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())
    {
        if (method != CASSANDRA_SEPARATE_PARTITION_KEY)
            writeString("(", out);
        writeString(where, out);
        if (method != CASSANDRA_SEPARATE_PARTITION_KEY)
            writeString(") AND (", out);
        else
            writeString(" AND ", out);
    }
    if (method == AND_OR_CHAIN)
    {
        first = true;
        for (const auto row : requested_rows)
        {
            if (!first)
                writeString(" OR ", out);
            first = false;
            writeString("(", out);
            composeKeyCondition(key_columns, row, out, 0, key_columns.size());