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Friday, October 21, 2011

MySQL Hacks: Preventing deletion of specific rows

Recently, someone emailed me:
I have a requirement in MYSQL as follows:
we have a table EMP and we have to restrict the users not delete employees with DEPT_ID = 10. If user executes a DELETE statement without giving any WHERE condition all the rows should be deleted except those with DEPT_ID = 10.

We are trying to write a BEFORE DELETE trigger but we are not able to get this functionality.

I have seen your blog where you explained about Using an UDF to Raise Errors from inside MySQL Procedures and/or Triggers. Will it helps me to get this functionality? Could you suggest if we have any other alternatives to do this as well?
Frankly, I usually refer people that write me these things to a public forum, but this time I felt like giving it a go. I figured it would be nice to share my solution and I'm also curious if others found other solutions still.

(Oh, I should point out that I haven't asked what the underlying reasons are for this somewhat extraordinary requirement. I normally would do that if I would be confronted with sucha a requirement in a professional setting. In this case I'm only interested in finding a crazy hack)

Attempt 1: Re-insert deleted rows with a trigger

My first suggestion was:
Raising the error won't help you achieve your goal: as soon as you raise the error, the statement will either abort (in case of a non-transactional table) or rollback all row changes made up to raising the error (in case of a transactional table)

Although I find the requirement strange, here's a trick you could try:

write a AFTER DELETE FOR EACH ROW trigger that re-inserts the rows back into the table in case the condition DEPT_ID = 10 is met.

Hope this helps...

Alas, I should've actually tried it myself before replying, because it doesn't work. If you do try it, a DELETEresults in this runtime error:
Can't update table 'emp' in stored function/trigger because it is already used by statement which invoked this stored function/trigger.
This is also known as "the mutating table problem".

Attempt 2: Re-insert deleted rows into a FEDERATED table

As it turns out, there is a workaround that meets all of the original demands. The workaround relies on the FEDERATED storage engine, which we can use to trick MySQL into thinking we're manipulating a different table than the one that fires the trigger. My first attempt went something like this:

CREATE TABLE t (
id INT AUTO_INCREMENT PRIMARY KEY,
dept_id INT,
INDEX(dept_id)
);

CREATE TABLE federated_t (
id INT AUTO_INCREMENT PRIMARY KEY,
dept_id INT,
INDEX(dept_id)
)
ENGINE FEDERATED
CONNECTION = 'mysql://root@localhost:3306/test/t';

DELIMITER //

CREATE TRIGGER adr_t
AFTER DELETE ON t
FOR EACH ROW
IF old.dept_id = 10 THEN
INSERT INTO t_federated
VALUES (old.id, old.dept_id);
END IF;
//

DELIMITER ;
So the idea is to let the trigger re-insert the deleted rows back into the federated table, which in turn points to the original table that fired the trigger to fool MySQL into thinking it isn't touching the mutating table. Although this does prevent one from deleting any rows that satisfy the DEPT_ID = 10 condition, it does not work as intended:

mysql> INSERT INTO t VALUES (1,10), (2,20), (3,30);
Query OK, 3 rows affected (0.11 sec)

mysql> DELETE FROM t;
ERROR 1159 (08S01): Got timeout reading communication packets

mysql> SELECT * FROM t;
+----+---------+
| id | dept_id |
+----+---------+
| 1 | 10 |
| 2 | 20 |
| 3 | 30 |
+----+---------+
3 rows in set (0.00 sec)
At this point I can only make an educated guess about the actual underlying reason for this failure. It could be that the deletion is locking the rows or even the table, thereby blocking the insert into the federated table until we get a timeout. Or maybe MySQL enters into an infinite loop of deletions and insertions until we hit a timeout. I didn't investigate, so I don't know, but it seems clear this naive solution doesn't solve he problem.

Attempt 3: Deleting from the FEDERATED table and re-inserting into the underlying table

It turns out that we can solve it with a FEDERATED table by turning the problem around: Instead of manipulating the original table, we can INSERT and DELETE from the FEDERATED table, and have an AFTER DELETE trigger on the FEDERATED table re-insert the deleted rows back into the original table:

DROP TRIGGER adr_t;

DELIMITER //

CREATE TRIGGER adr_federated_t
AFTER DELETE ON federated_t
FOR EACH ROW
IF old.dept_id = 10 THEN
INSERT INTO t
VALUES (old.id, old.dept_id);
END IF;
//

DELIMITER ;
Now, the DELETE does work as intended:

mysql> DELETE FROM federated_t;
Query OK, 3 rows affected (0.14 sec)

mysql> SELECT * FROM federated_t;
+----+---------+
| id | dept_id |
+----+---------+
| 1 | 10 |
+----+---------+
1 row in set (0.00 sec)
Of course, to actually use this solution, one would grant applications access only to the federated table, and "hide" the underlying table so they can't bypass the trigger by deleting rows directly from the underlying table.

Now, even though this solution does seem to fit the original requirements, I would not recommend it for several reasons:
  • It uses the FEDERATED storage engine, which hasn't been well supported. For that reason, it isn't enabled by default, and you need access to the MySQL configuration to enable it, limiting the applicability of this solution. Also, you could run into some nasty performance problems with the FEDERATED storage engine
  • The solution relies on a trigger. In MySQL, triggers can really limit performance
  • Perhaps the most important reason is that this solution performs "magic" by altering the behaviour of SQL statements. Arguably, this is not so much the fault of the solution as it is of the original requirement.

An Alternative without relying on magic: a foreign key constraint

If I were to encounter the original requirement in a professional situation, I would argue that we should not desire to alter the semantics of SQL commands. If we tell the RDBMS to delete all rows from a table, it should either succeed and result in all rows being deleted, or it should fail and fail completely, leaving the data unchanged.

So how would we go about implementing a solution for this changed requirement?

We certainly could try the approach that was suggested in the original request: create a trigger that raises an exception whenever we find the row should not be deleted. However, this would still rely on a trigger (which is slow). And if you're not on MySQL 5.5 (or higher), you would have to use one of the ugly hacks to raise an exception.

As it turns out, there is a very simple solution that does not rely on triggers. We can create a "guard table" that references the table we want to protect using a foreign key constraint:

mysql> CREATE TABLE t_guard (
-> dept_id INT PRIMARY KEY,
-> FOREIGN KEY (dept_id)
-> REFERENCES t(dept_id)
-> );
Query OK, 0 rows affected (0.11 sec)

mysql> INSERT INTO t_guard values (10);
Query OK, 1 row affected (0.08 sec)

mysql> DELETE FROM t;
ERROR 1451 (23000): Cannot delete or update a parent row: a foreign key constraint fails (`test`.`t_guard`, CONSTRAINT `t_guard_ibfk_1` FOREIGN KEY (`dept_id`) REFERENCES `t` (`dept_id`))
mysql> DELETE FROM t WHERE dept_id != 10;
Query OK, 2 rows affected (0.05 sec)
(Like in the prior example with the federated table, the guard table would not be accessible to the application, and the "guard rows" would have to be inserted by a privileged user)

Finally: what a quirkyy foreign key constraint!

You might have noticed that there's something quite peculiar about the foreign key constraint: typically, foreign key constraints serve to relate "child" rows to their respective "parent" row. To do that, the foreign key would typically point to a column (or set of columns) that make up either the primary key or a unique constraint in the parent table. But in this case, the referenced column dept_id in the t table is contained only in an index which is not unique. Strange as it may seem, this is allowed by MySQL (or rather, InnoDB). In this particular case, this flexibility (or is it a bug?) serves us quite well, and it allows us to guard many rows in the t table with dept_id = 10 with just one single row in the guard table.

Friday, October 07, 2011

Fighting Spam: Word Verification

Hi All,

this is a quick note to let you know that from now on, commenters on this blog will need to complete a word verification (captcha) step.

Personally, I regret to have to take this measure. Let me explain why I'm doing it anyway.

Since 3 months or so, moderating comments on this blog is becoming a real drag due to a surge in anonymous spam. While bloggers spam detection is quite good, I still get notificaton mails prompting me to moderate. I feel this is consuming more of my time than it's worth.

Except for requiring word verification, other policies (or lack thereof) are still in effect: all comments are moderated, but anyone can comment, even anonymously. In practice, all real comments get published - even negative or derogatory ones (should I receive them).

Sorry for the convenience, but I hope you'll understand.