incorrect result for SQL UDF with aggregate functions

[xxchan]

Describe the bug

create function mycount(x int[]) returns bigint language sql as 'select count(*) from unnest(x)';

select mycount('{1,2}');
┌─────────┐
│ mycount │
├─────────┤
│       1 │
└─────────┘

create table t(x int[]); insert into t values('{1,2,3}'),('{3,4}');

select mycount(x) from t;
┌─────────┐
│ mycount │
├─────────┤
│       2 │
└─────────┘

postgres:

+---------+
| mycount |
|---------|
| 2       |
+---------+

+---------+
| mycount |
|---------|
| 3       |
| 2       |
+---------+

Maybe we should ban agg in SQL UDF for now.

Error message/log

No response

To Reproduce

No response

Expected behavior

No response

How did you deploy RisingWave?

No response

The version of RisingWave

No response

Additional context

No response

[xxchan]

BTW, I found this when I was trying to solve the problem of casting a jsonb array to normal array (and then to use array functions like array_to_string). That may also require a aggregate SQL UDF.

https://risingwave-community.slack.com/archives/C03BW71523T/p1714818123417119

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