presto-main/src/main/java/io/prestosql/operator/aggregation/DecimalAverageAggregation.java/DecimalAverageAggregation/average
class DecimalAverageAggregation:    @VisibleForTesting
    public static BigDecimal average(LongDecimalWithOverflowAndLongState state, DecimalType type)
    {
        BigDecimal sum = new BigDecimal(Decimals.decodeUnscaledValue(state.getLongDecimal()), type.getScale());
        BigDecimal count = BigDecimal.valueOf(state.getLong());

        if (state.getOverflow() != 0) {
            BigInteger overflowMultiplier = TWO.shiftLeft(UNSCALED_DECIMAL_128_SLICE_LENGTH * 8 - 2);
            BigInteger overflow = overflowMultiplier.multiply(BigInteger.valueOf(state.getOverflow()));
            sum = sum.add(new BigDecimal(overflow));
        }
        return sum.divide(count, type.getScale(), ROUND_HALF_UP);
    }