title: lncreasing Threshold for Parallelism from 5 to 20 results in horrible query performance
body: <p>After watching a Brent Ozar video on query tuning, and also seeing agreement among index tuning blogs and articles, I increased Cost of Threshold for Parallelism on a SQL Server instance from 5 (Default) to 20.  Consequently, queries began to fail with a timeout error.  </p>
  
  <p>So, I started to investigate one of the queries.  And, I can see that when the cost threshold is 5, the query takes 11 seconds to run, but when the cost threshold is increased to 20, the execution plan changes and the same query takes over 8 minutes to run.  </p>
  
  <p>One of the differences is that, in the long running execution plan (cost threshold 20), the arrow after a Table Spool gets really fat, and when I hover over it, it can see that the number of rows read is 1,686,216,987.  Whereas, in the short running execution plan (cost threshold 5), the arrow after the same table spool is skinnier, and the numbers of rows read is 27769.</p>
  
  <p>In the long running execution plan, I do not see any parallelization.</p>
  
  <p>Any ideas how increasing the cost threshold might blow the logical reads through the roof?</p>