Window Functions with identical PARTITION BY and ORDER BY clauses evaluated separately

Relevant documentation:
https://www.postgresql.org/docs/9.4/queries-table-expressions.html#QUERIES-WINDOW
"When multiple window functions are used, all the window functions having
syntactically equivalent PARTITION BY and ORDER BY clauses in their window
definitions are guaranteed to be evaluated in a single pass over the data."

PostgreSQL version:

"PostgreSQL 17.4 on x86_64-windows, compiled by msvc-19.42.34436, 64-bit"

Machine information:

Windows server 2016
kernel version 10.0.14393.7783
12.00 GiB memory
4 cores


Reproduction (my_table_contents.csv attached to email as zip file):

   - CREATE TABLE my_table (champid SMALLINT, champmastery INT);
   - COPY my_table FROM 'path\to\my_table_contents.csv' WITH (FORMAT CSV);
   - CREATE INDEX my_idx ON my_table (champid, champmastery);
   - SELECT
     SUM(CAST(champmastery AS BIGINT)) OVER (
       PARTITION BY champid
       ORDER BY champmastery ASC
       ROWS BETWEEN CURRENT ROW AND UNBOUNDED FOLLOWING
     ) AS sumx,
     COUNT(1) OVER (
       PARTITION BY champid
       ORDER BY champmastery ASC
       RANGE BETWEEN 1000 PRECEDING AND 1000 FOLLOWING
     ) AS sampledensity
   FROM my_table;

I apologize for the email spacing. It may cause issues with copy paste.
Expected result: Given both window functions in the above SELECT query have
identical PARTITION BY and ORDER BY clauses, the execution plan should have
a single "Window Aggregation" operation.
Actual result: The execution plan generated for the above query has two
"Window Aggregation" operations
[image: image.png]