I've inherited some code at work that does some database updates. Basically does a $dbh->do() in the middle of a loop. Thousands of times. For testing, I just redirected the Mail output (w/sendmail) to a text file. 
#old.pl
  while (@emails = $sth->fetchrow) {
    $id       = $emails[0];
    $subject  = $emails[1];
    $body     = $emails[2];
    $mailto   = $emails[3];
    $mailfrom = $emails[4];
    $type     = $emails[5];

    push (@emailsToSend,"To: $mailto\nFrom: $mailfrom\nSubject: $subje
+ct\n\n$body\n");
    push (@emailIds,$id);
    push (@fromAddys,$mailfrom);

    if ($type eq 'weekly') {
      $totalweekly++;
     }
    elsif ($type eq 'daily') {
      $totaldaily++;
    }
    elsif ($type eq 'blast') {
      $totalnewsBlast++;
    }
    elsif ($type eq 'news') {
      $totalnews++;
    }
  }

  $emailcount = 0;
  foreach $singleEmail (@emailsToSend) {
    # SEND THE EMAIL
    $mailfrom = $fromAddys[$emailcount];
        open (OUT, ">>oldEmail.txt");
      print  OUT $singleEmail;
    close(OUT);
    $dbh->do(qq[UPDATE shawnTest set sent = 'Y' WHERE emailId=$emailId
+s[$emailcount]]) ||
&ErrorAlert("500E emailmonitor.pl DBI do error: ".DBI->errstr);
    $emailcount++;
  }

[download]
This seemed like quite a bit of overhead, with the extra variable assignment, as well as the DBI->do, so I did something like this: 
#new.pl
    my %total=map{$_,0}(qw(weekly daily blast custom));    
    my $updateSql=qq[UPDATE shawnTest set sent='Y' WHERE emailId=?];
    my $update=$dbh->prepare($updateSql);


while(@emails = $sth->fetchrow_array())
    {
        open (OUT, ">>newEmail.txt");
        print OUT "To: $emails[3]\nFrom: $emails[4]\nSubject:$emails[1
+]\n\n$emails[2]\n";
        close OUT;
        $update->execute($emails[0]);
        $total{$emails[5]}++;
    }

[download]
In both files, this code lives in a sub called fetchemail. I added some code like this to both files. 
my $t0 = new Benchmark;
fetchmail();
my $t1 = new Benchmark;
my $td = timediff($t1, $t0);
print "the code took:",timestr($td),"\n";

[download]
perl old.pl
the code took: 9 wallclock secs ( 1.43 usr 1.55 sys + 0.02 cusr 0.02 csys = 0.00 CPU)

#Reset database perl new.pl
the code took: 9 wallclock secs ( 1.59 usr + 1.58 sys = 3.17 CPU)

This code did 10,000 updates, so I didn't feel it necessary to Benchmark it over a thousand times or so. But I did expect, in light of the placeholders, for the results to be a bit more different. Is it my understanding of Benchmark (lack thereof), or do I underestimate the power of placeholders?

ÅßÅ×ÅßÅ
"It is a very mixed blessing to be brought back from the dead." -- Kurt Vonnegut

In reply to Unexpected Benchmark results with DBI-placeholders by abaxaba

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