From: Tomas Vondra <tomas.vondra@2ndquadrant.com>
To: Andrew Gierth <andrew@tao11.riddles.org.uk>
Cc: Alvaro Herrera <alvherre@2ndquadrant.com>
Cc: Surafel Temesgen <surafel3000@gmail.com>
Cc: PostgreSQL Hackers <pgsql-hackers@postgresql.org>
Subject: Re: FETCH FIRST clause WITH TIES option
Date: Sun, 5 Jan 2020 16:41:13 +0100
Message-ID: <20200105154113.l3mejzem3k663l4v@development> (raw)
In-Reply-To: <87sgm7z2z7.fsf@news-spur.riddles.org.uk>
References: <20191111145649.GA7829@alvherre.pgsql>
<87sgm7z2z7.fsf@news-spur.riddles.org.uk>
On Fri, Nov 29, 2019 at 05:19:00AM +0000, Andrew Gierth wrote:
>>>>>> "Alvaro" == Alvaro Herrera <alvherre@2ndquadrant.com> writes:
>
> Alvaro> First, I noticed that there's a significant unanswered issue
> Alvaro> from Andrew Gierth about this using a completely different
> Alvaro> mechanism, namely an implicit window function. Robert was
> Alvaro> concerned about the performance of Andrew's proposed approach,
> Alvaro> but I don't see any reply from Surafel on the whole issue.
>
> Alvaro> Andrew: Would you rather not have this feature in this form at
> Alvaro> all?
>
>I have done a proof-of-concept hack for my alternative approach, which I
>will post in another thread so as not to confuse the cfbot.
>
>The main advantage, as I see it, of a window function approach is that
>it can also work for PERCENT with only a change to the generated
>expression; the executor part of the code can handle any stopping
>condition. It can also be generalized (though the POC does not do this)
>to allow an SQL-level extension, something like "LIMIT WHEN condition"
>to indicate that it should stop just before the first row for which the
>condition is true. Whether this is a desirable feature or not is another
>question.
>
For the record, the alternative approach was posted here:
https://www.postgresql.org/message-id/flat/87o8wvz253.fsf@news-spur.riddles.org.uk
I think we need to make a decision about which road to take - shall we
continue with what Surafel originally proposed, or should we instead
adopt Andrew's patch?
Andrew's patch looks significantly simpler, although it probably will
need more work to get it committable. My main concern about using window
functions was performance, so I think we should benchmark. For example,
considering window functions are not parallel safe, would this have
negative impact on parallel queries?
regards
--
Tomas Vondra http://www.2ndQuadrant.com
PostgreSQL Development, 24x7 Support, Remote DBA, Training & Services
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-hackers@postgresql.org
Cc: tomas.vondra@2ndquadrant.com, andrew@tao11.riddles.org.uk, alvherre@2ndquadrant.com, surafel3000@gmail.com
Subject: Re: FETCH FIRST clause WITH TIES option
In-Reply-To: <20200105154113.l3mejzem3k663l4v@development>
* 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