Re: MD5 aggregate

On Fri, Jun 14, 2013 at 2:14 PM, Tom Lane <tgl(at)sss(dot)pgh(dot)pa(dot)us> wrote:

> Marko Kreen <markokr(at)gmail(dot)com> writes:
> > On Thu, Jun 13, 2013 at 12:35 PM, Dean Rasheed <dean(dot)a(dot)rasheed(at)gmail(dot)com>
> wrote:
> >> Attached is a patch implementing a new aggregate function md5_agg() to
> >> compute the aggregate MD5 sum across a number of rows.
>
> > It's more efficient to calculate per-row md5, and then sum() them.
> > This avoids the need for ORDER BY.
>
> Good point. The aggregate md5 function also fails to distinguish the
> case where we have 'xyzzy' followed by 'xyz' in two adjacent rows
> from the case where they contain 'xyz' followed by 'zyxyz'.
>
> Now, as against that, you lose any sensitivity to the ordering of the
> values.
>
> Personally I'd be a bit inclined to xor the per-row md5's rather than
> sum them, but that's a small matter.
>
> regards, tom lane
>
>
xor works but only if each row is different (e.g. at the very least all
columns together make a unique key).

>
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