Horizontal Partition. How affect other indexes?

[Yoo]

Err so no one responded to my post, so I suppose here's a more general Database question:

Let's say there is a table with two indexes. I horizontally partition by one of the indexes. How does this affect the other one? I need to utilize BOTH indexes and there is no way I'm able to vertically partition. Horizontal partition is basically a BRIN index, right? Gah, please help xD Dying... But thank you in advance!

This was my original question on here and on Reddit: https://www.reddit.com/r/django/comments/ce7olj/question_votes_table_with_partitions_brin_btree/