Description:
Setting myisam_repair_threads (globally or per-session) causes any repair of the table
(repair, alter, create as select) to fail to update the cardinality of the index to always
equal 1.

How to repeat:
Run this script:

>>>>>
drop table if exists at;
create table at (a int);
insert into at (a) values (1), (2), (3), (4), (5);

set session myisam_repair_threads=1;
drop table if exists mt;
create table mt (a int, index(a)) as select a from at;
show index from mt\G

set session myisam_repair_threads=2;
drop table if exists mt;
create table mt (a int, index(a)) as select a from at;
show index from mt\G
<<<<<

Note that the cardinality in the first show index is 5 (correct), in the second it is 1
(very wrong).  This holds true for any table definition and any number of rows.  This
effectively makes all indexes (and thus the table) useless.

Bad english.  I meant: Setting myisam_repair_threads (globally or per-session) causes any
repair of the table (repair, alter, create as select) to fail to update the cardinality of
the
index, instead making it always equal 1.

Thank you for a bug report. Verified just as described with 5.0.21-BK (ChangeSet@1.2143,
2006-04-10 08:29:24+02:00) on Linux:

Welcome to the MySQL monitor.  Commands end with ; or \g.
Your MySQL connection id is 4 to server version: 5.0.21

Type 'help;' or '\h' for help. Type '\c' to clear the buffer.

mysql> drop table if exists at;
Query OK, 0 rows affected, 1 warning (0.00 sec)

mysql> create table at (a int);
Query OK, 0 rows affected (0.01 sec)

mysql> insert into at (a) values (1), (2), (3), (4), (5);
Query OK, 5 rows affected (0.01 sec)
Records: 5  Duplicates: 0  Warnings: 0

mysql> set session myisam_repair_threads=1;
Query OK, 0 rows affected (0.00 sec)

mysql> drop table if exists mt;
Query OK, 0 rows affected, 1 warning (0.01 sec)

mysql> create table mt (a int, index(a)) as select a from at;
Query OK, 5 rows affected (0.01 sec)
Records: 5  Duplicates: 0  Warnings: 0

mysql> show index from mt\G
*************************** 1. row ***************************
       Table: mt
  Non_unique: 1
    Key_name: a
Seq_in_index: 1
 Column_name: a
   Collation: A
 Cardinality: 5
    Sub_part: NULL
      Packed: NULL
        Null: YES
  Index_type: BTREE
     Comment:
1 row in set (0.00 sec)

mysql> set session myisam_repair_threads=2;
Query OK, 0 rows affected (0.00 sec)

mysql> drop table if exists mt;
Query OK, 0 rows affected (0.01 sec)

mysql> create table mt (a int, index(a)) as select a from at;
Query OK, 5 rows affected (0.01 sec)
Records: 5  Duplicates: 0  Warnings: 0

mysql> show index from mt\G
*************************** 1. row ***************************
       Table: mt
  Non_unique: 1
    Key_name: a
Seq_in_index: 1
 Column_name: a
   Collation: A
 Cardinality: 1
    Sub_part: NULL
      Packed: NULL
        Null: YES
  Index_type: BTREE
     Comment:
1 row in set (0.00 sec)

It is in the same area as bug#11527.

Are you sure?  Looks different to me.  11527 requires >4GB of data, this one does not. 
Valeriy claims to have verified it on 5.0.21-BK, which should have the fix for 11527
(since almost two months ago).

Can you double check before calling this a dupe?

Jeremy - I did not make it a dup. It is still open.

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/10267

ChangeSet@1.2533, 2006-08-10 18:44:41+05:00, svoj@may.pils.ru +3 -0
  BUG#18874 - Setting myisam_repair_threads > 1, index cardinality always 1
  
  When myisam_repair_threads value to > 1, index repair operation always
  sets index cardinality to 1.
  
  Fixed wrong index cardinality calculation for index repair operation
  when myisam_repair_threads is > 1.

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/10357

ChangeSet@1.2533, 2006-08-14 17:05:02+05:00, svoj@may.pils.ru +3 -0
  BUG#18874 - Setting myisam_repair_threads > 1, index cardinality always 1
  Fixed by moving update_key_parts() down to be after write_index().

Pushed into 4.1.22, 5.0.25.

Pushed into 5.1.12.

Thank you for your bug report. This issue has been committed to our source repository of
that product and will be incorporated into the next release.

If necessary, you can access the source repository and build the latest available version,
including the bug fix. More information about accessing the source trees is available at

    http://dev.mysql.com/doc/en/installing-source.html

Documented bugfix in 4.1.22, 5.0.25, and 5.1.12 changelogs.