@Alena Melnikova Good to hear that you are happy with the results:)
1. You can go as low as 1k. Choose a balanced option on the average number of rows you query.
2. The usage of function to_date I believe will cause the orc index to stop working (Haven't tested that). Google "why function based index?"
I have a somewhat similar question, which I posted here:
I would really appreciate any ideas.