Нет необходимости ходить по петлям. Вы можете просто использовать generate_series
knayak=# select nextval('seq_myTable'), 'A' || index || '_x' as s
FROM generate_series(1,10) as index;
nextval | s
---------+----------
1 | A1_x
2 | A2_x
3 | A3_x
4 | A4_x
5 | A5_x
6 | A6_x
7 | A7_x
8 | A8_x
9 | A9_x
10 | A10_x
(10 rows)
Вставить заявление
INSERT INTO myTable (id, a) select nextval('seq_myTable') as id,
'A' || index || '_x' as a
FROM generate_series(1,10) as index;