Re: Weirdness (bug?) with aggregates and subqueries - Mailing list pgsql-general

Laurenz Albe <laurenz.albe@cybertec.at> writes:
> I found this in a blog (https://buttondown.email/jaffray/archive/sql-scoping-is-surprisingly-subtle-and-semantic/):
>   CREATE TABLE aa (a INT);
>   INSERT INTO aa VALUES (1), (2), (3);
>   CREATE TABLE xx (x INT);
>   INSERT INTO xx VALUES (10), (20), (30);

>   SELECT (SELECT sum(a) FROM xx LIMIT 1) FROM aa;

>    sum
>   ═════
>      6
>   (1 row)

> Huh?  Shouldn't that return three rows, just like

No.  The aggregate function is semantically of the closest query level
that contributes a Var to its argument, so it's evaluated at the "FROM
aa" level, causing that level to become an aggregated query that
returns just one row.  Then it acts like an outer reference as far
as the sub-select is concerned.  This is documented at the end of
Section 4.2.7 in our manual,

https://www.postgresql.org/docs/current/sql-expressions.html#SYNTAX-AGGREGATES

Thank the SQL spec for that weirdness.

            regards, tom lane