presto-main/src/main/java/io/prestosql/sql/planner/iterative/rule/PruneFilterColumns.java/PruneFilterColumns/pushDownProjectOff
class PruneFilterColumns:    @Override
    protected Optional<PlanNode> pushDownProjectOff(PlanNodeIdAllocator idAllocator, FilterNode filterNode, Set<Symbol> referencedOutputs)
    {
        Set<Symbol> prunedFilterInputs = Streams.concat(
                referencedOutputs.stream(),
                SymbolsExtractor.extractUnique(filterNode.getPredicate()).stream())
                .collect(toImmutableSet());

        return restrictChildOutputs(idAllocator, filterNode, prunedFilterInputs);
    }