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Re: Review: listagg aggregate

From: Robert Haas <robertmhaas(at)gmail(dot)com>
To: Takahiro Itagaki <itagaki(dot)takahiro(at)oss(dot)ntt(dot)co(dot)jp>
Cc: Pavel Stehule <pavel(dot)stehule(at)gmail(dot)com>, "David E(dot) Wheeler" <david(at)kineticode(dot)com>, "pgsql-hackers(at)postgresql(dot)org Hackers" <pgsql-hackers(at)postgresql(dot)org>
Subject: Re: Review: listagg aggregate
Date: 2010-01-28 13:53:26
Message-ID: 603c8f071001280553w30eb9daeua8e1b8aca7704d8a@mail.gmail.com (view raw or flat)
Thread:
Lists: pgsql-hackers
On Thu, Jan 28, 2010 at 3:59 AM, Takahiro Itagaki
<itagaki(dot)takahiro(at)oss(dot)ntt(dot)co(dot)jp> wrote:
> Pavel Stehule <pavel(dot)stehule(at)gmail(dot)com> wrote:
>
>> 2010/1/28 Pavel Stehule <pavel(dot)stehule(at)gmail(dot)com>:
>> > 2010/1/28 David E. Wheeler <david(at)kineticode(dot)com>:
>> >> On Jan 27, 2010, at 6:47 PM, Takahiro Itagaki wrote:
>> >>
>> >>> * I think we cannot cache the delimiter at the first call.
>> >>>   For example,
>> >>>     SELECT string_agg(elem, delim)
>> >>>       FROM (VALUES('A', ','), ('B', '+'), ('C', '*')) t(elem, delim);
>> >>>   should return 'A+B*C' rather than 'A,B,C'.
>> >
>> > no, has not.
>> What is use case for this behave??
>
> I also think this usage is nonsense, but seems to be the most consistent
> behavior for me. I didn't say anything about use-cases, but just capability.
> Since we allow such kinds of usage for now, you need to verify the
> delimiter is not changed rather than ignoring it if you want disallow
> to change the delimiter during an aggregation.
>
> Of course you can cache the first delimiter at start, and check delimiters
> are not changed every calls -- but I think it is just a waste of cpu cycle.

Agreed.  Not caching it seems the simplest solution.

...Robert

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