Boolean fields from PostGIS data type changed to QString in QQIS

I think it's impossible. But you can use the field like a boolean with the selection box (ex: myfield = 't' ). Result will be same as myfield is true in Postgresql.

I solved my problem with the following approach:

In the field which holds the boolean value, I defined a checkbox as edit widget under layer properties -> fieöds and set the representation for checked as t and for unchecked as f ( see image)

Another but similar approach is to use a value map field and use the two options (image 2)

I then defined a standard layer style for my layer in QGIS and saved this style into the database with properties -> style(the button down left) -> Save as default -> Save to datasource database

So every time I load the layer the bool field is shown as checkbox which is either t or f and which is understood by Postgres as a boolean value and if I query the field inside postgres with:

SELECT bool_field from my_table 

I get something like

bool_field: false, false. true, ...

Actually it is possible to doubleclick to the boolean field in the QGIS attribute table to get a checkbox where it is possible to change the value from true to false or vice versa. One must know this, since it isn't possible to edit this field otherwise. No chance to change the text from 'true' to 'false'.