Re: LAST_VALUE returns the entire partition

Thank you for such a quick response.
Why did last_value return ALL records to me and not one.
first_value(rg.spath) OVER (*PARTITION BY rg.id_path* *ORDER BY* rg.level
*DESC*) AS first_value
last_value(rg.spath) OVER (*PARTITION BY rg.id_path* *ORDER BY* rg.level)
AS last_value
last_value returns all rows, first_value one.? I don't understand this
behavior.
id_path | spath
---------+------------------------------------------------------
386 | group_root\group_level_1\group_level_2\group_level_3
386 | group_level_3
386 | group_level_2\group_level_3
386 | group_level_1\group_level_2\group_level_3

Thank you for your time.

пт, 2 июл. 2021 г. в 19:04, Tom Lane <tgl(at)sss(dot)pgh(dot)pa(dot)us>:

> =?UTF-8?B?0JTQvNC40YLRgNC40Lkg0JjQstCw0L3QvtCy?= <firstdismay(at)gmail(dot)com>
> writes:
> > I cannot understand the behavior of the last_value function. Conditions
> > opposite to first_value conditions. Why such a result?
>
> The default window frame isn't symmetrical is why. Per the manual [1]:
>
> 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.
>
> That is, if you do nothing to change it then first_value returns the
> value at the first row of the partition, while last_value returns the
> value at the last peer of the current row (which isn't even very
> well-defined when the current row has some peers).
>
> regards, tom lane
>
> [1] https://www.postgresql.org/docs/current/functions-window.html
>