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Hi @Klaster 

I just tested with the SQL Logs activated, the query is a union between 2 SELECT, and not a "JOIN" (it's not an intersection, wich will search the same key into the 2 tables, but a real union, wich will get ALL the lines from all the input tables).

 

INSERT INTO wkfXXXXX_W (iId) SELECT iId FROM wkfXXXXX_Y_Z union SELECT iId FROM wkfXXXXX_X_U

You should test with samples if you're not sure about performance (use the extract activity to get samples of your populations). This is usually a good way to get a performance overview.