presto-main/src/main/java/io/prestosql/cost/AssignUniqueIdStatsRule.java/AssignUniqueIdStatsRule/calculate
class AssignUniqueIdStatsRule:    @Override
    public Optional<PlanNodeStatsEstimate> calculate(AssignUniqueId assignUniqueId, StatsProvider statsProvider, Lookup lookup, Session session, TypeProvider types)
    {
        PlanNodeStatsEstimate sourceStats = statsProvider.getStats(assignUniqueId.getSource());
        return Optional.of(PlanNodeStatsEstimate.buildFrom(sourceStats)
                .addSymbolStatistics(assignUniqueId.getIdColumn(), SymbolStatsEstimate.builder()
                        .setDistinctValuesCount(sourceStats.getOutputRowCount())
                        .setNullsFraction(0.0)
                        .setAverageRowSize(BIGINT.getFixedSize())
                        .build())
                .build());
    }