Altering the Next Serial Value in a Typed Table

You can set the initial serial number or modify the next serial number for a ROW-type field with the MODIFY clause of the ALTER TABLE statement. (You cannot set the initial number for a serial field when you create a ROW data type.)

Suppose you have ROW types parent, child1, child2, and child3. 
CREATE ROW TYPE parent (a int);
CREATE ROW TYPE child1 (s serial) UNDER parent;
CREATE ROW TYPE child2 (b float, s8 serial8) UNDER child1;
CREATE ROW TYPE child3 (d int) UNDER child2;
You then create corresponding typed tables: 
CREATE TABLE OF TYPE parent;
CREATE TABLE OF TYPE child1 UNDER parent;
CREATE TABLE OF TYPE child2 UNDER child1;
CREATE TABLE OF TYPE child3 UNDER child2;
To change the next SERIAL and SERIAL8 numbers to 75, you can issue the following statement: 
ALTER TABLE child3 MODIFY (s serial(75), s8 serial8(75));

When the ALTER TABLE statement executes, the database server updates corresponding serial columns in the child1, child2, and child3 tables.