presto-main/src/main/java/io/prestosql/sql/planner/LogicalPlanner.java/LogicalPlanner/createExplainAnalyzePlan
class LogicalPlanner:    private RelationPlan createExplainAnalyzePlan(Analysis analysis, Explain statement)
    {
        RelationPlan underlyingPlan = planStatementWithoutOutput(analysis, statement.getStatement());
        PlanNode root = underlyingPlan.getRoot();
        Scope scope = analysis.getScope(statement);
        Symbol outputSymbol = symbolAllocator.newSymbol(scope.getRelationType().getFieldByIndex(0));
        root = new ExplainAnalyzeNode(idAllocator.getNextId(), root, outputSymbol, statement.isVerbose());
        return new RelationPlan(root, scope, ImmutableList.of(outputSymbol));
    }