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From Brian Hulette <brian.hule...@ccri.com>
Subject Re: Allow dictionary-encoded children?
Date Fri, 06 Apr 2018 19:12:36 GMT
Thanks Uwe, Wes, glad to hear I'm not too far out there :) The 
dictionary batch ordering seems like a reasonable requirement for this 
situation.

I made a JIRA to add something like this to the integration tests 
(https://issues.apache.org/jira/browse/ARROW-2412) and Ill put up a PR 
shortly.

On 04/06/2018 01:43 PM, Wes McKinney wrote:
> Having dictionaries-within-dictionaries does add some complexity, but
> I think the use case is valid and so it would be good to determine the
> best way to handle this in the IPC / messaging protocol.
>
> I would suggest: dictionaries can use other dictionaries, so long as
> those dictionaries occur earlier in the stream. I am not sure either
> the Java or C++ libraries will be able to properly handle these cases
> right now, but that's what we have integration tests for!
>
> On Fri, Apr 6, 2018 at 11:59 AM, Uwe L. Korn <uwelk@xhochy.com> wrote:
>> Hello Brian,
>>
>> I would also have considered this a legitimate use of the Arrow specification. We
only specify the DictionaryType to have a dictionary of any Arrow Type. In the context of
Arrow's IPC this seems to be a bit more complicated as we seem to have the assumption that
there is only one type of Dictionary per column. I would argue that we should be able to support
this once we work out a reliable way to transfer them via the IPC mechanism.
>>
>> Just as a related thought (might not produce the result you want): In Parquet, only
the values on the lowest level are dictionary-encoded. But this is also due to the fact that
Parquet uses repetition and definition levels to encode arbitrarily nested data types. These
are more space-efficient when they are correctly encoded but don't provide random access.
>>
>> Uwe
>>
>> On Fri, Apr 6, 2018, at 4:42 PM, Brian Hulette wrote:
>>> I've been considering a use-case with a dictionary-encoded struct
>>> column, which may contain some dictionary-encoded columns itself. More
>>> specifically, in this use-case each row represents a single observation
>>> in a geospatial track, which includes a position, a time, and some
>>> track-level metadata (track id, origin, destination, etc...). I would
>>> like to represent the metadata as a dictionary-encoded struct, since
>>> unique values will be repeated for each observation of that track, and I
>>> would _also_ like to dictionary-encode some of the metadata column's
>>> children, since unique values will typically be repeated in multiple tracks.
>>>
>>> I think one could make a (totally legitimate) argument that this is
>>> stretching a format designed for tabular data too far. This use-case
>>> could also be accomplished by breaking out the struct metadata column
>>> into its own arrow table, and managing a new integer column that
>>> references that table. This would look almost identical to what I
>>> initially described, it just wouldn't rely on the arrow libraries to
>>> manage the "dictionary".
>>>
>>>
>>> The spec doesn't have anything to say on this topic as far as I can
>>> tell, but our implementations don't currently allow a dictionary-encoded
>>> column's children to be dictionary-encoded themselves [1]. Is this just
>>> a simplifying assumption, or a hard rule that should be codified in the
>>> spec?
>>>
>>> Thanks,
>>> Brian
>>>
>>> [1]
>>> https://github.com/apache/arrow/blob/master/cpp/src/arrow/ipc/metadata-internal.cc#L824


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