I use an expression as `update_field` because this feature is not implemented yet: https://github.com/ClickHouse/ClickHouse/issues/6084

20.5 generates an incorrect query in this case:
```WHERE `date_add(updated_date, interval 15 minute)` >= '2020-07-02 16:40:06'```

The exception:
```mysqlxx::BadQuery: Unknown column 'date_add(updated_date, interval 15 minute)' in 'where clause'```

The problem is here: 
https://github.com/ClickHouse/ClickHouse/blob/b81c66a03148afc02647df3c11aec40382329912/src/Dictionaries/ExternalQueryBuilder.cpp#L172
The change was introduced in this PR: https://github.com/ClickHouse/ClickHouse/pull/11079

{
    switch (quoting_style)
    {
        case IdentifierQuotingStyle::None:
            writeString(s, out);
            break;
        case IdentifierQuotingStyle::Backticks:
            writeBackQuotedString(s, out);
            break;
        case IdentifierQuotingStyle::DoubleQuotes:
            writeDoubleQuotedString(s, out);
            break;
        case IdentifierQuotingStyle::BackticksMySQL:
            writeBackQuotedStringMySQL(s, out);
            break;
    }
}
std::string ExternalQueryBuilder::composeLoadAllQuery() const
{
    WriteBufferFromOwnString out;
    composeLoadAllQuery(out);
    writeChar(';', out);
    return out.str();
}
void ExternalQueryBuilder::composeLoadAllQuery(WriteBuffer & out) const