How should I specify work_mem/max_worker_processes if I want to do big queries now and then?

Hello,

I am doing a query to fetch about 10000000 records in one time. But the query seems very slow, like "mission impossible".

I am very confident that these records should be fit into my shared_buffers settings(20G), and my query is totally on my index, which is this big:(19M x 100 partitions), this index size can also be put into shared_buffers easily.(actually I even made a new partial index which is smaller and delete the bigger old index)

This kind of situation makes me very disappointed.How can I make my queries much faster if my data grows more than 10000000 in one partition? I am using pg11.6.

Many thanks,

James