Insert line break in postgresql when updating text field

You want chr(10) instead of char(10).

Be careful with this, because that might be the wrong newline. The “right” newline depends on the client that consumes it. Macs, Windows, and Linux all use different newlines. A browser will expect <br />.

It might be safest to write your update using an escape string for PostgreSQL 9.1+. But read the docs linked below.

UPDATE public.table 
SET long_text = E'First Line\nSecond line.' 
WHERE id = 19;

The default value of ‘standard_conforming_strings’ is ‘on’ in 9.1+.

show standard_conforming_strings;