Does myisampack affect the query cache?

[Philip Smolen]

I can't find this documented anywhere, but the results are consistent. As soon as I run myisampack on a table, the query cache doesn't want to work on the table any more. Is this right? Or am I missing something?

I have a MyISAM table with about 6 million rows. I type
select count(*) from xxx where description = "xxx"
It takes over 30 seconds the first time I run the query. The second time takes 0 seconds, as expected. I run myisampack on the table, and *every* time I run the query it takes over 30 seconds. The query cache is not working on this table. Running myisamchk -rq doesn't change anything, the query cache still ignores this table. Using myisamchck to unpack the table does not change anything; the query cache still ignores this table.

If I recreate the table using "create table like ... insert into ... select * from ...", then the query cache works on the new copy of the table.

I can't see any obvious difference between the unpacked table (which doesn't cache) and the new copy of the table (which works as expected). The frm files are identical. The length of the MYD files are identical. "myisamck -dvi" and "show table status" give identical results for both tables.

All this time the query cache continues to work on my other tables.

mysql --version
mysql Ver 14.12 Distrib 5.0.51a, for redhat-linux-gnu (x86_64) using readline 5.0

I'd appreciate any advice on this subject.