CI 1.7 - ci_sessions table error

[eluser]gokure[/eluser]
Upgrade CI form 1.6.3 to 1.7, and added sessions tables field "user_data", but when accessing the site, it's have NOT NULL constraint.
I used database is postgres 8.3.1, and update ci_sessions table

Code:

ALTER TABLE ci_sessions ADD COLUMN user_data text NOT NULL;

Error: In the field "user_data" have NOT NULL constraint.

Code:

A Database Error Occurred
Error Number:

错误: 在字段 "user_data" 中空值违反了非空约束

INSERT INTO "ci_sessions" ("session_id", "ip_address", "user_agent", "last_activity") VALUES ('93513eddb16621fbcb236fa24b9a7bfb', '192.168.0.1', 'Mozilla/5.0 (Windows; U; Windows NT 5.1; en-US) Ap', 1224910414)

[eluser]gokure[/eluser]
Maybe set default values

Code:

ALTER TABLE ci_sessions ADD COLUMN user_data text NOT NULL DEFAULT ''::text;

that's worked.