You may want to look at some database, e.g. DBD::CSV.

UPDATE: I was thinking of something like this but I do not have a 20GB test file to see how fast it is. 


use strict;
use warnings;
use DBI;
my $dbn = DBI->connect ("dbi:CSV:", "", "", { f_dir => "dbdcsv" } );

$dbn->do ("DROP TABLE data");
$dbn->do ("CREATE TABLE data (key1 CHAR (10), key2 CHAR(10), data CHAR
+(255) )");

my $sth = $dbn->prepare( "INSERT INTO data (key1, key2, data) VALUES (
+?, ?, ?)" );
while(<DATA>){
  chomp;
  $sth->execute( split /\s+/, $_, 3 );
}

$sth = $dbn->prepare ( "SELECT * FROM data ORDER BY key1, key2" );
$sth->execute( );
$"="\t";
while( my $row = $sth->fetchrow_arrayref() ) {
  print "@$row\n";
}

__DATA__
key1  key2  ndnjfgdsjfjjkjjfjf...
key1  key2  kdfkjdfgdfugbjndkfgkjgndkjfjkd
key43 key21 sdkjfhdghdbgbd
key1  key3  jujdejnsduhffnjj
key2  key2  jhzezhdjjf...

[download]

UPDATE 2: I have tested this on some random 20GB test file. I cannot comment on performance but it consumes a LOT of memory...

In reply to Re: sorting type question- space problems by hdb
in thread sorting type question- space problems by baxy77bax

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