There are a few reasons why I don't want it supported in MySQL.
- It will make the MySQL server much more complex and bugs accompany complexity.
- MySQL doesn't need it for many workloads. It favors throughput over response time for queries on large data sets. That model works for many customers. The simplicity and scalability of replication further enable throughput.
- There are alternatives for parallel query processing including Greenplum, if they open source all of their code, and Hadoop, if you don't need SQL and you can figure out how to get your data into it from MySQL.
There are more limited forms of support for parallel operations that require fewer changes to MySQL.
- InnoDB already supports parallel IO because it issues prefetch requests during table and index scan and the requests are processed by a background thread.
- Table and index scans on partitioned tables can be done in parallel.
- Filesort can use async IO, real or simulated, to overlap IO with sorting.