Function run_length 

pub fn run_length(bytes: &[u8]) -> bool

Whether this is a run length chunk, which decode_selected reads without expanding its runs.

It still decodes every run length, and the run values when they are deltas, so it is not pointed. What it skips is writing out every row and picking a few of them afterwards, which for a key kept as runs, such as the order key of TPC-H lineitem, was most of the cost.