In case this helps anyone... I had to figure it out recently and though it isn't hard, it isn't obvious either.
Here's how to truncate a string column in Postgres. Assume the column is of length
M > N and we want it to be
N with simple trim:
alter table T alter column C type varchar(N)The key is that
using substring(C from 1 for N);
Creferences both the column name (in the first instance) and the value (in the second instance).