fetchrow_hashref is the slowest of the fetch methods. If you have a large number of rows, you can improve efficiency by using fetchrow_arrayref, or, even better, by using bind_columns.

  my $sth = $dbh->prepare( $sql );
  $sth->execute;
  my ($pointer);
  my $rv = $sth->bind_columns(\$pointer);
  while ($sth->fetch) {
      # do something with $pointer
  }
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If you have under a few thousand fetches, it's unlikely this would gain you much time, but for larger fetches, it's the fastest.