Introduction

Recently a customer claimed that his queries were slow some times and sometimes they were fast.

First idea: Flipping query execution plan caused by InnoDB could be skipped because it affected mainly MyISAM tables.
Second idea: Caching effects by either the file system cache caching MyISAM data or the MyISAM key buffer caching MyISAM indexes were examined: File system cache was huge and MyISAM key buffer was only used up to 25%.

I was a bit puzzled...

Following question came up recently: What happens if you drop or add a partition of an existing already partitioned table with ALTER TABLE? Will it be copied or will just the single partition be dropped and added? How are the index(es) rebuild after such a drop/add?

In the MySQL documentation were not to many details mentioned: