presto-main/src/main/java/io/prestosql/cost/SemiJoinStatsRule.java/SemiJoinStatsRule/calculate
class SemiJoinStatsRule:    @Override
    public Optional<PlanNodeStatsEstimate> calculate(SemiJoinNode node, StatsProvider statsProvider, Lookup lookup, Session session, TypeProvider types)
    {
        PlanNodeStatsEstimate sourceStats = statsProvider.getStats(node.getSource());

        // For now we just propagate statistics for source symbols.
        // Handling semiJoinOutput symbols requires support for correlation for boolean columns.

        return Optional.of(sourceStats);
    }