Method executeImpl

src/Interpreters/InterpreterDescribeQuery.cpp:107–249

BlockInputStreamPtr InterpreterDescribeQuery::executeImpl()
{
    ColumnsDescription columns;
    StorageSnapshotPtr storage_snapshot;

    const auto & ast = query_ptr->as<ASTDescribeQuery &>();
    const auto & table_expression = ast.table_expression->as<ASTTableExpression &>();
    const auto & settings = getContext()->getSettingsRef();

    if (table_expression.subquery)
    {
        auto names_and_types = InterpreterSelectWithUnionQuery::getSampleBlock(
            table_expression.subquery->children.at(0), getContext()).getNamesAndTypesList();
        columns = ColumnsDescription(std::move(names_and_types));
    }
    else if (table_expression.table_function)
    {
        TableFunctionPtr table_function_ptr = TableFunctionFactory::instance().get(table_expression.table_function, getContext());
        columns = table_function_ptr->getActualTableStructure(getContext());
    }
    else
    {
        auto table_id = getContext()->resolveStorageID(table_expression.database_and_table_name);
        getContext()->checkAccess(AccessType::SHOW_COLUMNS, table_id);
        auto table = DatabaseCatalog::instance().getTable(table_id, getContext());
        auto table_lock = table->lockForShare(getContext()->getInitialQueryId(), settings.lock_acquire_timeout);
        auto metadata_snapshot = table->getInMemoryMetadataPtr();
        storage_snapshot = table->getStorageSnapshot(metadata_snapshot, getContext());
        columns = metadata_snapshot->getColumns();
    }

    bool extend_object_types = settings.describe_extend_object_types && storage_snapshot;
    bool include_subcolumns = settings.describe_include_subcolumns;
    Block sample_block = getSampleBlock(getContext(), include_subcolumns);
    MutableColumns res_columns = sample_block.cloneEmptyColumns();

    auto dialect_type = getContext()->getSettingsRef().dialect_type;

    for (const auto & column : columns)
    {
        size_t i = 0;
        res_columns[i++]->insert(column.name);
        if (extend_object_types)
            res_columns[i++]->insert(storage_snapshot->getConcreteType(column.name)->getName());
        else if (dialect_type != DialectType::CLICKHOUSE)
        {
            /// Under ANSI mode, data type will be parsed by ANSI type parser, which converts Nullable to
            /// null modifiers. And the nullability of root type will be demonstrated in the separate
            /// field: `nullable`. For instance, the type field Nullable(Array(Array(Nullable(Array(String)))))
            /// will be divided into two fields under ANSI mode:
            ///     type: Array(Array(Array(String NOT NULL) NULL) NOT NULL
            ///     nullable: true
            ParserDataType type_parser(ParserSettings::ANSI);
            String type_name = column.type->getName();
            const char * type_name_pos = type_name.data();
            const char * type_name_end = type_name_pos + type_name.size();
            auto type_ast = parseQuery(type_parser, type_name_pos, type_name_end, "data type", 0, DBMS_DEFAULT_MAX_PARSER_DEPTH);