An alternative approach is Microsoft Log Parser. While it is old in internet time (2005), it works brilliantly on very large files. I've successfully used it on 10+ GigaByte files. The documentation is a little sparse but once you get the hang of it it's pretty simple. Clearly it only works on windows boxen.

Example: #Find all unique from col1 logparser -i:csv -o:csv -stats:off -dtlines:2000 -headers:off "select distinct col1 from input.csv" >out.csv