Обсуждение: Wrong behaviour of array comparison when arrays contain nulls
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
For the record, while deviations from the standard related to ROW expressions are documented here:
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/3476For 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
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