presto-main/src/main/java/io/prestosql/cost/FilterStatsCalculator.java/FilterStatsCalculator/visitIsNullPredicate
class FilterStatsCalculator:        @Override
        protected PlanNodeStatsEstimate visitIsNullPredicate(IsNullPredicate node, Void context)
        {
            if (node.getValue() instanceof SymbolReference) {
                Symbol symbol = Symbol.from(node.getValue());
                SymbolStatsEstimate symbolStats = input.getSymbolStatistics(symbol);
                PlanNodeStatsEstimate.Builder result = PlanNodeStatsEstimate.buildFrom(input);
                result.setOutputRowCount(input.getOutputRowCount() * symbolStats.getNullsFraction());
                result.addSymbolStatistics(symbol, SymbolStatsEstimate.builder()
                        .setNullsFraction(1.0)
                        .setLowValue(NaN)
                        .setHighValue(NaN)
                        .setDistinctValuesCount(0.0)
                        .build());
                return result.build();
            }
            return PlanNodeStatsEstimate.unknown();
        }