And this is necessary to apply some transformation to the value from database and show it in the barcode, right?

Yes,
but it's also usefull to have the ability to generate bar codes
from external values ​​passed to parameters.

Agree, it will be useful.

Also it is still possible to add parameter to the query
SELECT field1, $P(Param1) AS barcode

=========== Mantis Info =====================
It is better to report such issue(s) into Mantis - our bug tracker.
http://valentina-db.com/bt

Why it is better? Described in these FAQs:
http://www.valentina-db.com/dokuwiki/doku.php?id=valentina:faqs:vbugs

* Each separate issue report into separate issue report.

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