Re: [SQL] RULE questions.

>
> Hi,
>
> I have what I first thought would be a trivial problem, in that I require
> the 2 VARCHAR columns in the following table to have the data stored in
> upper case.
>
> test_table
> +-----------------------------+------------------------------+------+
> | Field | Type |Length|
> +-----------------------------+------------------------------+------+
> | user_id | int4 | 4 |
> | name | varchar() | 10 |
> | password | varchar() | 10 |
> +-----------------------------+------------------------------+------+
>
>
> I considered just using UPPER() in every SELECT statement, or creating a
> view that SELECTed the 2 VARCHAR cols using UPPER but I assume that this
> would have more performance issues than having the data converted to
> uppercase only once during an insert or update. (Please correct me if I am
> wrong).

It's right.

>
> After looking at triggers and rules I came up with the following solution:
>
> CREATE VIEW test AS SELECT * FROM test_table;
>
> CREATE RULE insert_test AS
> ON INSERT TO test DO INSTEAD
> INSERT INTO test_table (user_id, name, password) VALUES(new.user_id,
> UPPER(new.name), UPPER(new.password));
>
> CREATE RULE update_test AS
> ON UPDATE TO test DO INSTEAD
> UPDATE test_table SET name = UPPER(new.name), password = UPPER(new.password)
> WHERE user_id = new.user_id;

1. Make sure user_id is unique or extend the WHERE clause in
the UPDATE rule. To explain why:

user_id | name
--------+----------
1 | aaa
1 | bbb
2 | ccc

UPDATE test SET name = 'ddd' WHERE name = 'aaa';

user_id | name
--------+----------
1 | ddd
1 | ddd
2 | ccc

This is because the rule will find the user_id 1 for name
'aaa' and then updates any row with user_id 1.

2. Change the WHERE clause in the UPDATE rule to compare
against old.user_id and add "user_id = new.user_id" to
the SET clause. Otherwise it would not be possible to
change the user_id because this thrown away by the rule.

3. Don't forget the ON DELETE rule. Maybe you don't want
once given user_id's to be changed or deleted. Then 2.
and 3. aren't right.

>
> which means that any insert or update to the test view is stored in upper
> case as required.
>
> However, I still have two concerns about this.
>
> 1) What impact on performance does using a VIEW in this way have?

Only the rewriting overhead per query. The rewrite system
changes the querytree generated by the parser in such a way
that the planner/optimizer will get the same input as if the
query really was the SELECT from test_table. If you have a
view

CREATE VIEW test AS SELECT * FROM test_table;

the two statements

SELECT * FROM test;
SELECT * FROM test_table;

are totally equivalent from the planners/optimizers (and so
from the executors) point of view. The rewriting overhead
depends on how complex the statements and rule definitions
are. But not on the number of rows affected in the statement.
Selecting thousands of rows has the same speed than doing it
from the real tables behind a view. It's very small because
compared against parser/planner/optimizer it has to do very
few system cache lookups and works mostly with the data that
is already in memory.

> 2) Users can still enter data straight into test_table in lower case
> bypassing the "rules"

Not necessarily. Since v6.4 rule actions (in contrast to
triggers up to now) inherit the access permissions of the
owner of the relation they're fired on.

CREATE TABLE test_table ...;
CREATE VIEW test AS SELECT * FROM test_table;

REVOKE ALL ON test_table FROM public;
GRANT ALL ON test_table TO me;

REVOKE ALL ON test FROM public;
GRANT ALL ON test TO me;
GRANT SELECT, INSERT, UPDATE, DELETE ON test TO public;

Now any user can access test, but nobody but me can access
test_table. Not even a SELECT does work. They can do most
things on test. But the rule actions are executed under the
permissions of me, so they work silently.

YOU MUST NOT GRANT ALL TO PUBLIC. ALL includes RULE
permission, so a user could change the rules on test, do some
things (maybe on any of your other tables) and reinstall the
original state of rules!

In addition to that, consider the case you really don't want
once given user_id's ever to change. Nor you like them to be
ever reused. But they should disappear on DELETE.

CREATE TABLE test_table (user_id int,
name varchar(10),
pass varchar(10),
alive bool);

CREATE UNIQUE INDEX test_user_id ON test_table (user_id);

CREATE VIEW test AS SELECT * FROM test_data
WHERE alive;

CREATE RULE ins_test AS ON INSERT TO test
DO INSTEAD INSERT INTO test_table
VALUES (new.user_id, UPPER(new.name), UPPER(new.pass), 't');

CREATE RULE upd_test AS ON UPDATE TO test
DO INSTEAD UPDATE test_table
SET name = UPPER(new.name), pass = UPPER(new.pass)
WHERE user_id = old.user_id AND alive;

CREATE RULE del_test AS ON DELETE TO test
DO INSTEAD UPDATE test_table
SET alive = 'f'
WHERE user_id = old.user_id AND alive;

Plus all the REVOKE and GRANT. This setup denies changes to
user_id, makes the row's disappear on DELETE but throw's an
error 'cannot insert duplicate ...' if someone tries to reuse
a user_id. Only the owner of the test_table can reincarnate a
once deleted account.

>
> First off, is there an easier way to ensure that data is stored in uppercase
> for certain columns (not the whole table). And if not does anyone have
> comments on performance issues, or ways of stopping users accidentally or
> intentionally inserting lower case data straight into the table rather than
> the view?

The Postgres rewrite rule system is the most powerful way to
do that.

Jan

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