Search Part of the Index

Good day,
I have a Index with huge products list.
I want to search only specific part of the Index, is there any way to do it ?
For example, i have client A which uses 100 K products only for them i don't
want to search entire index.
Any help in this regard is appreciated, thanks in advance.

Add a filter on clientA. It will be super efficient

Thanks.. we were able to figure it out using parent-child relationship mapping

Are you sure you absolutely need parent/ child for your use case?