Note for bounty - START:
赏金注意事项 - START:
PARAMETERS SNIFFING (that is the only "idea" that was reported in pre-bounty questions) is not the issue here, as you can read in the "update" section at the end of the question. The problem is really related to how sql server creates execution plans for a parametrized query when distinct is used. I uploaded a very simple database backup (it works with sql server 2008 R2) PA
