| From: | Tomas Vondra <tomas(dot)vondra(at)2ndquadrant(dot)com> |
|---|---|
| To: | Andres Freund <andres(at)anarazel(dot)de> |
| Cc: | David Fetter <david(at)fetter(dot)org>, Tom Lane <tgl(at)sss(dot)pgh(dot)pa(dot)us>, David Rowley <david(dot)rowley(at)2ndquadrant(dot)com>, Heikki Linnakangas <hlinnaka(at)iki(dot)fi>, pgsql-hackers <pgsql-hackers(at)postgresql(dot)org>, Jeff Davis <pgsql(at)j-davis(dot)com> |
| Subject: | Re: Spilling hashed SetOps and aggregates to disk |
| Date: | 2018-06-06 14:06:18 |
| Message-ID: | 83b9a11d-bb27-3c33-ebeb-f332b5de31c7@2ndquadrant.com |
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| Lists: | pgsql-hackers |
On 06/06/2018 04:01 PM, Andres Freund wrote:
> Hi,
>
> On 2018-06-06 15:58:16 +0200, Tomas Vondra wrote:
>> The other issue is that serialize/deserialize is only a part of a problem -
>> you also need to know how to do "combine", and not all aggregates can do
>> that ... (certainly not in universal way).
>
> There are several schemes where only serialize/deserialize are needed,
> no? There are a number of fairly sensible schemes where there won't be
> multiple transition values for the same group, no?
>
Possibly, not sure what schemes you have in mind exactly ...
But if you know there's only a single transition value, why would you
need serialize/deserialize at all. Why couldn't you just finalize the
value and serialize that?
regards
--
Tomas Vondra http://www.2ndQuadrant.com
PostgreSQL Development, 24x7 Support, Remote DBA, Training & Services
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