Re: Confusing reference to MERGE use inside WITH

On Sun, Jul 16, 2023 at 8:48 PM PG Doc comments form
<noreply@postgresql.org> wrote:
>
> The following documentation comment has been logged on the website:
>
> Page: https://www.postgresql.org/docs/15/queries-with.html
> Description:
>
> Hi.
>
> In the queries WITH documentation
> https://www.postgresql.org/docs/15/queries-with.html it is a bit confusing
> that MERGE is referenced in several places as being something you can use
> inside with, even though it can't.
>
> If you search for MERGE on the page, you will find the 2 first results is a
> reference saying MERGE can be used inside WITH, the same as SELECT, INSERT,
> UPDATE, or DELETE. The 3rd result then correctly states that MERGE cannot be
> used inside WITH.
>
> If I'm misunderstanding the meaning of the first 2 results, then I apologise
> :)
>
> Greetings
> Rasmus Porsager


I agree. The second one is kind of confusing.

> WITH w AS (
>     SELECT key, very_expensive_function(val) as f FROM some_table
> )
> SELECT * FROM w AS w1 JOIN w AS w2 ON w1.f = w2.f;
> Here, materialization of the WITH query ensures that very_expensive_function is evaluated only once per table row,
nottwice. 
> The examples above only show WITH being used with SELECT, but it can be attached in the same way to INSERT, UPDATE,
DELETE,> or MERGE. 

There are two "SELECT".   " only show WITH being used with SELECT",
not mention of "SELECT" refer to the auxiliary statements or primary
statements.


--
 I recommend David Deutsch's <<The Beginning of Infinity>>

  Jian