Result:
  System sort: 59 minutes.
  Postgres load (6m) + select (59m): 66 minutes 

##</code><code>##

$ ls -lh dna2.txt 
-rw-rw-r-- 1 xxxxx xxxxx 12G Dec 21 16:38 dna2.txt

1588    mm_ref_chrY.fa  2902494 R       CTTGACTTTTATGTGTCACTGTGTTCAGTTCCTGGT
939     mm_ref_chrY.fa  2902495 F       CCAGGAACTGACCACAGTGACACATAAGAGTCAAAT
742     mm_ref_chrY.fa  2902497 F       AGGAACTGACCACAGTGACACATAAGAGTCAAATAG
1097    mm_ref_chrY.fa  2902497 F       AGGAACTGACCACAGTGACACATAAGAGTCAAATAG
100     mm_ref_chrY.fa  2902499 F       GAACTGACCACAGTGACACATAAAAGTCAAGTAGTG
1184    mm_ref_chrY.fa  2902499 F       GAACTGACCACAGTGACACATAAAAGTCAAGTAGTG
286     mm_ref_chrY.fa  2902505 F       ACCACAGTGCCACATAAAAGTCAAGTAGGGAATCCT
235     mm_ref_chrY.fa  2902513 R       ACCAGGACAGGATCCCCTACTTGACTTTTATGTGGC
1744    mm_ref_chrY.fa  2902516 F       ACATAAAAGTCAAGTAGTGGACCCTGTCCTGGTCTG
1029    mm_ref_chrY.fa  2902519 F       TAAAAGTCAAGTAGGGGATCCTGTCCTGGTCTGGCA

#
#  bash + sort version:
#

$ time sort -k5 dna2.txt > dna2.sortk5.out
real    59m48.641s

#
#  Postgres version:
#

#
# loading the data:
#

$ time < dna2.txt \
  psql -d test -c "
    drop table if exists dna_test;
    create table dna_test(
      y integer, 
      chromosome text,
      genomic_location integer,
      direction text,
      seq text);
     copy dna_test from stdin csv delimiter E'\t';
  " ;
real    6m20.430s

echo "
select to_char(count(*), '999G999G999') as rowcount
from dna_test" | psql -d test
   rowcount
--------------
  181,261,572
(1 row)

#
#  querying a resultset into a sorted file:
#
$ time echo " copy (select * from dna_test order by seq) to stdout" \
  | psql -1qtAd test > dna_test.order_by_seq.out
real    59m12.569s



##</code><code>##

$ time echo "
       create index dna_test_seq_idx on dna_test (seq)
" | psql -d test
CREATE INDEX
real    63m20.451s

##</code><code>##


$ time echo "
   explain analyze select * from dna_test order by seq
" | psql -1qtAd test > dna_test.order_by_seq.explain_analyze.txt

real    57m44.054s

$ cat dna_test.order_by_seq.explain_analyze.txt
Sort  (cost=35550611.84..36003765.76 rows=181261568 width=62) (actual time=1834228.291..3428773.879 rows=181261572 loops=1)
  Sort Key: seq
  Sort Method:  external merge  Disk: 12933032kB
  ->  Seq Scan on dna_test  (cost=0.00..3872406.68 rows=181261568 width=62) (actual time=17.966..65802.621 rows=181261572 loops=1)
Total runtime: 3463580.243 ms