Re: The document contains false statements - Mailing list pgsql-docs

Hello David,

Saturday, September 13, 2025, 7:26:21 PM, you wrote:

> On Saturday, September 13, 2025, PG Doc comments form <noreply@postgresql.org> wrote:
> The following documentation comment has been logged on the website:

> Page: https://www.postgresql.org/docs/17/sql-createfunction.html
> Description:

>> This is effectively another way of declaring a named OUT parameter

> Actually OUT works twice slower in compare to RETURNS TABLE
> https://stackoverflow.com/q/79763947/4632019

> DB<>fiddle for [`OUT`](https://dbfiddle.uk/fz9L_wm0) and [`RETURNS
> TABLE`](https://dbfiddle.uk/uTkU1MT8) cases.

> *I hope after the fix, data centers will consume 2 times less electricity



> The statement is not false - it contains an “except” clause that
> you’ve ignored which makes it true for exactly this reason.  The
> fact you are comparing a set-returning function to one that doesn’t
> return a set has invalidated the test.
> The fundamental issue here is “select (composite_func()).*” where
> the function is not set-returning if known to be broken - the “*”
> expansion during planning results in the function being executed
> multiple times once for each output column. (I may be missing some
> nuances here as, since the inclusion of lateral joins, this almost never comes up anymore.)

> Non-trivial function calls should be placed in the FROM clause of a
> query; in part to ensure avoidance of this problematic behavior.
> This is not at all limited to RLS.
> In short, I don’t know how to improve the documentation to prevent
> people from writing bad queries of this type.  Concrete suggestions
> are welcome, but removing this sentence, or re-wording it, doesn’t
> seem like it would make any difference.
> David J.

Thanks for more information on this. You and Tom both pointed me to `RETURNS SET OF` part. I agree with Tom that
documentationis not the place to teach user how to write SQL. 
But I would appeal that the documentation should be meaning full. And for me the part after "except" looks the same as
itwould be written in Arabic "إرجاع مجموعة من" (RETURNS SETOF). 

The question from David: How it could be done better? is good. Let me describe how I see it from my point of view and
experience.

The documentation above highlights as RETURNS SETOF as something special. I never used SETOF. I tried to google and
findalmost nothing in the official documentation, except these two 

https://www.postgresql.org/docs/current/xfunc-sql.html#XFUNC-SQL-FUNCTIONS-RETURNING-SET
https://www.postgresql.org/docs/current/xfunc-sql.html#XFUNC-SQL-FUNCTIONS-RETURNING-TABLE
with just a mention that it can return 0, 1 or more rows.

And the most informative one is
https://www.postgresql.org/docs/current/queries-table-expressions.html#QUERIES-TABLEFUNCTIONS

The latest one my my mind should be reffered from
https://www.postgresql.org/docs/current/xfunc-sql.html#XFUNC-SQL-FUNCTIONS-RETURNING-TABLE, Eg. 
Please read 7.2.1.4 Table Functions for more information.


For the original problem:
>The name of an output column in the RETURNS TABLE syntax. This is effectively another way of declaring a named OUT
parameter,
> except that RETURNS TABLE also implies RETURNS SETOF.

My proposition is to add link to 7.2.1.4 Table Functions for more information and extend "7.2.1.4 Table Functions for
moreinformation" with the information 
that "in certain circumstances RETURNS TABLE is a subject for optimizer and could be inlined
https://wiki.postgresql.org/wiki/Inlining_of_SQL_functions#Inlining_conditions_for_table_functions".
 Having this it would be clear that `RETURNS TABLE` and `RETURNS SETOF` are sort of fuctions which are called Table
functions.


Probably official documentation should have in future a section somewhere "How to optimize your queries" and links to
benchmarkslike this I did in my question on SO: 
https://pastebin.com/n3sxBxt6
https://dbfiddle.uk/xfy-qw75

Without this information and statement that these two: OUT and RETURNS TABLE are just taste of syntax, users will use
eitherwithout knowing consequences. (like me until I benchmarked it) 

--
Best regards,
Eugen Konkov

--
Best regards,
Eugen Konkov