Re: no partition pruning when partitioning using array type

On 2017/12/11 14:31, Amit Langote wrote:
> On 2017/12/09 3:46, Robert Haas wrote:
>> On Fri, Dec 8, 2017 at 5:40 AM, Amit Langote wrote:
>>> I noticed that if you partition using a array type column, partition
>>> pruning using constraint exclusion fails to work due to a minor problem.
>>
>> I guess the question is whether that's guaranteed to be safe.  I spent
>> a little bit of time thinking about it and I don't see a problem.  The
>> function is careful to check that the opclasses and collations of the
>> OpExprs are compatible, and it is the behavior of the operator that is
>> in question here, not the column type, so your change seems OK to me.
>> But I hope somebody else will also study this, because this stuff is
>> fairly subtle and I would not like to be the one who breaks it.
> 
> Thanks for taking a look at it.
> 
> I will try to say a little more on why it seems safe.  RelabelType node
> exists (if any) on top of a given expression node only to denote that the
> operator for which the node is an input will interpret its result as of
> the type RelableType.resulttype, instead of the node's original type.  No
> conversion of values actually occurs before making any decisions that this
> function is in charge of making, because the mismatching types in question
> are known to be binary coercible.  Or more to the point, the operator that
> will be used in the proof will give correct answers for the values without
> having to do any conversion of values.  IOW, it's okay if we simply drop
> the RelabelType, because it doesn't alter in any way the result of the
> proof that operator_predicate_proof() performs.
> 
> That said, I've to come think in this particular case that the
> partitioning code that generates the predicate expression should be a bit
> smarter about the various types it manipulates such that RelabelType won't
> be added in the first place.  In contrast, make_op(), that generates an
> OpExpr from the parser representation of a = '{1}' appearing in the
> query's WHERE clause, won't add the RelabelType because the underlying
> type machinery that it invokes is able to conclude that that's
> unnecessary.  The original patch may still be worth considering as a
> solution though.
> 
> I hope someone else chimes in as well. :)

Bug #15042 [1] seems to be caused by this same problem.  There, a
RelabelType node is being slapped (by the partitioning code) on a Var node
of a partition key on enum.

Attached updated patch.

Thanks,
Amit

[1]
https://www.postgresql.org/message-id/151747550943.1247.2111555422760537959%40wrigleys.postgresql.org