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Re: Review: listagg aggregate

From: Pavel Stehule <pavel(dot)stehule(at)gmail(dot)com>
To: "David E(dot) Wheeler" <david(at)kineticode(dot)com>
Cc: Takahiro Itagaki <itagaki(dot)takahiro(at)oss(dot)ntt(dot)co(dot)jp>, "pgsql-hackers(at)postgresql(dot)org Hackers" <pgsql-hackers(at)postgresql(dot)org>
Subject: Re: Review: listagg aggregate
Date: 2010-01-28 08:40:01
Message-ID: 162867791001280040r4f531ed7v141462b85fc17a33@mail.gmail.com (view raw or flat)
Thread:
Lists: pgsql-hackers
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'.
>>
>> Ooh, nice.
>>
>>> * Can we use StringInfo directly as the aggregate context instead of
>>>  StringAggState? For the first reason, we need to drop 'delimiter' field
>>>  from struct StringAggState. Now it has only StringInfo field.
>>
>> Makes sense.
>
> no, has not.

What is use case for this behave??

Pavel


>
> Pavel
>
>>
>>> * We'd better avoiding to call text_to_cstring() for delimitors and elements
>>>  for performance reason. We can use appendBinaryStringInfo() here.
>>>
>>> My proposal patch attached.
>>>
>>> Also, I've not changed it yet, but it might be considerable:
>>>
>>> * Do we need better names for string_agg1_transfn and string_agg2_transfn?
>>>  They are almost "internal names", but we could have more
>>>  like string_agg_with_sep_transfn.
>>
>> Yes please.
>>
>>> Comments?
>>
>> Patch looks great, thank you!
>>
>> David
>>
>>
>>
>

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