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
