Hello Tokbanov,
in order to write a KNIME JSON cell into a Postgres JSON column you need to do the following:
In the PostgreSQL Connector node you need to add the speciy the JDBC Parameter
stringtype=unspecified
and you need to add the following mapping
JSON->String->Varchar
to the Output Type Mapping tab
Once this is done you should be able to write JSON cells into JSON columns in the database.
When you want to read the data back into KNIME you can use the DB Type Mapper node to specify the mapping rule for each individuell JSON column (in this example called JSON) as shown below
You can find an example workflow on the KNIME Hub
Bye
Tobias