On Fri, Jun 14, 2013 at 2:14 PM, Tom Lane <t...@sss.pgh.pa.us> wrote:
> Marko Kreen <mark...@gmail.com> writes: > > On Thu, Jun 13, 2013 at 12:35 PM, Dean Rasheed <dean.a.rash...@gmail.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). > > -- > Sent via pgsql-hackers mailing list (pgsql-hackers@postgresql.org) > To make changes to your subscription: > http://www.postgresql.org/mailpref/pgsql-hackers >
