$ perl -E 'say join",",map{int rand 1000000} 1..20 for 1..1000000' > sample.csv

##</code><code>##

$ time cut -d"," -f 17 sample.csv | sort | uniq > out

real	0m4.391s
user	0m4.788s
sys	0m0.060s
$ time perl -F, -E '$s{$F[16]}++ }{ say for sort keys %s' sample.csv > out

real	0m6.716s
user	0m6.668s
sys	0m0.048s