Bug #30234 Unexpected behavior using DELETE with AS and USING
Submitted: 3 Aug 2007 21:39 Modified: 18 Apr 20:56
Reporter: Dan Rue
Status: Closed
Category:Server: DML Severity:S3 (Non-critical)
Version:5.0.41/5.1/4.1 OS:Any
Assigned to: Davi Arnaut Target Version:
Tags: USING, as, delete
Triage: D1 (Critical)

[3 Aug 2007 21:39] Dan Rue 
Description:
When executing the following query: 

DELETE FROM foo AS b 
USING foo AS f, bar AS b 
WHERE f.id=b.id;

The most unexpected behavior happens.  It actually will delete the matching records from
the bar table!  This is completely unexpected, and extremely dangerous.

How to repeat:
DROP TABLE IF EXISTS foo;
DROP TABLE IF EXISTS bar;

CREATE TABLE foo (
|       id INT
);
INSERT INTO foo (id) VALUES (1);
INSERT INTO foo (id) VALUES (2);
INSERT INTO foo (id) VALUES (3);
INSERT INTO foo (id) VALUES (4);
INSERT INTO foo (id) VALUES (5);

CREATE TABLE bar (
|       id INT
);
INSERT INTO bar (id) VALUES (4);
INSERT INTO bar (id) VALUES (5);
INSERT INTO bar (id) VALUES (6);
INSERT INTO bar (id) VALUES (7);
INSERT INTO bar (id) VALUES (8);

-- Show the overlapping rows
SELECT foo.* FROM foo, bar WHERE foo.id=bar.id;

-- What do you think will happen?  
DELETE FROM foo AS b USING foo AS f, bar AS b WHERE f.id=b.id;

-- User intended to delete records 4 and 5 from foo, but nothing was deleted.
SELECT * FROM foo;

-- Instead, 4 and 5 were deleted from bar.  How astonishing!
SELECT * FROM bar;

Suggested fix:
Simple: it should be a syntax error to cross AS references.
[3 Aug 2007 22:42] Miguel Solorzano 
Thank you for the bug report.
[15 Aug 2007 14:41] Bugs System 
A patch for this bug has been committed. After review, it may
be pushed to the relevant source trees for release in the next
version. You can access the patch from:

  http://lists.mysql.com/commits/32557

ChangeSet@1.2502, 2007-08-15 14:41:19+02:00, mhansson@linux-st28.site +4 -0
  Bug #30234: Unexpected behavior using DELETE with AS and USING
  
  DELETE statements with the following type of non-unique aliases
  gave unexpected results:
  DELETE FROM t1 AS alias USING t1, t2 AS alias WHERE t1.a = alias.a;
  This query would leave table t1 intact but delete rows from t2.
  Fixed by prohibiting non-unique aliases.
[24 Aug 2007 15:39] Bugs System 
A patch for this bug has been committed. After review, it may
be pushed to the relevant source trees for release in the next
version. You can access the patch from:

  http://lists.mysql.com/commits/33024

ChangeSet@1.2502, 2007-08-24 15:40:16+02:00, mhansson@linux-st28.site +4 -0
  Bug #30234: Unexpected behavior using DELETE with AS and USING
  
  DELETE FROM ... USING ... statements with the following type of 
  ambiguous aliasing gave unexpected results:
  DELETE FROM t1 AS alias USING t1, t2 AS alias WHERE t1.a = alias.a;
  This query would leave table t1 intact but delete rows from t2.
  Fixed by changing DELETE FROM ... USING syntax so that only alias 
  references (as opposed to alias declarations) may be used in FROM.
[30 Aug 2007 9:04] Martin Hansson 
Trudy says the following about which version to push this bug to:

I've looked at bug#30234. It's tagged for 5.2 so if
you feel the patch is too big/risky for 5.1, then
please push only to 5.2. It would be great if this
can be fixed in 5.1, of course, but there are
workarounds for the functionality.

Note: If you decide to fix only in 5.2, please ask
Docs team to provide some warning in the Reference
Manual that deleting from multiple tables in this
way doesn't work. Thanks!
[31 Aug 2007 3:00] Marc Alff 
Patch approved by email
[14 Sep 2007 9:45] Bugs System 
Pushed into 5.1.23-beta
[14 Sep 2007 9:46] Bugs System 
Pushed into 5.0.50
[9 Oct 2007 2:52] Paul DuBois 
Noted in 5.0.50, 5.1.23 changelogs.

Multiple-table DELETE statements could delete rows from the wrong
table.
[30 Nov 2007 10:11] Bugs System 
A patch for this bug has been committed. After review, it may
be pushed to the relevant source trees for release in the next
version. You can access the patch from:

  http://lists.mysql.com/commits/38935

ChangeSet@1.2589, 2007-11-30 10:11:46+01:00, mhansson@linux-st28.site +3 -0
  Bug#30234: Unexpected behavior using DELETE with AS and USING
  
  Anti-patch. This patch undoes the previously pushed patch.
[30 Nov 2007 10:14] Martin Hansson 
There, an 'undo' patch is committed. I will check in with  the reviewers if this should be
pushed to 5.0.
[5 Dec 2007 20:13] Davi Arnaut 
In order to properly implement Bug#27525 it's needed that a DELETE statement must have no
ambiguous alias. This bug only fixed it for one of the forms of a DELETE statement -- the 
DELETE FROM <list> USING <list> WHERE ..., and the DELETE <list> FROM <list> WHERE ...
form was left unfixed, so this bug fix is incomplete.

So I'm taking over this bug to fix both DELETE forms and to address the ambiguous cases,
as it is a necessary step for Bug#27525.
[14 Dec 2007 9:14] Bugs System 
Pushed into 5.0.54
[14 Dec 2007 9:17] Bugs System 
Pushed into 5.1.23-rc
[14 Dec 2007 9:20] Bugs System 
Pushed into 6.0.5-alpha
[19 Dec 2007 20:32] Davi Arnaut 
Patch committed in Bug#27525
[28 Feb 0:14] Davi Arnaut 
Queued in 6.0-runtime

  http://lists.mysql.com/commits/43122

ChangeSet@1.2575, 2008-02-27 19:59:48-03:00, davi@mysql.com +9 -0
  Bug#27525 table not found when using multi-table-deletes with aliases over several
  Bug#30234 Unexpected behavior using DELETE with AS and USING
[3 Mar 19:19] Bugs System 
Pushed into 6.0.5-alpha
[18 Apr 20:56] Paul DuBois 
There are multiple changelog entries because this bugfix has a more complicated history
than most:

- Original push was into 5.0.50 and 5.1.23.
- The 5.0.50 push was reverted in 5.0.54, leaving the 5.0 server unchanged relative to
pre-5.0.50.
- The 5.1.23 push is not reverted (the second push into 5.1.23 and 6.0.5 was a null
merge)

Changelog entry for 5.0.50:

Multiple-table DELETE statements containing ambiguous aliases could
have unintended side effects such as deleting rows from the wrong 
table. Example:  

DELETE FROM t1 AS a2 USING t1 AS a1 INNER JOIN t2 AS a2;

This bugfix enables alias declarations to be declared only in the
table_references part. Elsewhere in the statement, alias references
are allowed but not alias declarations. However, this patch was
reverted in MySQL 5.0.54 because it changed the behavior of a General
Availability MySQL release.

Changelog entry for 5.0.54:

The MySQL 5.0.50 patch for this bug was reverted because it changed
the behavior of a General Availability MySQL release.

Changelog entry for 5.1.23:

Multiple-table DELETE statements containing ambiguous aliases could
have unintended side effects such as deleting rows from the wrong
table. Example:

DELETE FROM t1 AS a2 USING t1 AS a1 INNER JOIN t2 AS a2;

Now alias declarations can be declared only in the table_references
part. Elsewhere in the statement, alias references are allowed but
not alias declarations. Statements containing aliases that are no
longer allowed must be rewritten.
[18 Apr 20:57] Paul DuBois 
The work begun for this bug report was extended in 6.0 by Bug#27525, which see.