ClickHouse/src/Interpreters/InterpreterDescribeQuery.cpp

#include <Storages/IStorage.h>
#include <DataStreams/OneBlockInputStream.h>
#include <DataStreams/BlockIO.h>
#include <DataTypes/DataTypeString.h>
#include <Parsers/queryToString.h>
#include <Common/typeid_cast.h>
#include <TableFunctions/ITableFunction.h>
#include <TableFunctions/TableFunctionFactory.h>
#include <Interpreters/InterpreterSelectWithUnionQuery.h>
#include <Interpreters/Context.h>
#include <Interpreters/InterpreterDescribeQuery.h>
#include <Interpreters/IdentifierSemantic.h>
#include <Access/AccessFlags.h>
#include <Parsers/ASTIdentifier.h>
#include <Parsers/ASTFunction.h>
#include <Parsers/ASTTablesInSelectQuery.h>
#include <Parsers/TablePropertiesQueriesASTs.h>


namespace DB
{

BlockIO InterpreterDescribeQuery::execute()
{
    BlockIO res;
    res.in = executeImpl();
    return res;
}


Block InterpreterDescribeQuery::getSampleBlock()
{
    Block block;

    ColumnWithTypeAndName col;
    col.name = "name";
    col.type = std::make_shared<DataTypeString>();
    col.column = col.type->createColumn();
    block.insert(col);

    col.name = "type";
    block.insert(col);

    col.name = "default_type";
    block.insert(col);

    col.name = "default_expression";
    block.insert(col);

    col.name = "comment";
    block.insert(col);

    col.name = "codec_expression";
    block.insert(col);

    col.name = "ttl_expression";
    block.insert(col);

    return block;
}


BlockInputStreamPtr InterpreterDescribeQuery::executeImpl()
{
    ColumnsDescription columns;

    const auto & ast = query_ptr->as<ASTDescribeQuery &>();
    const auto & table_expression = ast.table_expression->as<ASTTableExpression &>();
    if (table_expression.subquery)
    {
        auto names_and_types = InterpreterSelectWithUnionQuery::getSampleBlock(
            table_expression.subquery->children.at(0), context).getNamesAndTypesList();
        columns = ColumnsDescription(std::move(names_and_types));
    }
    else
    {
        StoragePtr table;
        if (table_expression.table_function)
        {
            const auto & table_function = table_expression.table_function->as<ASTFunction &>();
            TableFunctionPtr table_function_ptr = TableFunctionFactory::instance().get(table_function.name, context);
            /// Run the table function and remember the result
            table = table_function_ptr->execute(table_expression.table_function, context, table_function_ptr->getName());
        }
        else
        {
            auto table_id = context.resolveStorageID(table_expression.database_and_table_name);
            context.checkAccess(AccessType::SHOW_COLUMNS, table_id);
            table = DatabaseCatalog::instance().getTable(table_id);
        }

        auto table_lock = table->lockStructureForShare(false, context.getInitialQueryId());
        columns = table->getColumns();
    }

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

    for (const auto & column : columns)
    {
        if (column.is_virtual)
            continue;

        res_columns[0]->insert(column.name);
        res_columns[1]->insert(column.type->getName());

        if (column.default_desc.expression)
        {
            res_columns[2]->insert(toString(column.default_desc.kind));
            res_columns[3]->insert(queryToString(column.default_desc.expression));
        }
        else
        {
            res_columns[2]->insertDefault();
            res_columns[3]->insertDefault();
        }

        res_columns[4]->insert(column.comment);

        if (column.codec)
            res_columns[5]->insert(column.codec->getCodecDesc());
        else
            res_columns[5]->insertDefault();

        if (column.ttl)
            res_columns[6]->insert(queryToString(column.ttl));
        else
            res_columns[6]->insertDefault();
    }

    return std::make_shared<OneBlockInputStream>(sample_block.cloneWithColumns(std::move(res_columns)));
}

}