Did Microsoft change query optimizer regarding number of files and parallelism Did Microsoft change query optimizer regarding number of files and parallelism? Does the optimizer no longer factor in the number of files to determine the degree of parallelism for a query? If so, does anyone know when the change was made? If not, can anyone provide a link to Microsoft documentation that discusses the topic (current documentation for either SQL Server 2014 or 2016)?
That was an urban legend that Microsoft's Bob Dorr debunked back in 2007:


  This information has been taken out of context and propagated
  incorrectly as SQL Server has a thread per database file so adding
  more files to a database can increase I/O performance.   Adding more
  files and properly aligning them with storage can increase the I/O
  performance for a variety of reasons but achieving new I/O threads per
  file is NOT one of them.


Read the post for much more detail.