Sort a data.table fast by Ascending/Descending order

Question

I have a data.table with about 3 million rows and 40 columns. I would like to sort this table by descending order within groups like the following sql mock code:

<         


        

Answer 1

The comment was mine, so I'll post the answer. I removed it because I couldn't test whether it was equivalent to what you already had. Glad to hear it's faster.

X <- X[order(Year, MemberID, -Month)]

Summarizing shouldn't depend on the order of your rows.