Primary Key with auto increment field

I would like to increment one field, depending of the value of the other. here´s an example:

id     variable
1      1
2      1 
1      2  
2      2

can i do that with the following commands?


SELECT setval('sequence',(SELECT max(id) FROM table)) INTO variable;
insert into table values(variable, ..., ...);  ?

Will this be transactional? Cause, they say that setval is a command that its transactional. Using this way i


don´t will need to use a sequence anymore. Is that correct?


Thanks you all.