Although this seems like the eternal question, as there are many variations on this problem posted on PM, some of which include Benchmarks, I've just found something a little curious when it comes to benchmarking two approaches to this.

I'm converting a 2xn array reference, which is the result of a DBI call (selectall_arrayref), into a hashref. At first, I thought the easy way might be to do something like: 
sub AssembleByHash
{
        my ($array_ref) = @_;
        my (%result);

        foreach my $row (@$array_ref)
        {
                $result{$row->[0]} = $row->[1];
        }

        return \%result;
}

[download]
Which I thought would be far to easy, since it didn't harness the Power of Map. So, instead, I supposed that this might be faster: 
sub AssembleByArray
{
        my ($array_ref) = @_;
        my (%result);

        %result = map { @$_ } @$array_ref;

        return \%result;
}

[download]
After all, it's quick and to the point. When applying Benchmark to validate my theory, though, I got the following result using a 10_000 row test-table and 100 iterations: 
Benchmark: timing 10 iterations of AssembleByArray, AssembleByHash...
AssembleByArray: 10 wallclock secs ( 9.64 usr +  0.04 sys =  9.68 CPU)
+ @  1.03/s (n=10)
AssembleByHash:  1 wallclock secs ( 1.31 usr +  0.00 sys =  1.31 CPU) 
+@  7.63/s (n=10)
                  Rate AssembleByArray  AssembleByHash
AssembleByArray 1.03/s              --            -86%
AssembleByHash  7.63/s            639%              --

[download]
So, I can only suppose that the temporary array created by the map call is killing performance. Or, am I missing something?

In reply to Multi-Dimensional Array to Hash Conversion Optimization by tadman

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