presto-main/src/main/java/io/prestosql/operator/AggregationOperator.java/AggregationOperator/addInput
class AggregationOperator:    @Override
    public void addInput(Page page)
    {
        checkState(needsInput(), "Operator is already finishing");
        requireNonNull(page, "page is null");

        long memorySize = 0;
        for (Aggregator aggregate : aggregates) {
            aggregate.processPage(page);
            memorySize += aggregate.getEstimatedSize();
        }
        if (useSystemMemory) {
            systemMemoryContext.setBytes(memorySize);
        }
        else {
            userMemoryContext.setBytes(memorySize);
        }
    }