"SQL Server allows statistical information regarding the distribution of values in a column to be created. This statistical information can be used by the query processor to determine the optimal strategy for evaluating a query. When you create an index, SQL Server automatically stores statistical information regarding the distribution of values in the indexed column(s).

As the data in a column changes, index and column statistics can become out-of-date and cause the query optimizer to make less-than-optimal decisions on how to process a query. For example, if you create a table with an indexed column and 1,000 rows of data, all with unique values in the indexed column, the query optimizer considers the indexed column a good way to collect the data for a query. If you update the data in the column so there are many duplicated values, the column is no longer an ideal candidate for use in a query. However, the query optimizer still considers it to be a good candidate based on the index's outdated distribution statistics, which are based on the data before the update."

UPDATE STATISTICS
sp_updatestats