Re: MERGE examples not clear

On Tue, Feb 21, 2023 at 08:56:50AM -0700, David G. Johnston wrote:
> On Tue, Feb 21, 2023 at 8:35 AM PG Doc comments form <noreply(at)postgresql(dot)org>
> wrote:
>
> The following documentation comment has been logged on the website:
>
> Page: https://www.postgresql.org/docs/15/sql-merge.html
> Description:
>
> On this page: https://www.postgresql.org/docs/15/sql-merge.html
> the first and second examples seems to be contrasted (by "this would be
> exactly equivalent to the following statement"), however the difference
> does
> not seem to related to the stated reason ("the MATCHED result does not
> change"). It seems like the difference should involve the order of WHEN
> clauses?
> Of course, it might be that I don't understand the point, in which case
> maybe the point could be stated more clearly?
>
>
> Yeah, that is a pretty poor pair of examples.  Given that a given customer can
> reasonably be assumed to have more than one recent transaction the MERGE has a
> good chance of failing.
>
> The only difference between the two is the second one uses an explicit subquery
> as the source while the first simply names a table.  If the subquery had a
> GROUP BY customer_id that would be a good change explaining that the second
> query is different because it is resilient in the face of duplicate customer
> recent transactions.
>
> While here...source_alias (...completely hides...the fact that a query was
> issued).  What?  Probably it should read (not verified) that it is actually
> required when the source is a query (maybe tweaking the syntax to match).

The attached patch removes the second example, which doesn't seem to add
much.

--
Bruce Momjian <bruce(at)momjian(dot)us> https://momjian.us
EDB https://enterprisedb.com

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