As part of your upgrade procedure, you need to run:
update statistics table_name
on all tables as soon as possible following upgrade. Statistics that are copied from the old distribution page by the upgrade are not as accurate as those created by update statistics, because the weights are average, not actual. Running this command updates default statistics on the leading columns of your indexes.
However, be cautious about adding new statistics to the production environment until you have determined your needs and tested any changes. Not all new statistics are needed or helpful in every environment.
WARNING! Making unneeded changes to statistics may adversely
affect performance. In addition, running the
statistics command with incorrect parameters
may change the number of steps in a histogram and impact performance.
Apply changes first in a test environment. See the Performance and Tuning Guide for information.