Cannot update view in PostgreSQL? - postgresql

Question

My site was developed using Drupal 6 running on a Postgresql 8.3 server on Ubuntu 11.10. Also webmin version 1.590.
Now I want to update records in a table, but when I run:
UPDATE uac_institution_view SET status = '2' WHERE nid = '9950'
it gives me an error like:
Failed to execute SQL : SQL UPDATE uac_institution_view SET status =
'2' WHERE nid = '9950' failed : ERROR: cannot update a view HINT: You
need an unconditional ON UPDATE DO INSTEAD rule.
The problem is that only SELECT queries work. UPDATE, INSERT and DELETE commands are not working; they fail with the above error.
Is this a permisssion problem? A syntax error? Something else?

Answer 1

PostgreSQL views are not updateable by default. You must tell PostgreSQL how you want the view to be updated.
Do this using "an unconditional ON UPDATE DO INSTEAD rule" (as the error message you pasted said) or preferably on PostgreSQL 9.1 and above using a view trigger. I provided links to all that in my answer to your previous post, but here's some more info:
updateable views in PostgreSQL 9.1 using INSTEAD OF trigger
updateable views (for Pg 9.0 and below using rules)
CREATE TRIGGER
CREATE VIEW
rules vs triggers
rules
triggers in PL/pgSQL
In many cases it's better to leave the view read-only and just update the underlying table. Since you have not provided a definition of the view it's hard to say what that would actually involve. Update your question with the output of running \d uac_institution_view in psql and comment to say you've done so; maybe I can point out a way to run the update directly on the underlying table(s).
You are using a very obsolete version of PostgreSQL (8.3) so you cannot use the preferred INSTEAD OF trigger approach, you must either use rules or update the underlying table directly.

Answer 2

FYI, after the answer involving rules/triggers was posted, PostgreSQL 9.3 came out with auto-updatable views. Version 9.3 is in beta 2 as of June 27, 2013, so it's not yet GA.
Here is an example: https://web.archive.org/web/20160322164044/http://michael.otacoo.com/postgresql-2/postgres-9-3-feature-highlight-auto-updatable-views/

Answer 3

I am on postgres 9.5 and views are updatable by default.
Example :
CREATE TABLE UP_DATE (id number, name varchar2(29));
insert into up_date values(1, 'Foo');
select * from up_date;
CREATE OR REPLACE VIEW UPDATE
AS
Select
name from up_date;
select * from update;
insert into update values('Bar');
select * from update;
Will out put Foo and Bar

Answer 4

Everything just works from PG 9.3 onwards as noted by Jeff French... With some exceptions (more info below).
Simple example
You can test this code on your PostgreSQL. Use cascade drop when you are done (to drop view with the table).
-- create table
--DROP TABLE user_table CASCADE;
CREATE TABLE user_table (
id serial,
lastname varchar(100),
user_type varchar(2) DEFAULT 'nn',
PRIMARY KEY (id)
);
-- initial data
INSERT INTO user_table(lastname) VALUES('Foo');
SELECT * FROM user_table;
-- simple view (note, no id here)
CREATE OR REPLACE VIEW user_view
AS
SELECT lastname, user_type
FROM user_table
;
-- check view (will have initial data)
SELECT * FROM user_view;
-- insert into user_table via view
INSERT INTO user_view VALUES('Bar');
-- check (both will have both records)
SELECT * FROM user_view;
SELECT * FROM user_table;
-- you can run above many times
-- (id will auto-increment even though it is not in the view)
-- update user_table via view
UPDATE user_view SET user_type='v' WHERE lastname = 'Bar';
SELECT * FROM user_table;
Limitations
There are some limitations though and that will depend on you PG version.
In PG 9.3 views cannot have any expressions etc. Also only one table is allowed... So more or less a simple select limiting, reordering, or renaming columns.
In PG 9.4 views can be partially updatable. So you can have expression but you will not be able to update them (not out of the box at least).
If you have WHERE in your view then you might get a bit weird results. So this will still work with insert:
CREATE OR REPLACE VIEW user_view
AS
SELECT lastname as last_name, user_type
FROM user_table
WHERE user_type = 'v'
;
INSERT INTO user_view VALUES('Bar');
But update might not work. At least this will not work in that it will update 0 rows:
UPDATE user_view SET user_type='v';
Because effectively that would be equivalent to below query (so makes sense if you think about it):
UPDATE user_view SET user_type='v' WHERE user_type = 'v';
I wonder if at some point they might support joins... But at the time of writing PG 14 is out and it doesn't support joined tabled in views (for updates I mean).
Alternatives
You can still use INSTEAD OF triggers, especially for more complicated views. And you can use rules... But (as noted in the CREATE RULE docs) automatically updatable views will be faster then manually created rules.