On Mon, 21 Nov 2011 09:55:15 +0000, Mark Leith <mark.leith@stripped> wrote:
> This doesn't necessarily relate to the limitation of partitioned
> tables not having a multi-threaded read of partitions under the
> covers, as at the handler / SQL layer, MySQL is just waiting for the
> storage engine to return rows to it at the current step of JOIN
> execution - under the covers the storage engine could choose to access
> the partitions of a single table concurrently if it wanted to (we just
> don't have that in MySQL at the moment).
NDB does this. Multiple data nodes may be sending data back to the MySQL
server at once.
This is a pretty limited form of parallelism though.
Basically, concurrent query execution is hard and for the vast majority
of queries that MySQL processes, wouldn't see any or much improvement -
avoiding the added complexity (and performance impact for non
parallelized queries) of locking is a pretty big benefit.