my $dbh = DBI->connect('...', { RaiseError => 0 })
    or die "Unable to connect to datasource: $DBI::errstr\n";
my $sth = $dbh->prepare('SELECT PK, Name, Memo...');
    or die "Unable to prepare query statement: $DBI::errstr\n";
my $rv = $sth->execute()
    or die "Unable to execute query statement: $DBI::errstr\n";

my $data = $sth->fetchall_arrayref;
my %hash = %{ &checksums($data) };

# some time later...

foreach(@$data) {
    print "PK:\t", $_->[0], "\t";
    print "Name:\t", $_->[1], "\t";
    print "Digest:\t", $hash{$_->[0]}, "\n";
}

sub checksums {
    my $recordset = shift;
    my %checksums;
    foreach(@$recordset) {
        $checksums{$_->[0]} = &digest($_->[2]);
    }
    return \%checksums;
}
[download]

...untested, of course. That may not even work, it depends on exactly what fetchall_arrayref does.

Otherwise, maybe you can alias or copy the statement handle, although I'm not sure how that would work. If you're feeling ambitious, you could write a wrapper for DBI that caches the statement handle in memory and allows you to manipulate the cursor. Actually, there may already be a module to do that. Check CPAN. =) If you're feeling lazy, maybe you can just run the same query twice and generate two equivalent recordsets. Eew!

When I hear about problems like this I can't help but imagine there's a better way to approach the issue. If you can rewrite your code to avoid having to loop through the recordset twice, that might be the best solution. (see below for a rewrite of the above code)

Good luck,

'kaboo

(this node has been updated)

my @data;
while($sth->fetch) {
    push @data, {
        PK      => $_->[0],
        Name    => $_->[1],
        Memo    => $_->[2],
        Digest  => &digest($_->[2])
    };
}

# some time later...

foreach(@data) {
    print "PK:\t", $_->{PK}, "\t";
    print "Name:\t", $_->{Name}, "\t";
    print "Digest:\t", $_->{Digest}, "\n";
}
[download]