Bug #9509 Optimizer: wrong result after AND with latin1_german2_ci comparisons

Description:
When I AND a TRUE expression with a TRUE expression, the result should be TRUE.
But when one of the expressions is "LENGTH(opd)=1" and the other expression is
"opd=two-character-literal", MySQL decides it's an impossible WHERE clause.
(I can see this with EXPLAIN.) So the result is wrong when I use latin1_german2_ci.

How to repeat:
mysql> create table ti (s1 char(5) character set latin1 collate latin1_german2_ci);
Query OK, 0 rows affected (0.01 sec)

mysql> insert into ti values (0xf6) /* this is o-umlaut */;
Query OK, 1 row affected (0.00 sec)

mysql> select * from ti where length(s1)=1;
+------+
| s1   |
+------+
| ö   |
+------+
1 row in set (0.00 sec)

mysql> select * from ti where s1='oe';
+------+
| s1   |
+------+
| ö   |
+------+
1 row in set (0.00 sec)

mysql> select * from ti where length(s1)=1 and s1='oe';
Empty set (0.00 sec)

I get wrong results with Lithuanian and Czech too.
I do not claim it's always the same bug, but I hope
that the fixer of bug#9509 will check these too.

mysql> create table tli (s1 char(5) character set
utf8 collate utf8_lithuanian_ci);
Query OK, 0 rows affected (0.01 sec)

mysql> insert into tli values ('I'),('K'),('Y');
Query OK, 3 rows affected (0.00 sec)
Records: 3  Duplicates: 0  Warnings: 0

/* although 'Y' < 'K' in Lithuanian, I get nothing when
I look for a 'Y' */

mysql> select * from tli where s1 < 'K' and s1 = 'Y';
Empty set (0.01 sec)

mysql> create table tcz (s1 char(5) character set utf8 collate
utf8_czech_ci);
Query OK, 0 rows affected (0.00 sec)

mysql> insert into tcz values
('c'),('d'),('h'),('ch'),('CH'),('cH'),('Ch'),('i');
Query OK, 8 rows affected (0.00 sec)
Records: 8  Duplicates: 0  Warnings: 0

/* although 'CH' > 'd', and 'CH' = 'CH',
I get nothing when I use both conditions, ANDed. */

mysql> select * from tcz where s1 > 'd' and s1 = 'CH';
Empty set (0.00 sec)

/* although 'cH' <> 'ch', and s1 = 'cH',
I get nothing when I use both conditions, ANDed. */

mysql> select * from tcz where s1 = 'cH' and s1 <> 'ch';
Empty set (0.00 sec)

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/5199

Pushed into 4.1.22

TODO: merge to 5.0 and 5.1

Merged to 5.0 (will be in 5.0.25).

TODO: merge to 5.1

This isn't a showstopper, so I've set the status to patch pending until the fix is merged to 5.1.

Pushed to 5.1.12.

Noted in 4.1.22, 5.0.25, 5.1.12 changelogs.

The optimizer could produce an incorrect result after AND with
collations such as latin1_german2_ci, utf8_czech_ci, and
utf8_lithianian_ci.