When you create an index, a histogram is generated for the leading column in the index. Examples in earlier chapters have shown how statistics for other columns can increase the accuracy of optimizer statistics. For example, see “Using statistics on multiple search arguments”.
You should consider adding statistics for virtually all columns that are frequently used as search arguments, as long as your maintenance schedule allows time to keep these statistics up to date.
In particular, adding statistics for minor columns of composite indexes can greatly improve cost estimates when those columns are used in search arguments or joins along with the leading index key.