Postgres also returns false for this, needs careful consideration.

@hannes I got it! Postgres returns '' when executing select concat(null), but duckdb(main) returns null when executing select concat(null). Can I modify it to also return ''?

Yes, that makes sense

Yes, that makes sense

I noticed that select concat(null) return single '' in v1.4, but now it returns null in the main branch. This change seems to originate from this pr https://github.com/duckdb/duckdb/pull/19565/files. Is this normal? Should this sql return null or single ''?

PostgreSQL:

postgres=# SELECT concat(NULL::VARCHAR) IS NULL r;
 r 
---
 f
(1 row)

postgres=# SELECT concat(NULL::VARCHAR, NULL::VARCHAR) IS NULL r;
 r 
---
 f
(1 row)

postgres=# SELECT NULL::VARCHAR || NULL::VARCHAR IS NULL r;
 r 
---
 t
(1 row)

postgres=# SELECT array_cat(NULL::INT[]) IS NULL r;
ERROR:  function array_cat(integer[]) does not exist
LINE 1: SELECT array_cat(NULL::INT[]) IS NULL r;
               ^
HINT:  No function matches the given name and argument types. You might need to add explicit type casts.
postgres=# SELECT array_cat(NULL::INT[], NULL::INT[]) IS NULL r;
 r 
---
 t
(1 row)

postgres=# SELECT NULL::INT[] || NULL::INT[] IS NULL r;
 r 
---
 t
(1 row)

DuckDB v1.4.3:

D SELECT concat(NULL::VARCHAR) IS NULL r;
┌─────────┐
│    r    │
│ boolean │
├─────────┤
│ false   │
└─────────┘
D SELECT concat(NULL::VARCHAR, NULL::VARCHAR) IS NULL r;
┌─────────┐
│    r    │
│ boolean │
├─────────┤
│ false   │
└─────────┘
D SELECT NULL::VARCHAR || NULL::VARCHAR IS NULL r;
┌─────────┐
│    r    │
│ boolean │
├─────────┤
│ true    │
└─────────┘
D SELECT array_cat(NULL::INT[]) IS NULL r;
┌─────────┐
│    r    │
│ boolean │
├─────────┤
│ false   │
└─────────┘
D SELECT array_cat(NULL::INT[], NULL::INT[]) IS NULL r;
┌─────────┐
│    r    │
│ boolean │
├─────────┤
│ false   │
└─────────┘
D SELECT NULL::INT[] || NULL::INT[] IS NULL r;
┌─────────┐
│    r    │
│ boolean │
├─────────┤
│ true    │
└─────────┘

DuckDB main:

memory D SELECT concat(NULL::VARCHAR) IS NULL r;
┌─────────┐
│    r    │
│ boolean │
├─────────┤
│ false   │
└─────────┘
memory D SELECT concat(NULL::VARCHAR, NULL::VARCHAR) IS NULL r;
┌─────────┐
│    r    │
│ boolean │
├─────────┤
│ false   │
└─────────┘
memory D SELECT NULL::VARCHAR || NULL::VARCHAR IS NULL r;
┌─────────┐
│    r    │
│ boolean │
├─────────┤
│ true    │
└─────────┘
memory D SELECT array_cat(NULL::INT[]) IS NULL r;
┌─────────┐
│    r    │
│ boolean │
├─────────┤
│ false   │
└─────────┘
memory D SELECT array_cat(NULL::INT[], NULL::INT[]) IS NULL r;
┌─────────┐
│    r    │
│ boolean │
├─────────┤
│ false   │
└─────────┘
memory D SELECT NULL::INT[] || NULL::INT[] IS NULL r;
┌─────────┐
│    r    │
│ boolean │
├─────────┤
│ true    │
└─────────┘

EDIT: in PostgreSQL the concat function is NULL-tolerant, whereas the concatenation operator (||) is not. Any operator acting on a NULL value in PostgreSQL will return NULL, while functions aren't guaranteed to. I'm not sure if I understand this PR, as it seems like we already have the correct behaviour.

PostgreSQL:

postgres=# SELECT concat(NULL::VARCHAR) IS NULL r;
 r 
---
 f
(1 row)

postgres=# SELECT concat(NULL::VARCHAR, NULL::VARCHAR) IS NULL r;
 r 
---
 f
(1 row)

postgres=# SELECT NULL::VARCHAR || NULL::VARCHAR IS NULL r;
 r 
---
 t
(1 row)

postgres=# SELECT array_cat(NULL::INT[]) IS NULL r;
ERROR:  function array_cat(integer[]) does not exist
LINE 1: SELECT array_cat(NULL::INT[]) IS NULL r;
               ^
HINT:  No function matches the given name and argument types. You might need to add explicit type casts.
postgres=# SELECT array_cat(NULL::INT[], NULL::INT[]) IS NULL r;
 r 
---
 t
(1 row)

postgres=# SELECT NULL::INT[] || NULL::INT[] IS NULL r;
 r 
---
 t
(1 row)

DuckDB v1.4.3:

D SELECT concat(NULL::VARCHAR) IS NULL r;
┌─────────┐
│    r    │
│ boolean │
├─────────┤
│ false   │
└─────────┘
D SELECT concat(NULL::VARCHAR, NULL::VARCHAR) IS NULL r;
┌─────────┐
│    r    │
│ boolean │
├─────────┤
│ false   │
└─────────┘
D SELECT NULL::VARCHAR || NULL::VARCHAR IS NULL r;
┌─────────┐
│    r    │
│ boolean │
├─────────┤
│ true    │
└─────────┘
D SELECT array_cat(NULL::INT[]) IS NULL r;
┌─────────┐
│    r    │
│ boolean │
├─────────┤
│ false   │
└─────────┘
D SELECT array_cat(NULL::INT[], NULL::INT[]) IS NULL r;
┌─────────┐
│    r    │
│ boolean │
├─────────┤
│ false   │
└─────────┘
D SELECT NULL::INT[] || NULL::INT[] IS NULL r;
┌─────────┐
│    r    │
│ boolean │
├─────────┤
│ true    │
└─────────┘

DuckDB main:

memory D SELECT concat(NULL::VARCHAR) IS NULL r;
┌─────────┐
│    r    │
│ boolean │
├─────────┤
│ false   │
└─────────┘
memory D SELECT concat(NULL::VARCHAR, NULL::VARCHAR) IS NULL r;
┌─────────┐
│    r    │
│ boolean │
├─────────┤
│ false   │
└─────────┘
memory D SELECT NULL::VARCHAR || NULL::VARCHAR IS NULL r;
┌─────────┐
│    r    │
│ boolean │
├─────────┤
│ true    │
└─────────┘
memory D SELECT array_cat(NULL::INT[]) IS NULL r;
┌─────────┐
│    r    │
│ boolean │
├─────────┤
│ false   │
└─────────┘
memory D SELECT array_cat(NULL::INT[], NULL::INT[]) IS NULL r;
┌─────────┐
│    r    │
│ boolean │
├─────────┤
│ false   │
└─────────┘
memory D SELECT NULL::INT[] || NULL::INT[] IS NULL r;
┌─────────┐
│    r    │
│ boolean │
├─────────┤
│ true    │
└─────────┘

EDIT: in PostgreSQL the concat function is NULL-tolerant, whereas the concatenation operator (||) is not. Any operator acting on a NULL value in PostgreSQL will return NULL, while functions aren't guaranteed to. I'm not sure if I understand this PR, as it seems like we already have the correct behaviour.

the title of this PR is no longer appropriate. the results of executing SELECT concat(NULL::VARCHAR) are different in main and V1.4; In V1.4 and PostgreSQL, it returns ''; in main, it returns NULL.
The original problem was that select concat(null) returned NULL in the main, but select concat(null) is null returned false. Although I don't know why this happens(select concat(null) is null return false in main), PostgreSQL and v1.4 both return '' when select concat(null) is executed. Therefore, I tried to directly fix it so that select concat(null) returns ''.