ClickHouse/dbms/src/Storages/transformQueryForExternalDatabase.cpp

#include <Common/typeid_cast.h>
#include <Parsers/IAST.h>
#include <Parsers/ASTFunction.h>
#include <Parsers/ASTIdentifier.h>
#include <Parsers/ASTLiteral.h>
#include <Parsers/ASTSelectQuery.h>
#include <Parsers/ASTExpressionList.h>
#include <Parsers/queryToString.h>
#include <Interpreters/ExpressionAnalyzer.h>
#include <Storages/transformQueryForExternalDatabase.h>


namespace DB
{

static bool isCompatible(const IAST & node)
{
    if (const ASTFunction * function = typeid_cast<const ASTFunction *>(&node))
    {
        String name = function->name;

        if (!(name == "and"
            || name == "or"
            || name == "not"
            || name == "equals"
            || name == "notEquals"
            || name == "greater"
            || name == "less"
            || name == "lessOrEquals"
            || name == "greaterOrEquals"))
            return false;

        for (const auto & expr : function->arguments->children)
            if (!isCompatible(*expr.get()))
                return false;

        return true;
    }

    if (const ASTLiteral * literal = typeid_cast<const ASTLiteral *>(&node))
    {
        /// Foreign databases often have no support for Array and Tuple literals.
        if (literal->value.getType() == Field::Types::Array
            || literal->value.getType() == Field::Types::Tuple)
            return false;

        return true;
    }

    if (typeid_cast<const ASTIdentifier *>(&node))
        return true;

    return false;
}


String transformQueryForExternalDatabase(
    const IAST & query,
    const NamesAndTypesList & available_columns,
    const String & database,
    const String & table,
    const Context & context)
{
    ExpressionAnalyzer analyzer(query.clone(), context, {}, available_columns);
    const Names & used_columns = analyzer.getRequiredColumns();

    auto select = std::make_shared<ASTSelectQuery>();

    select->replaceDatabaseAndTable(database, table);

    auto select_expr_list = std::make_shared<ASTExpressionList>();
    for (const auto & name : used_columns)
        select_expr_list->children.push_back(std::make_shared<ASTIdentifier>(StringRange(), name));

    select->select_expression_list = std::move(select_expr_list);

    /** If there was WHERE,
      * copy it to transformed query if it is compatible,
      * or if it is AND expression,
      * copy only compatible parts of it.
      */

    const ASTPtr & original_where = typeid_cast<const ASTSelectQuery &>(query).where_expression;
    if (original_where)
    {
        if (isCompatible(*original_where))
        {
            select->where_expression = original_where;
        }
        else if (const ASTFunction * function = typeid_cast<const ASTFunction *>(original_where.get()))
        {
            if (function->name == "and")
            {
                auto new_function_and = std::make_shared<ASTFunction>();
                auto new_function_and_arguments = std::make_shared<ASTExpressionList>();
                new_function_and->arguments = new_function_and_arguments;
                new_function_and->children.push_back(new_function_and_arguments);

                for (const auto & elem : function->arguments->children)
                    if (isCompatible(*elem))
                        new_function_and_arguments->children.push_back(elem);

                select->where_expression = std::move(new_function_and);
            }
        }
    }

    return queryToString(select);
}

}