MySQL uses PRIMARY key when it should? use another index

[Vasil Dimov]

CREATE TABLE `t1` (
  `a` int(11) NOT NULL DEFAULT '0',
  `b` blob NOT NULL,
  `c` text NOT NULL,
  `d` text NOT NULL,
  PRIMARY KEY (`a`,`b`(255),`c`(255)),
  KEY `b` (`b`(767)),
  KEY `t1ba` (`b`(10),`a`)
) ENGINE=InnoDB;

EXPLAIN SELECT a FROM t1 ORDER BY b;

 +----+-------------+-------+-------+---------------+---------+---------+------+------+-----------------------------+
 | id | select_type | table | type  | possible_keys | key     | key_len | ref  | rows |
Extra                       |
 +----+-------------+-------+-------+---------------+---------+---------+------+------+-----------------------------+
 |  1 | SIMPLE      | t1    | index | NULL          | PRIMARY | 518     | NULL |    1 |
Using index; Using filesort | 
 +----+-------------+-------+-------+---------------+---------+---------+------+------+-----------------------------+

The result is the same even for the command:

EXPLAIN SELECT b,a FROM t1 FORCE INDEX FOR ORDER BY (t1ba) ORDER BY b,a;

Some time ago it would have used the t1ba index. I am trying to find the
changeset that changed this behavior but am experiencing some problems,
here is what I did:

* get a list of the "recent" changesets with "bkf changes" on the latest
bk://mysql.bkbits.net/mysql-5.1 tree (same list can be retrieved via
the BK web interface).

* did a "binary search" in that list by:
  bkf clone -r... bk://mysql.bkbits.net/mysql-5.1 mysql-5.1
  compile
  test
  repeat for another revision.

The outcome is that the tree, cloned with -r1.2604.71.1 has the "old"
behavior (t1ba index is used) while the tree cloned with -r1.2635.3.5
has the "new" behavior. The thing is that both trees differ in 7692
lines. I would expect they to differ only by the change that was
introduced in 1.2635.3.5 (one line fix) because they are adjacent in the
list of changesets:
http://mysql.bkbits.net:8080/mysql-5.1/?DATE=-4w..&PAGE=changes

What am I missing here?

How can I further dig and find the changeset that caused the behavior to
change?

-- 
Vasil Dimov
moc.elcaro@stripped        Software Developer @ Oracle/Innobase Oy
gro.DSBeerF@dv                Committer @ FreeBSD.org
gro.d5v@dv                    Home @ Sofia, Bulgaria

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