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From: Guyren Howe <[email protected]>
To: Erik Wienhold <[email protected]>
Cc: PG-General Mailing List <[email protected]>
Subject: Re: nth_value out of more than n values returns null
Date: Mon, 4 Nov 2024 15:38:05 -0800
Message-ID: <4e60ede0-86a5-4900-b415-05d68ad75cb1@Spark> (raw)
In-Reply-To: <[email protected]>
References: <3ed0759d-c332-4f96-a147-499a694e9204@Spark>
	<[email protected]>

I’m trying to get the id of the 5,000,000th record, so I can join against it to get a name. I didn’t fully understand what the docs say there. What am I missing?
On 4 Nov 2024 at 15:36 -0800, Erik Wienhold <[email protected]>, wrote:
> On 2024-11-05 00:17 +0100, Guyren Howe wrote:
> > This query:
> >
> > SELECT NTH_VALUE(id, 5000000) OVER (ORDER BY created_at, id ASC) FROM table
> >
> > in a table where SELECT COUNT(*) returns a value a few thousand over 5
> > million, where id is the primary key, returns null.
> >
> > The inclusion of the primary key should make the order by a total
> > order. So there should be a 5 millionth row.
> >
> > How can this happen?
>
> https://www.postgresql.org/docs/current/functions-window.html explains
> it:
>
> Note that first_value, last_value, and nth_value consider only the rows
> within the "window frame", which by default contains the rows from the
> start of the partition through the last peer of the current row. This is
> likely to give unhelpful results for last_value and sometimes also
> nth_value. You can redefine the frame by adding a suitable frame
> specification (RANGE, ROWS or GROUPS) to the OVER clause. See Section
> 4.2.8 for more information about frame specifications.
>
> You probably want to extend the window frame with this:
>
> SELECT NTH_VALUE(id, 5000000) OVER (
> ORDER BY created_at, id ASC
> ROWS BETWEEN UNBOUNDED PRECEDING AND UNBOUNDED FOLLOWING
> ) FROM table
>
> --
> Erik

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