WL#2709: Speed up ALTER TABLE (part 8: Use multiple threads to rebuild index)

Affects: Server-7.1   —   Status: Un-Assigned   —   Priority: Medium

See WL#2289 for task background.

Overall description:

- Use multiple threads to rebuild one index for faster creation of
  a table with one big index.  This can be done by using parallel
  sorting algorithms on the index and use one thread for reading
  row data and another for creating the index. (MyISAM)