presto-main/src/main/java/io/prestosql/sql/planner/QueryPlanner.java/QueryPlanner/planFrom
class QueryPlanner:    private PlanBuilder planFrom(QuerySpecification node)
    {
        RelationPlan relationPlan;

        if (node.getFrom().isPresent()) {
            relationPlan = new RelationPlanner(analysis, symbolAllocator, idAllocator, lambdaDeclarationToSymbolMap, metadata, session)
                    .process(node.getFrom().get(), null);
        }
        else {
            relationPlan = planImplicitTable();
        }

        return planBuilderFor(relationPlan);
    }