data.table - does setkey(…) create an index or physically reorder the rows in a data table?

Question

This (very basic) question is the result of an exchange here.

The documentation for setkey() states:

setkey() sorts a data.table

Answer 1

1. The rows are sorted. "Changed by reference" here means there is no copying of the entire table and rows are just swapped.

2. setkey(DT, NULL) is equivalent to setattr(DT, "sorted", NULL). It simply unsets the "sorted" attribute.