Developers often need to merge data from external sources to the base table. The expectation of this merge operation is that data from external sources refresh data at the main table. Refresh means inserting new records, updating existing records, and deleting if a record is not found. 

Since the 9.4 version release, PostgreSQL has supported the INSERT command with the ‘ON CONFLICT’ clause. While this proved to be a workaround for the MERGE command, many features, such as conditional delete and simplicity of query, were missing. Other databases, such as Oracle and SQL Server, already support MERGE commands. Until PostgreSQL 15, it was still one of the long-awaited features for PostgreSQL users.