presto-main/src/main/java/io/prestosql/sql/planner/RelationPlan.java/RelationPlan/RelationPlan
class RelationPlan:    public RelationPlan(PlanNode root, Scope scope, List<Symbol> fieldMappings)
    {
        requireNonNull(root, "root is null");
        requireNonNull(fieldMappings, "outputSymbols is null");
        requireNonNull(scope, "scope is null");

        int allFieldCount = getAllFieldCount(scope);
        checkArgument(allFieldCount == fieldMappings.size(),
                "Number of outputs (%s) doesn't match number of fields in scopes tree (%s)",
                fieldMappings.size(),
                allFieldCount);

        this.root = root;
        this.scope = scope;
        this.fieldMappings = ImmutableList.copyOf(fieldMappings);
    }