Correlated hash table scan icon

Correlated Hash Table Scan

The Correlated Hash Table Scan icon indicates that the entire temporary hash table will be scanned and all of the entries contained with the hash table will be processed. A correlated hash table scan is generally considered when optimizer is considering a plan that requires the data values to be collated together but the sequence of the data is not required. In addition, the some of the values used to create the correlated hash table can change from one scan to another. The use of a correlated hash table scan will allow the optimizer to generate a plan that can take advantage of any non-join selection while creating the temporary correlated hash table. An additional benefit of using a correlated hash table scan is that the data structure of the temporary correlated hash table will usually cause the table data within the hash table to remain resident within main memory after creation thus reducing paging on the subsequent scan operations.