Function count_rows_from_slice_par 

pub fn count_rows_from_slice_par(
    buffer: Buffer<u8>,
    quote_char: Option<u8>,
    comment_prefix: Option<&CommentPrefix>,
    eol_char: u8,
    has_header: bool,
    skip_lines: usize,
    skip_rows_before_header: usize,
    skip_rows_after_header: usize,
    raise_if_empty: bool,
) -> PolarsResult<usize>

Available on crate features csv or json only.

Read the number of rows without parsing columns. Useful for count(*) queries. Supports transparent decompression.