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

{
        writeQuoted(db, out);
        writeChar('.', out);
    }
    if (!schema.empty())
    {
        writeQuoted(schema, out);
        writeChar('.', out);
    }
    writeQuoted(table, out);
    writeString(" WHERE ", out);
    if (!where.empty())
    {
        writeString(where, out);
        writeString(" AND ", out);
    }
    writeQuoted(dict_struct.id->name, out);
    writeString(" IN (", out);
    auto first = true;
    for (const auto id : ids)
    {
        if (!first)
            writeString(", ", out);
        first = false;
        writeString(DB::toString(id), out);
    }
    writeString(");", out);
    return out.str();
}
std::string