In the previous two chapters, you learned about indexes and index statistics. The purpose of indexes is to reduce the reads required to access the row required for the query and for index statistics to help the optimizer determine the optimal query plan. That is all great, but indexes are not for free and there are cases where indexes are not very effective and do not warrant the overhead, but you still need the optimizer to be aware of the data distribution. That is where histograms can be useful.
This chapter starts out discussing what histograms are and for which workloads ...
