# Defining a blocking key

Available in...

* Big Data Platform
* Cloud API Services Platform
* Cloud Big Data Platform
* Cloud Data Fabric
* Cloud Data Management Platform
* Data Fabric
* Data Management Platform
* Data Services Platform
* Qlik Talend Cloud Enterprise Edition
* Qlik Talend Cloud Premium Edition
* Real-Time Big Data Platform

## About this task

Defining a blocking key is not mandatory but strongly advisable. Using a blocking key to partition data in blocks reduces the number of records that need to be examined as comparisons are restricted to record pairs within each block. Using blocking columns is very useful when you are processing a big dataset.

## Procedure

1. From the Blocking key menu, click ![Add new item](https://help.qlik.com/talend/en-US/studio-user-guide/Resources/images/add.svg) in the Blocking algorithm section.
2. Select an algorithm for the blocking key, and set the other parameters in the Blocking Key table as needed.

   In this example, only one blocking key is used. The first character of each word in the country column is retrieved and listed in the BLOCK\_KEY column.
3. Click the name of the columns you want to use to partition the processed data in blocks.

   ![Examples of blocking keys and their parameters in the Blocking key menu.](https://help.qlik.com/talend/en-US/studio-user-guide/Resources/images/match_analysis3.png)

   You can define more than one column in the table, but only one blocking key will be generated and listed in the BLOCK\_KEY column in the Data table.

   For example, if you use an algorithm on the country and lnamecolumns to process records that have the same first character, data records that have the same first letter in the country and last names are grouped together in the same block. Comparison is restricted to record within each block.

   To remove a column from the Blocking key table, right-click it and select Delete or click on its name in the Data table.
4. Click Chart to compute the generated key, and display the results in a chart.

   This chart allows you to visualize the statistics regarding the number of blocks and to adapt the blocking parameters according to the results you want to get.