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Re: Review: listagg aggregate

From: Pavel Stehule <pavel(dot)stehule(at)gmail(dot)com>
To: Hitoshi Harada <umi(dot)tanuki(at)gmail(dot)com>
Cc: Robert Haas <robertmhaas(at)gmail(dot)com>, 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 17:17:50
Message-ID: 162867791001280917k55f4637eub1c0f5eb9db06708@mail.gmail.com (view raw or flat)
Thread:
Lists: pgsql-hackers
2010/1/28 Hitoshi Harada <umi(dot)tanuki(at)gmail(dot)com>:
> 2010/1/29 Pavel Stehule <pavel(dot)stehule(at)gmail(dot)com>:
>> 2010/1/28 Robert Haas <robertmhaas(at)gmail(dot)com>:
>>> On Thu, Jan 28, 2010 at 9:01 AM, Pavel Stehule <pavel(dot)stehule(at)gmail(dot)com> wrote:
>>>> simplest could not be a best. There have to be only a const
>>>> expression. But we have not possibility to check it in pg.
>>>
>>> Well... that's an entirely arbitrary limitation.  I admit that it
>>> doesn't seem likely that someone would want to have a variable
>>> delimiter, but putting extra effort and code complexity into
>>> preventing it seems pointless.
>>
>> It is only a few lines with zero complexity.
>>
>> The main issue of Takahiro proposal is  "unclean" behave.
>>
>> we can have a content
>>
>> c1    c2
>> -----------
>> c11, c12,
>> c21, c22
>>
>> and result of string_agg(c1, c2)
>>
>> have to be ?? c11 c12 c21 or c11 c22 c21 ?? What if some content of c2
>> will be NULL ?? I checked oracle. Oracle doesn't allow variable as
>> delimiter. We can't check it. But we can fix first value and using it
>> as constant.
>
> What about get_fn_expr_arg_stable() to check if the argument is stable
> during aggregate?

I newer know so this function exists. Now we can

a) check and allow only stable params
b) when second parameter is stable, then store it and use it as constant.

I prefer a)

Pavel

>
> Regards,
>
>
> --
> Hitoshi Harada
>

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