Re: [HACKERS] A bug in mapping attributes in ATExecAttachPartition()

May be we should pass a flag to predicate_implied_by() to handle NULL
behaviour for CHECK constraints. Partitioning has shown that it needs
to use predicate_implied_by() for comparing constraints and there may
be other cases that can come up in future. Instead of handling it
outside predicate_implied_by() we may want to change it under a flag.

On Fri, Jun 9, 2017 at 11:43 AM, Amit Langote
<Langote_Amit_f8@lab.ntt.co.jp> wrote:
> On 2017/06/08 18:43, Amit Langote wrote:
>> On 2017/06/08 1:44, Robert Haas wrote:
>>> On Wed, Jun 7, 2017 at 7:47 AM, Ashutosh Bapat
>>> <ashutosh.bapat@enterprisedb.com> wrote:
>>>> In ATExecAttachPartition() there's following code
>>>>
>>>> 13715         partnatts = get_partition_natts(key);
>>>> 13716         for (i = 0; i < partnatts; i++)
>>>> 13717         {
>>>> 13718             AttrNumber  partattno;
>>>> 13719
>>>> 13720             partattno = get_partition_col_attnum(key, i);
>>>> 13721
>>>> 13722             /* If partition key is an expression, must not skip
>>>> validation */
>>>> 13723             if (!partition_accepts_null &&
>>>> 13724                 (partattno == 0 ||
>>>> 13725                  !bms_is_member(partattno, not_null_attrs)))
>>>> 13726                 skip_validate = false;
>>>> 13727         }
>>>>
>>>> partattno obtained from the partition key is the attribute number of
>>>> the partitioned table but not_null_attrs contains the attribute
>>>> numbers of attributes of the table being attached which have NOT NULL
>>>> constraint on them. But the attribute numbers of partitioned table and
>>>> the table being attached may not agree i.e. partition key attribute in
>>>> partitioned table may have a different position in the table being
>>>> attached. So, this code looks buggy. Probably we don't have a test
>>>> which tests this code with different attribute order between
>>>> partitioned table and the table being attached. I didn't get time to
>>>> actually construct a testcase and test it.
>>
>> There seem to be couple of bugs here:
>>
>> 1. When partition's key attributes differ in ordering from the parent,
>>    predicate_implied_by() will give up due to structural inequality of
>>    Vars in the expressions.  By fixing this, we can get it to return
>>    'true' when it's really so.
>>
>> 2. As you said, we store partition's attribute numbers in the
>>    not_null_attrs bitmap, but then check partattno (which is the parent's
>>    attribute number which might differ) against the bitmap, which seems
>>    like it might produce incorrect result.  If, for example,
>>    predicate_implied_by() set skip_validate to true, and the above code
>>    failed to set skip_validate to false where it should have, then we
>>    would wrongly end up skipping the scan.  That is, rows in the partition
>>    will contain null values whereas the partition constraint does not
>>    allow it.  It's hard to reproduce this currently, because with
>>    different ordering of attributes, predicate_refute_by() never returns
>>    true (as mentioned in 1 above), so skip_validate does not need to be
>>    set to false again.
>>
>> Consider this example:
>>
>> create table p (a int, b char) partition by list (a);
>>
>> create table p1 (b char not null, a int check (a in (1)));
>> insert into p1 values ('b', null);
>>
>> Note that not_null_attrs for p1 will contain 1 corresponding to column b,
>> which matches key attribute of the parent, that is 1, corresponding to
>> column a.  Hence we end up wrongly concluding that p1's partition key
>> column does not allow nulls.
>
> [ ... ]
>
>> I am working on a patch to fix the above mentioned issues and will post
>> the same no later than Friday.
>
> And here is the patch.
>
> Thanks,
> Amit



-- 
Best Wishes,
Ashutosh Bapat
EnterpriseDB Corporation
The Postgres Database Company