Re: LISTAGG à la Oracle in PostgreSQL

On 09.03.26 21:21, Pierre Forstmann wrote:
> Hello,
>
> I can write a LISTAGG aggregate for:
>
> create table emp(deptno numeric, ename text);
>
> SELECT deptno, LISTAGG(ename, ','::text ORDER BY ename) AS employees
> FROM   emp GROUP BY deptno ORDER BY deptno;
>
> I would like to know if is possible to create an aggregate LISTAGG that
> would work like in Oracle:
>
> SELECT deptno,
>        listagg(ename, ',') WITHIN GROUP (ORDER BY ename) AS employees
> FROM emp
> GROUP BY deptno
> ORDER BY deptno;
>
> I failed and IA also failed. Claude says:
>
> It is not possible to exactly replicate listagg(ename, ',') WITHIN GROUP
> (ORDER BY ename) as a custom PostgreSQL aggregate
> because PostgreSQL strictly forbids ungrouped columns as direct
> arguments to ordered-set aggregates.
>
> Do you agree ?

One of the reasons that PostgreSQL hasn't implemented LISTAGG is that it
is a misdesign. It uses ordered-set aggregate syntax even
though it is not very similar to the other ordered-set aggregates.
Its syntax should be more similar to ARRAY_AGG or
JSON_ARRAYAGG, for example. But it's too late to fix the standard on this.