@MohammadArmanK Hi,

Indexing helps in improving database performance and helps in reporting. A Declare Index rule is used to define a criteria based on which the Pega Platform automatically maintains index instances for faster access. An index can improve search and reporting access for properties that cannot be exposed as database columns because they are embedded within an aggregate property. Please check the below link to get the details on how to create and use the Declare Index

Declare Index

@MohammadArmanK  we need to create Index class and with in that class we need to create the properties as we have in the source class, 

Then while create we need to specify source page context, source class and index class to write.

and map the properties of source class with Index class properties

Mainly we use declare index to optimize the Aggregate properties, we will have seperate columns created once we configure this with in the db table