On Wednesday, May 13, 2020, Tom Ellis <
tom-postgresql.org@jaguarpaw.co.uk> wrote:
Hello,
The code under 1 gives me the error message "aggregate functions are
not allowed in FROM clause of their own query level" whereas the code
under 2 is permitted. Unless I am much mistaken the latter is
equivalent to the former because it just makes a new "local" name for
`v`.
Because step 2 precedes step 4.
A. Am I right in thinking that the two forms are equivalent?
In so far as if the first one could be executed it would provide the same result, yes...i think
A1. And am I right to presume that it's always possibly to rewrite
more complicated examples that yield the same error to valid versions,
just by coming up with a local name for the problematic fields?
Don’t feel like figuring out a counter-example, your given example is not compelling enough
B. If they are indeed equivalent, what is the rationale for forbidding
the former? It seems like it would be more convenient to allow users
to write the former form.
It wasn’t directly intentional. Lateral came long after from/group by. But since it extends the from clause the processing order puts it before aggregation.
1. Causes error:
SELECT * FROM
((SELECT 0 as "v") as "T1"
INNER JOIN LATERAL
(SELECT
SUM("v") as "r"
FROM (SELECT 0) as "T1") as "T2"
ON TRUE) as "T1"
2. Runs successfully
SELECT * FROM
((SELECT 0 as "v") as "T1"
INNER JOIN LATERAL
(SELECT
SUM("v_again") as "r"
FROM (SELECT "v" as "v_again") as "T1") as "T2"
ON TRUE) as "T1"
By the way, the only reference to this issue that I can find on the
entire internet is the following old mailing list post:
https://www.postgresql.org/message-id/1375925710.17807.13.camel%40vanquo.pezone.net
Yeah, because usually one just writes your example:
Select sum(“v”) from (select 0 as “v”) as t1 group by “v”;
I also posted on DBA.StackExchange
https://dba.stackexchange.com/questions/266988/why-is-it-that-aggregate-functions-are-not-allowed-in-from-clause-of-their-own
I did not.
David J.