Re: Why overhead of SPI is so large?

From: Konstantin Knizhnik <k(dot)knizhnik(at)postgrespro(dot)ru>
To: Pavel Stehule <pavel(dot)stehule(at)gmail(dot)com>
Cc: "Tsunakawa, Takayuki" <tsunakawa(dot)takay(at)jp(dot)fujitsu(dot)com>, PostgreSQL Hackers <pgsql-hackers(at)postgresql(dot)org>
Subject: Re: Why overhead of SPI is so large?
Date: 2019-08-23 14:32:02
Message-ID: ae38c01f-13cb-0e0a-bd35-c5ce92406425@postgrespro.ru
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On 23.08.2019 14:42, Pavel Stehule wrote:
>
> In reality it is not IMMUTABLE function. On second hand, there are lot
> of application that depends on this behave.
>
> It is well know trick how to reduce estimation errors related to
> JOINs. When immutable function has constant parameters, then it is
> evaluated in planning time.
>
> So sometimes was necessary to use
>
> SELECT ... FROM tab WHERE foreign_key = immutable_function('constant
> parameter')
>
> instead JOIN.
>
> It is ugly, but it is working perfectly. I think so until we will have
> multi table statistics, this behave should be available in Postgres.
>
> Sure, this function should not be used for functional indexes.
>

What about the following version of the patch?

--
Konstantin Knizhnik
Postgres Professional: http://www.postgrespro.com
The Russian Postgres Company

Attachment Content-Type Size
plpgsql_exec_expr-2.patch text/x-patch 2.7 KB

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