PostgreSQL sequence based on another column

Question

Lets say I have a table as such:

Column   |     Type    |                        Notes
---------+------------ +----------------------------------------------         


        

Answer 1

Here's a simple way using standard SQL:

INSERT INTO mytable (id, seq, data)
SELECT << your desired ID >>,
       COUNT(*) + 1,
       'Quick brown fox, lorem ipsum, lazy dog, etc etc.'
FROM mytable
WHERE id = << your desired ID (same as above) >>;

See SQL Fiddle Demo.

(If you wanted to be a bit cleverer you could consider creating a trigger to update the row using the same method immediately after an insert.)