Thread

1. Wrong behaviour of array comparison when arrays contain nulls

   Lukas Eder <lukas.eder@gmail.com> — 2022-04-06T12:22:45Z

   Hello,
   
   A discussion has been brought to my attention about the behaviour of H2 vs
   PostgreSQL when it comes to comparing arrays that contain NULL values, see:
   https://github.com/h2database/h2database/issues/3476
   
   For the following query:
   
   SELECT array[1, NULL] = array[1, NULL]
   
   H2 returns NULL whereas PostgreSQL returns TRUE. In my opinion and
   intuition, as well as according to ISO/IEC 9075-2:2016(E) 8.2 <comparison
   predicate> GR 1) b) ii), H2 is right and PostgreSQL is wrong.
   
   On the above H2 github issue, a slack discussion was linked, to which I
   have no access, but the gist of the slack discussion was that NULL is
   *identical* to NULL according to the SQL standard, but identical doesn't
   mean equal, and the aboe 8.2 GR 1) b) ii) clearly requires all array
   elements Xi and Yi to be equal for the arrays to be equal.
   
   Best Regards,
   Lukas
   

2. Re: Wrong behaviour of array comparison when arrays contain nulls

   Lukas Eder <lukas.eder@gmail.com> — 2022-04-06T13:13:16Z

   For the record, while deviations from the standard related to ROW
   expressions are documented here:
   https://www.postgresql.org/docs/current/functions-comparisons.html#COMPOSITE-TYPE-COMPARISON
   
   This particular deviation isn't documented on that page. Might be worth
   adding?
   
   On Wed, Apr 6, 2022 at 2:22 PM Lukas Eder <lukas.eder@gmail.com> wrote:
   
   > Hello,
   >
   > A discussion has been brought to my attention about the behaviour of H2 vs
   > PostgreSQL when it comes to comparing arrays that contain NULL values, see:
   > https://github.com/h2database/h2database/issues/3476
   >
   > For the following query:
   >
   > SELECT array[1, NULL] = array[1, NULL]
   >
   > H2 returns NULL whereas PostgreSQL returns TRUE. In my opinion and
   > intuition, as well as according to ISO/IEC 9075-2:2016(E) 8.2 <comparison
   > predicate> GR 1) b) ii), H2 is right and PostgreSQL is wrong.
   >
   > On the above H2 github issue, a slack discussion was linked, to which I
   > have no access, but the gist of the slack discussion was that NULL is
   > *identical* to NULL according to the SQL standard, but identical doesn't
   > mean equal, and the aboe 8.2 GR 1) b) ii) clearly requires all array
   > elements Xi and Yi to be equal for the arrays to be equal.
   >
   > Best Regards,
   > Lukas
   >
   

3. Re: Wrong behaviour of array comparison when arrays contain nulls

   Tom Lane <tgl@sss.pgh.pa.us> — 2022-04-06T13:37:52Z

   Lukas Eder <lukas.eder@gmail.com> writes:
   > For the following query:
   > SELECT array[1, NULL] = array[1, NULL]
   > H2 returns NULL whereas PostgreSQL returns TRUE. In my opinion and
   > intuition, as well as according to ISO/IEC 9075-2:2016(E) 8.2 <comparison
   > predicate> GR 1) b) ii), H2 is right and PostgreSQL is wrong.
   
   If we don't impose a total order on array values, then we cannot build
   btree indexes on such columns.  So yes, this is a deviation from the
   SQL standard, and no we are not going to change it.
   
   As for documentation, section 9.19 says
   
       The comparison operators compare the array contents
       element-by-element, using the default B-tree comparison function for
       the element data type, and sort based on the first difference.
   
   which implies this behavior but perhaps could be more explicit.
   
   			regards, tom lane