On Tue, Feb 21, 2023 at 8:35 AM PG Doc comments form <nore...@postgresql.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). David J.
