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Re: Review: listagg aggregate

From: Pavel Stehule <pavel(dot)stehule(at)gmail(dot)com>
To: Robert Haas <robertmhaas(at)gmail(dot)com>
Cc: Takahiro Itagaki <itagaki(dot)takahiro(at)oss(dot)ntt(dot)co(dot)jp>, "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 14:01:19
Message-ID: (view raw, whole thread or download thread mbox)
Lists: pgsql-hackers
2010/1/28 Robert Haas <robertmhaas(at)gmail(dot)com>:
> 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.

simplest could not be a best. There have to be only a const
expression. But we have not possibility to check it in pg.


> ...Robert

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