From: Kong Man <kong_mansatiansin@hotmail.com>
To: tgl@sss.pgh.pa.us
Cc: vyegorov@gmail.com
Cc: pgsql-sql@postgresql.org
Subject: Re: Writeable CTE Not Working?
Date: Tue, 29 Jan 2013 18:45:56 -0800
Message-ID: <DUB116-W353526CD12D2106765BAE08B1E0@phx.gbl> (raw)
In-Reply-To: <18861.1359508600@sss.pgh.pa.us>
References: <DUB116-W6555FD44F7B4D966C07B48B1F0@phx.gbl>
<CAGnEbohaO2r8Zg=PXNsAOEG+c+8oUwO_2z7X6QKj+3QE=wtDbA@mail.gmail.com>
<DUB116-W10DDC6C15D2D368AD772C08B1F0@phx.gbl>
<18861.1359508600@sss.pgh.pa.us>
List-Unsubscribe: <mailto:majordomo@postgresql.org?body=unsub%20pgsql-sql>
> I think this explanation is wrong --- if you run the query with EXPLAIN
> ANALYZE, you can see from the rowcounts that the writable CTE *does* get
> run to completion, as indeed is stated to be the behavior in the fine
> manual.
>
> However, for a case like this where the main query isn't reading from
> the CTE, the CTE will get cycled to completion after the main query is
> done. I think what is happening is that the main query is updating all
> the rows in the table, and then when the CTE comes along it thinks the
> rows are already updated in the current command, so it doesn't replace
> 'em a second time. This is a consequence of the fact that the same
> command-counter ID is used throughout the query. My recollection is
> that that choice was intentional and that doing it differently would
> break use-cases that are less outlandish than this one. I don't recall
> specific examples though.
Cool. Now I understand it much better.
> Why are you trying to update the same table in two different parts of
> this query, anyway? The best you can really hope for with that is
> unspecified behavior --- we will surely not promise that one of them
> completes before the other starts, so in general there's no way to be
> sure which one would process a particular row first.
It was just my misuse of writable CTE thinking it would be more efficient than separate statements.
Best regards,
-Kong
=
Reply instructions:
You may reply publicly to this message via plain-text email
using any one of the following methods:
* Reply to all the recipients using the --to and --cc options:
reply via email
To: pgsql-sql@postgresql.org
Cc: kong_mansatiansin@hotmail.com, tgl@sss.pgh.pa.us, vyegorov@gmail.com
Subject: Re: Writeable CTE Not Working?
In-Reply-To: <DUB116-W353526CD12D2106765BAE08B1E0@phx.gbl>
* Save the following mbox file, import it into your mail client,
and reply-to-all from there: mbox
This inbox is served by DDX for PostgreSQL; see mirroring instructions
for how to clone and mirror all data and code used for this inbox