Thanks Michael und Adrian for your answers.
I think that
"
My guess is that the SERIAL was defined for the column, then the data
was added with id values, but the counter for the sequence behind the
SERIAL was not updated to a value greater the the last id added.
"
is a correct description of what happens.
For avoiding the need of an additional command for updating the sequence
before every insert statement:
Is there a way to specify the desired behaviour
(that PG always provides conflict-free id values, eg. max(id)+1 when
id values are not given explicitly in the INSERT statement)
already whend defining(!) the table in the CREATE TABLE statement ?
I've tried to using the newer definition:
id int GENERATED BY DEFAULT AS IDENTITY PRIMARY KEY
but that does not solve the problem.
Is there a way to define id columns to that when the database provides values,
it recognizes already existing values avoiding conflicts.
You'll need to run something like the below to set the next value to the max current value. You'll just have to figure out the name of the sequence that is automatically created whether you use the pseudo type serial, or the newer IDENTITY option. Both are implemented with a sequence.
--set sequence to max ID on a table
select setval( 'table_name_id_seq', ( select max(id) + 1 from table_name ) );