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Re: Review: listagg aggregate

From: Robert Haas <robertmhaas(at)gmail(dot)com>
To: Tom Lane <tgl(at)sss(dot)pgh(dot)pa(dot)us>
Cc: Pavel Stehule <pavel(dot)stehule(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 16:01:23
Message-ID: 603c8f071001280801w1ea13203ucabb1b6fe97403fe@mail.gmail.com (view raw or flat)
Thread:
Lists: pgsql-hackers
On Thu, Jan 28, 2010 at 10:56 AM, Tom Lane <tgl(at)sss(dot)pgh(dot)pa(dot)us> wrote:
> Robert Haas <robertmhaas(at)gmail(dot)com> writes:
>> 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.
>
> Yeah.  The real issue here is that in some cases you'd like to have
> non-aggregated parameters to an aggregate, but SQL has no notation
> to express that.

Right.

> I think Pavel's underlying complaint is that if the delimiter
> argument isn't constant, then we're exposing an implementation
> dependency in terms of just which values get separated by which
> delimiters.  The most practical implementation seems to be that
> the first-call delimiter isn't actually used at all, and on
> subsequent calls the delimiter *precedes* the associated value,
> which is a bit surprising given the order in which one writes
> them.  Not sure if this is worth documenting though.  Those two
> or three people who actually try it will figure it out soon enough.

Yeah, I'm thoroughly unworried about it.

...Robert

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