MySQL Bugs: #28747: ALTER TABLE wipes cardinality statistics

Bug #28747	ALTER TABLE wipes cardinality statistics
Submitted:	29 May 2007 15:18	Modified:	8 Jun 2007 9:10
Reporter:	Domas Mituzas	Email Updates:
Status:	Closed	Impact on me:	None
Category:	MySQL Server: MyISAM storage engine	Severity:	S3 (Non-critical)
Version:	5.0-bk, 5.1-bk	OS:	Any
Assigned to:		CPU Architecture:	Any

Description:
Once ALTER TABLE is run, 'SHOW INDEX' reports NULL cardinalities on MyISAM tables, though after OPTIMIZE they are not wiped.

How to repeat:
create table xt(a int primary key, b int unique, c int unique) engine=MyISAM;
insert into xt values (1,1,1),(2,2,2),(3,3,3),(4,4,4),(5,5,5);
alter table xt engine=MyISAM;
show index from xt;
optimize table xt;
show index from xt;

Suggested fix:
n/a

Thank you for taking the time to write to us, but this is not a bug. Please double-check the documentation available at http://dev.mysql.com/doc/ and the instructions on how to report a bug at http://bugs.mysql.com/how-to-report.php

The manual only says that after ANALYZE TABLE index cardinalities will be calculated. It does not claim that ALTER will preserve them or update them.

Then documentation should be added about ALTER TABLE destroying index cardinality statistics and the need to run ANALYZE TABLES afterwards?

Thank you for your bug report. This issue has been addressed in the documentation. The updated documentation will appear on our website shortly, and will be included in the next release of the relevant products.