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Re: Review: listagg aggregate

From: Tom Lane <tgl(at)sss(dot)pgh(dot)pa(dot)us>
To: Pavel Stehule <pavel(dot)stehule(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 17:16:59
Message-ID: 17245.1264699019@sss.pgh.pa.us (view raw or flat)
Thread:
Lists: pgsql-hackers
Pavel Stehule <pavel(dot)stehule(at)gmail(dot)com> writes:
> in 99.99% the second argument will be a constant. Can we use this
> information and optimize function for this case?

> The detoast on every row can take some percent from a performance.

What detoast?  There won't be one for a constant, nor even for a
variable in any sane situation --- who's going to be using
multi-kilobyte delimiter values?  And if they do, aren't they likely
to run out of memory for the result long before the repeated detoasts
become an interesting problem?  You're arguing about a case that
seems quite irrelevant to the real world.

			regards, tom lane

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