchTo(old_context);

		memcpy(state1->dnulls, state2->dnulls, sizeof(bool) * state2->nelems);

		state1->nelems = state2->nelems;

		PG_RETURN_POINTER(state1);
	}
	else if (state2->nelems > 0)
	{
		/* We only need to combine the two states if state2 has any elements */
		int			reqsize = state1->nelems + state2->nelems;
		MemoryContext oldContext = MemoryContextSwitchTo(state1->mcontext);