Method explain

src/Interpreters/InterpreterExplainQuery.cpp:238–374

}

BlockInputStreamPtr InterpreterExplainQuery::explain()
{
    const auto & ast = query->as<ASTExplainQuery &>();

    Block sample_block = getSampleBlock();
    MutableColumns res_columns = sample_block.cloneEmptyColumns();

    WriteBufferFromOwnString buf;
    bool single_line = false;

    // if settings.enable_optimizer = true && query is supported by optimizer, print plan with optimizer.
    // if query is not supported by optimizer, settings `settings.enable_optimizer` in context will be disabled.
    if (ast.getKind() == ASTExplainQuery::ParsedAST)
    {
        if (ast.getSettings())
            throw Exception("Settings are not supported for EXPLAIN AST query.", ErrorCodes::UNKNOWN_SETTING);

        dumpAST(*ast.getExplainedQuery(), buf);
    }
    else if (ast.getKind() == ASTExplainQuery::AnalyzedSyntax)
    {
        if (ast.getSettings())
            throw Exception("Settings are not supported for EXPLAIN SYNTAX query.", ErrorCodes::UNKNOWN_SETTING);

        ExplainAnalyzedSyntaxVisitor::Data data(getContext());
        ExplainAnalyzedSyntaxVisitor(data).visit(query);

        ast.getExplainedQuery()->format(IAST::FormatSettings(buf, false));
    }
    else if (ast.getKind() == ASTExplainQuery::QueryPlan)
    {
        if (!dynamic_cast<const ASTSelectWithUnionQuery *>(ast.getExplainedQuery().get()))
            throw Exception("Only SELECT is supported for EXPLAIN query", ErrorCodes::INCORRECT_QUERY);

        auto settings = checkAndGetSettings<QueryPlanSettings>(ast.getSettings());
        QueryPlan plan;

        InterpreterSelectWithUnionQuery interpreter(ast.getExplainedQuery(), getContext(), SelectQueryOptions());
        interpreter.buildQueryPlan(plan);

        if (settings.optimize)
            plan.optimize(QueryPlanOptimizationSettings::fromContext(getContext()));

        if (settings.json)
        {
            /// Add extra layers to make plan look more like from postgres.
            auto plan_map = std::make_unique<JSONBuilder::JSONMap>();
            plan_map->add("Plan", plan.explainPlan(settings.query_plan_options));
            auto plan_array = std::make_unique<JSONBuilder::JSONArray>();
            plan_array->add(std::move(plan_map));

            auto format_settings = getFormatSettings(getContext());
            format_settings.json.quote_64bit_integers = false;

            JSONBuilder::FormatSettings json_format_settings{.settings = format_settings};
            JSONBuilder::FormatContext format_context{.out = buf};

            plan_array->format(json_format_settings, format_context);