presto-main/src/main/java/io/prestosql/cost/ProjectStatsRule.java/ProjectStatsRule/doCalculate
class ProjectStatsRule:    @Override
    protected Optional<PlanNodeStatsEstimate> doCalculate(ProjectNode node, StatsProvider statsProvider, Lookup lookup, Session session, TypeProvider types)
    {
        PlanNodeStatsEstimate sourceStats = statsProvider.getStats(node.getSource());
        PlanNodeStatsEstimate.Builder calculatedStats = PlanNodeStatsEstimate.builder()
                .setOutputRowCount(sourceStats.getOutputRowCount());

        for (Map.Entry<Symbol, Expression> entry : node.getAssignments().entrySet()) {
            calculatedStats.addSymbolStatistics(entry.getKey(), scalarStatsCalculator.calculate(entry.getValue(), sourceStats, session, types));
        }
        return Optional.of(calculatedStats.build());
    }