Hi @shrutisingh - Welcome to AWS QuickSight community and thanks for posting the question. At present QS is not having UNION or UNION ALL capabilities and I believe this is already in feature request. As a work around solution, you need to transfer one source file ( possibly smaller one) to another system and using custom SQL approach you can do the union from one data source.
Hi @Xclipse - Can you please check whether this feature ( UNION and UNION ALL) is already in feature request or not. if not, please add it as this is one of the important ask as well.
Regards - Sanjeeb