Alt index search on partitioned table

[CHap Harrison]

I have a myISAM table with 30 partitions. The primary key is a SHA1 hash value, which is also used in the partition selection function. I also have an alternate key - an autoincrement integer. Since the alternate key does not include the value used in the partition selection function, selecting on the alternate index can't use a process called (I think) "partition pruning". Therefore, a select for an alternate key must involve searches on all 30 partitions. This much makes sense.

I was recently told that such a search is carried out *serially* -- on one partition after another. I find this hard to believe, since it sounds like something that could be done concurrently. Can anyone confirm this behavior?

mySQL 5.6.14