Have you used Semi Join in PostgreSQL and in what cases is it applicable?

Semi Join in PostgreSQL is a way to optimize queries when you need to check for the existence of related records in another table, but do not need to retrieve the related data itself.

For example, if you want to get all records from table A for which related records exist in table B, you can use a construct with EXISTS or the IN operator, which PostgreSQL optimizes as a Semi Join.

Example:

SELECT * FROM A WHERE EXISTS (SELECT 1 FROM B WHERE B.a_id = A.id);

Semi Join is effective when it is important to check for the existence of relations without the need to join and return data from both tables, which reduces load and speeds up query execution.