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

{
    writeString("SELECT ", out);
    if (dict_struct.id)
    {
        if (!dict_struct.id->expression.empty())
        {
            writeParenthesisedString(dict_struct.id->expression, out);
            writeString(" AS ", out);
        }
        writeQuoted(dict_struct.id->name, out);
        if (dict_struct.range_min && dict_struct.range_max)
        {
            writeString(", ", out);
            if (!dict_struct.range_min->expression.empty())
            {
                writeParenthesisedString(dict_struct.range_min->expression, out);
                writeString(" AS ", out);
            }
            writeQuoted(dict_struct.range_min->name, out);
            writeString(", ", out);
            if (!dict_struct.range_max->expression.empty())
            {
                writeParenthesisedString(dict_struct.range_max->expression, out);
                writeString(" AS ", out);
            }
            writeQuoted(dict_struct.range_max->name, out);
        }
    }