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From: Tomas Vondra <tomas.vondra@2ndquadrant.com>
To: Tom Lane <tgl@sss.pgh.pa.us>
Cc: David Rowley <david.rowley@2ndquadrant.com>
Cc: Stephen Frost <sfrost@snowman.net>
Cc: Andres Freund <andres@anarazel.de>
Cc: PostgreSQL-development <pgsql-hackers@postgresql.org>
Subject: Re: Parallel Aggregates for string_agg and array_agg
Date: Wed, 28 Mar 2018 16:05:05 +0200
Message-ID: <1ffa699d-ff84-4c21-10cb-4d8b36017bab@2ndquadrant.com> (raw)
In-Reply-To: <18741.1522245278@sss.pgh.pa.us>
References: <CAKJS1f8cB3cc5AK=wPvByhU_jKkgd81iRt2kE6cru8uxx2HCuA@mail.gmail.com>
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	<1b6ee15c-c218-761e-0120-c1ed438e2db2@2ndquadrant.com>
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	<20180326022647.GL24540@tamriel.snowman.net>
	<CAKJS1f-axkksX3XzduLJuTL=o8CSwf1NaQYO6=iHSXR_B3pCKQ@mail.gmail.com>
	<20180326132009.GN24540@tamriel.snowman.net>
	<CAKJS1f8JVpUUkYP2c+RyVRdpS1dnjB_DA5fbcDSL3qfMRqc81Q@mail.gmail.com>
	<6538.1522096067@sss.pgh.pa.us>
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	<452e8b7f-418b-4e61-c212-c0e4410b94ad@2ndquadrant.! com>
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	<18741.1522245278@sss.pgh.pa.us>



On 03/28/2018 03:54 PM, Tom Lane wrote:
> Tomas Vondra <tomas.vondra@2ndquadrant.com> writes:
>> On 03/28/2018 05:28 AM, Tom Lane wrote:
>>> Getting a solution that would work for other polymorphic serialization
>>> functions seems like a bit of a research project to me.  In the meantime,
>>> I think David's right that what we need to look at is the actual input
>>> type of the aggregate, and then assume that what's to be serialized is
>>> an array of that.  Conceivably an aggregate could be built that uses
>>> these serial/deserial functions and yet its input type is something else
>>> than what it constructs an array of ... but I find it a bit hard to
>>> wrap my brain around what that would be exactly.
> 
>> But David's fix doesn't check the aggregate to produce an array of the
>> input type (or anyarray). It could easily be an aggregate computing a
>> bloom filter or something like that, which has no such issues in the
>> serial/deserial functions.
> 
> Oh, if he's not restricting it to these serialization functions, I 
> agree that seems wrong. I thought the discussion was about what to
> do after checking the functions.
> 

Nope, David's patch does not check what the serial/deserial functions
are (and checks aggref->aggargtypes directly, not the exprType thing).

>> Also, if it's checking aggref->aggargtypes, it'll reject anyelement
>> parameters, no?
> 
> I had in mind to look at exprType() of the argument.
> 

Right, I'm fine with that.

regards

-- 
Tomas Vondra                  http://www.2ndQuadrant.com
PostgreSQL Development, 24x7 Support, Remote DBA, Training & Services




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