| From: | Tom Lane <tgl(at)sss(dot)pgh(dot)pa(dot)us> |
|---|---|
| To: | Marko Kreen <markokr(at)gmail(dot)com> |
| Cc: | Dean Rasheed <dean(dot)a(dot)rasheed(at)gmail(dot)com>, PostgreSQL Hackers <pgsql-hackers(at)postgresql(dot)org> |
| Subject: | Re: MD5 aggregate |
| Date: | 2013-06-14 13:14:32 |
| Message-ID: | 8110.1371215672@sss.pgh.pa.us |
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| Lists: | pgsql-hackers |
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
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