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* nth_value out of more than n values returns null
@ 2024-11-04 23:17 Guyren Howe <guyren@gmail.com>
0 siblings, 2 replies; 8+ messages in thread
From: Guyren Howe @ 2024-11-04 23:17 UTC (permalink / raw)
To: pgsql-general
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?
^ permalink raw reply [nested|flat] 8+ messages in thread
* Re: nth_value out of more than n values returns null
@ 2024-11-04 23:36 Adrian Klaver <adrian.klaver@aklaver.com>
parent: Guyren Howe <guyren@gmail.com>
1 sibling, 0 replies; 8+ messages in thread
From: Adrian Klaver @ 2024-11-04 23:36 UTC (permalink / raw)
To: Guyren Howe <guyren@gmail.com>; pgsql-general
On 11/4/24 15:17, 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?
If I where to hazard a guess it has to do with this:
https://www.postgresql.org/docs/current/functions-window.html
"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."
Further I am pretty sure that this ORDER BY created_at, id ASC is
creating a window frame over created_at, id and that there are duplicate
created_at values which means the frame has less then 5000000 rows. Try
a smaller number and see what happens.
--
Adrian Klaver
adrian.klaver@aklaver.com
^ permalink raw reply [nested|flat] 8+ messages in thread
* Re: nth_value out of more than n values returns null
@ 2024-11-04 23:36 Erik Wienhold <ewie@ewie.name>
parent: Guyren Howe <guyren@gmail.com>
1 sibling, 1 reply; 8+ messages in thread
From: Erik Wienhold @ 2024-11-04 23:36 UTC (permalink / raw)
To: Guyren Howe <guyren@gmail.com>; +Cc: pgsql-general
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
^ permalink raw reply [nested|flat] 8+ messages in thread
* Re: nth_value out of more than n values returns null
@ 2024-11-04 23:38 Guyren Howe <guyren@gmail.com>
parent: Erik Wienhold <ewie@ewie.name>
0 siblings, 1 reply; 8+ messages in thread
From: Guyren Howe @ 2024-11-04 23:38 UTC (permalink / raw)
To: Erik Wienhold <ewie@ewie.name>; +Cc: pgsql-general
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 <ewie@ewie.name>, 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
^ permalink raw reply [nested|flat] 8+ messages in thread
* Re: nth_value out of more than n values returns null
@ 2024-11-04 23:44 David G. Johnston <david.g.johnston@gmail.com>
parent: Guyren Howe <guyren@gmail.com>
0 siblings, 2 replies; 8+ messages in thread
From: David G. Johnston @ 2024-11-04 23:44 UTC (permalink / raw)
To: Guyren Howe <guyren@gmail.com>; +Cc: Erik Wienhold <ewie@ewie.name>; pgsql-general
On Monday, November 4, 2024, Guyren Howe <guyren@gmail.com> wrote:
> 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?
>
So just use “offset 5_000_000 limit 1”. Bringing in a window function here
seems unhelpful.
David J.
^ permalink raw reply [nested|flat] 8+ messages in thread
* Re: nth_value out of more than n values returns null
@ 2024-11-04 23:46 Guyren Howe <guyren@gmail.com>
parent: David G. Johnston <david.g.johnston@gmail.com>
1 sibling, 1 reply; 8+ messages in thread
From: Guyren Howe @ 2024-11-04 23:46 UTC (permalink / raw)
To: David G. Johnston <david.g.johnston@gmail.com>; +Cc: Erik Wienhold <ewie@ewie.name>; pgsql-general
Wouldn’t it be offset 4_999_999?
I’d still like to understand why nth_value doesn’t work.
On 4 Nov 2024 at 15:44 -0800, David G. Johnston <david.g.johnston@gmail.com>, wrote:
> On Monday, November 4, 2024, Guyren Howe <guyren@gmail.com> wrote:
> > 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?
>
> So just use “offset 5_000_000 limit 1”. Bringing in a window function here seems unhelpful.
>
> David J.
>
^ permalink raw reply [nested|flat] 8+ messages in thread
* Re: nth_value out of more than n values returns null
@ 2024-11-04 23:53 David G. Johnston <david.g.johnston@gmail.com>
parent: Guyren Howe <guyren@gmail.com>
0 siblings, 0 replies; 8+ messages in thread
From: David G. Johnston @ 2024-11-04 23:53 UTC (permalink / raw)
To: Guyren Howe <guyren@gmail.com>; +Cc: Erik Wienhold <ewie@ewie.name>; pgsql-general
On Monday, November 4, 2024, Guyren Howe <guyren@gmail.com> wrote:
> Wouldn’t it be offset 4_999_999?
>
Probably. I tend to expect off-by-one for these kinds of things and test
my way out.
>
> I’d still like to understand why nth_value doesn’t work.
>
When you perform an order by in a window clause the frame you get by
default ends at the current row. Consider “count(*) over ()” versus
“count(*) over (order by id)”.
You need to not use defaults for the window frame if this doesn’t suit you.
David J.
^ permalink raw reply [nested|flat] 8+ messages in thread
* Re: nth_value out of more than n values returns null
@ 2024-11-05 00:49 Tom Lane <tgl@sss.pgh.pa.us>
parent: David G. Johnston <david.g.johnston@gmail.com>
1 sibling, 0 replies; 8+ messages in thread
From: Tom Lane @ 2024-11-05 00:49 UTC (permalink / raw)
To: David G. Johnston <david.g.johnston@gmail.com>; +Cc: Guyren Howe <guyren@gmail.com>; Erik Wienhold <ewie@ewie.name>; pgsql-general
"David G. Johnston" <david.g.johnston@gmail.com> writes:
> So just use “offset 5_000_000 limit 1”. Bringing in a window function here
> seems unhelpful.
Yeah, that. A bite-size example might help clarify what the window
function is doing:
regression=# create table zed(f1 int) ;
CREATE TABLE
regression=# insert into zed select generate_series(1, 10);
INSERT 0 10
regression=# select f1, nth_value(f1, 5) over (order by f1) from zed;
f1 | nth_value
----+-----------
1 |
2 |
3 |
4 |
5 | 5
6 | 5
7 | 5
8 | 5
9 | 5
10 | 5
(10 rows)
For the first four rows, the window frame doesn't include the row
you want, so you get NULL. You can fix that with a non-default
window frame:
regression=# select f1, nth_value(f1, 5) over (order by f1 ROWS BETWEEN UNBOUNDED PRECEDING AND UNBOUNDED FOLLOWING) from zed;
f1 | nth_value
----+-----------
1 | 5
2 | 5
3 | 5
4 | 5
5 | 5
6 | 5
7 | 5
8 | 5
9 | 5
10 | 5
(10 rows)
So yeah, you can get the fifth (or five million'th) row this way, but
you'll get N copies of it, which I assume is not what you want.
Better
regression=# select f1 from zed order by f1 offset 4 limit 1;
f1
----
5
(1 row)
which gets you just the one row and is a lot cheaper too.
regards, tom lane
^ permalink raw reply [nested|flat] 8+ messages in thread
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-- links below jump to the message on this page --
2024-11-04 23:17 nth_value out of more than n values returns null Guyren Howe <guyren@gmail.com>
2024-11-04 23:36 ` Adrian Klaver <adrian.klaver@aklaver.com>
2024-11-04 23:36 ` Erik Wienhold <ewie@ewie.name>
2024-11-04 23:38 ` Guyren Howe <guyren@gmail.com>
2024-11-04 23:44 ` David G. Johnston <david.g.johnston@gmail.com>
2024-11-04 23:46 ` Guyren Howe <guyren@gmail.com>
2024-11-04 23:53 ` David G. Johnston <david.g.johnston@gmail.com>
2024-11-05 00:49 ` Tom Lane <tgl@sss.pgh.pa.us>
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