Re: Ordered Partitioned Table Scans - Mailing list pgsql-hackers

On 1 November 2018 at 22:05, Antonin Houska <ah@cybertec.at> wrote:
> I think these conditions are too restrictive:
>
>         /*
>          * Determine if these pathkeys match the partition order, or reverse
>          * partition order.  It can't match both, so only go to the trouble of
>          * checking the reverse order when it's not in ascending partition
>          * order.
>          */
>         partition_order = pathkeys_contained_in(pathkeys,
>                                                 partition_pathkeys);
>         partition_order_desc = !partition_order &&
>                                 pathkeys_contained_in(pathkeys,
>                                                         partition_pathkeys_desc);
>
>
> In the example above I wanted to show that your new feature should work even
> if "pathkeys" is not contained in "partition_pathkeys".

Okay, after a bit more time looking at this I see what you're saying
now and I agree, but; see below.

>> > Another problem I see is that build_partition_pathkeys() continues even if it
>> > fails to create a pathkey for some partitioning column. In the example above
>> > it would mean that the table can have "partition_pathkeys" equal to {j}
>> > instead of {i, j} on some EC-related conditions. However such a key does not
>> > correspond to reality - this is easier to imagine if another partition is
>> > considered.
>> >
>> > partition 3:
>> >
>> >  i | j | k
>> > ---+---+---
>> >  1 | 0 | 1
>> >  1 | 0 | 0
>> >
>> > So I think no "partition_pathkeys" should be generated in that case. On the
>> > other hand, if the function returned the part of the list it could construct
>> > so far, it'd be wrong because such incomplete pathkeys could pass the checks I
>> > proposed above for reasons unrelated to the partitioning scheme.
>>
>> Oops. That's a mistake. We should return what we have so far if we
>> can't make one of the pathkeys. Will fix.
>
> I think no "partition_pathkeys" should be created in this case, but before we
> can discuss this in detail there needs to be an agreement on the evaluation of
> the relationship between "pathkeys" and "partition_pathkeys", see above.

The problem with doing that is that if the partition keys are better
than the pathkeys then we'll most likely fail to generate any
partition keys at all due to lack of any existing eclass to use for
the pathkeys. It's unsafe to use just the prefix in this case as the
eclass may not have been found due to, for example one of the
partition keys having a different collation than the required sort
order of the query. In other words, we can't rely on a failure to
create the pathkey meaning that a more strict sort order is not
required.

I'm a bit unsure on how safe it would be to pass "create_it" as true
to make_pathkey_from_sortinfo(). We might be building partition path
keys for some sub-partitioned table. In this case the eclass should
likely have a its member added with em_is_child = true.  The existing
code always sets em_is_child to false. It's not that clear to me that
setting up a new eclass with a single em_is_child = true member is
correct.

-- 
 David Rowley                   http://www.2ndQuadrant.com/
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