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
