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Thread: BUG #17575: row(NULL) IS NULL inconsistent with IS NOT DISTINCT FROM NULL

BUG #17575: row(NULL) IS NULL inconsistent with IS NOT DISTINCT FROM NULL

From
PG Bug reporting form
Date:
The following bug has been logged on the website:

Bug reference:      17575
Logged by:          Alexey Borschev
Email address:      aborschev@gmail.com
PostgreSQL version: 14.4
Operating system:   Ubuntu
Description:

Hi, PG hackers!
I noticed strange behavior of ROW(NULL): 
I expect, that  IS NULL  operator should give the same result as  IS NOT
DISTINCT FROM NULL
similarly, IS NOT NULL  operator should give the same result as  IS DISTINCT
FROM NULL:     

SELECT row(NULL::int) = row(NULL::int)              AS "test= "
      , row(NULL::int) IS NULL                      AS IS_NULL   
      , row(NULL::int) IS NOT NULL                  AS NOT_NULL  
      , row(NULL::int) IS DISTINCT FROM NULL        AS IS_DISTINCT_FROM_NULL
   
      , row(NULL::int) IS NOT DISTINCT FROM NULL    AS
NOT_DISTINCT_FROM_NULL   
    
 test=  | is_null | not_null | is_distinct_from_null |
not_distinct_from_null  
--------+---------+----------+-----------------------+------------------------
        | t       | f        | t                     | f
 

But here row(NULL::int) IS NULL -> true, and row(NULL::int) IS NOT DISTINCT
FROM NULL -> false !
        
        
Functions num_nulls and num_nonnulls consider row(...) as non-nulls:
 
SELECT num_nulls(row(NULL::int), row(NULL::int, NULL::TEXT), row(NULL::int,
'Bob'::TEXT)) 
  , num_nonnulls(row(NULL::int), row(NULL::int, NULL::TEXT), row(NULL::int,
'Bob'::TEXT)) ;
 num_nulls | num_nonnulls 
-----------+--------------
         0 |            3

Next point:
I expected that IS NULL and IS NOT NULL operators must always return
opposite results, but:

SELECT row(NULL::int, 'Bob'::TEXT) IS NULL                        AS
Row_IsNULL  
     , row(NULL::int, 'Bob'::TEXT) IS NOT NULL                    AS
Row_NotNULL ;

 row_isnull | row_notnull 
------------+-------------
 f          | f
- They both return False on same input!
 
Can we fix or document this PG issue?

These tests was done on fresh installation of PG 14 vanilla, Ubuntu, no
additional configuration: 

postgres=# select version();
                                                              version
                                                      

-----------------------------------------------------------------------------------------------------------------------------------
 PostgreSQL 14.4 (Ubuntu 14.4-1.pgdg20.04+1) on x86_64-pc-linux-gnu,
compiled by gcc (Ubuntu 9.4.0-1ubuntu1~20.04.1) 9.4.0, 64-bit


P.S.
It would be nice to have an abbreviation for  IS NOT DISTINCT FROM operator,
for example == , 
 and have this operator supported in  == ANY(...) and JOINs (hash, merge,
nested loops)

Re: BUG #17575: row(NULL) IS NULL inconsistent with IS NOT DISTINCT FROM NULL

From
"David G. Johnston"
Date:


On Friday, August 5, 2022, PG Bug reporting form <noreply@postgresql.org> wrote:
The following bug has been logged on the website:

Bug reference:      17575
Logged by:          Alexey Borschev
Email address:      aborschev@gmail.com
PostgreSQL version: 14.4
Operating system:   Ubuntu
Description:       

Hi, PG hackers!
I noticed strange behavior of ROW(NULL):
I expect, that  IS NULL  operator should give the same result as  IS NOT
DISTINCT FROM NULL
similarly, IS NOT NULL  operator should give the same result as  IS DISTINCT
FROM NULL:     

SELECT row(NULL::int) = row(NULL::int)              AS "test= "
      , row(NULL::int) IS NULL                      AS IS_NULL   
      , row(NULL::int) IS NOT NULL                  AS NOT_NULL 
      , row(NULL::int) IS DISTINCT FROM NULL        AS IS_DISTINCT_FROM_NULL

      , row(NULL::int) IS NOT DISTINCT FROM NULL    AS
NOT_DISTINCT_FROM_NULL   

 test=  | is_null | not_null | is_distinct_from_null |
not_distinct_from_null 
--------+---------+----------+-----------------------+------------------------
        | t       | f        | t                     | f                   


But here row(NULL::int) IS NULL -> true, and row(NULL::int) IS NOT DISTINCT
FROM NULL -> false !

 

 

 
Next point:
I expected that IS NULL and IS NOT NULL operators must always return
opposite results, but:

SELECT row(NULL::int, 'Bob'::TEXT) IS NULL                        AS
Row_IsNULL 
     , row(NULL::int, 'Bob'::TEXT) IS NOT NULL                    AS
Row_NotNULL ;

 row_isnull | row_notnull
------------+-------------
 f          | f
- They both return False on same input!

Can we fix or document this PG issue?

It is documented.  The paragraph just below the tip.


David J.

Re: BUG #17575: row(NULL) IS NULL inconsistent with IS NOT DISTINCT FROM NULL

From
Tom Lane
Date:
PG Bug reporting form <noreply@postgresql.org> writes:
> I noticed strange behavior of ROW(NULL): 
> I expect, that  IS NULL  operator should give the same result as  IS NOT
> DISTINCT FROM NULL
> similarly, IS NOT NULL  operator should give the same result as  IS DISTINCT
> FROM NULL:     

I don't see any particular reason to expect that.  row(NULL)
is a row object containing one null field, which is in fact not
identical to a null composite value.  The SQL spec dictates that
IS NULL should return true for both cases, but that doesn't mean
that no other operator is allowed to distinguish them.  I'd say
that this is a wart of IS NULL rather than desirable behavior
we should copy elsewhere.

> Next point:
> I expected that IS NULL and IS NOT NULL operators must always return
> opposite results, but:

You have not read the SQL standard, then.  It's quite clear
about that.
 
> Can we fix or document this PG issue?

... nor our documentation.  See

https://www.postgresql.org/docs/current/functions-comparison.html

para beginning "If the expression is row-valued,", near the
bottom of the page.

            regards, tom lane

Re: BUG #17575: row(NULL) IS NULL inconsistent with IS NOT DISTINCT FROM NULL

From
Алексей Борщёв
Date:
Thank You, David and Tom!
Now these tricks around row(null) are much clearer to me!
I think this issue should be closed.

пт, 5 авг. 2022 г. в 17:23, Tom Lane <tgl@sss.pgh.pa.us>:
>
> PG Bug reporting form <noreply@postgresql.org> writes:
> > I noticed strange behavior of ROW(NULL):
> > I expect, that  IS NULL  operator should give the same result as  IS NOT
> > DISTINCT FROM NULL
> > similarly, IS NOT NULL  operator should give the same result as  IS DISTINCT
> > FROM NULL:
>
> I don't see any particular reason to expect that.  row(NULL)
> is a row object containing one null field, which is in fact not
> identical to a null composite value.  The SQL spec dictates that
> IS NULL should return true for both cases, but that doesn't mean
> that no other operator is allowed to distinguish them.  I'd say
> that this is a wart of IS NULL rather than desirable behavior
> we should copy elsewhere.
>
> > Next point:
> > I expected that IS NULL and IS NOT NULL operators must always return
> > opposite results, but:
>
> You have not read the SQL standard, then.  It's quite clear
> about that.
>
> > Can we fix or document this PG issue?
>
> ... nor our documentation.  See
>
> https://www.postgresql.org/docs/current/functions-comparison.html
>
> para beginning "If the expression is row-valued,", near the
> bottom of the page.
>
>                         regards, tom lane