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From Wes McKinney <wesmck...@gmail.com>
Subject Re: Allow dictionary-encoded children?
Date Fri, 06 Apr 2018 17:43:24 GMT
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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