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https://issues.apache.org/jira/browse/ARROW-3247?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=17047692#comment-17047692
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Martin Durant commented on ARROW-3247:
--------------------------------------
The spark-type maps:
{code:java}
- spark_schema:
| - map_op_op: MAP, OPTIONAL
| - key_value: REPEATED
| | - key: BYTE_ARRAY, UTF8, REQUIRED
| - value: type, OPTIONAL{code}
The minimal required (because MAP implies repeated):
{code:java}
- scheme
| - map_op_op: MAP, OPTIONAL
| | - key: BYTE_ARRAY, UTF8, REQUIRED
| - value: type, OPTIONAL{code}
The first allow for values: NULL, {}, \{key: NULL}, \{key: value}
The latter allows for: NULL, \{key: NULL}, \{key: value} but no empty map. Note
that this may be more efficiently stored, since the number of available
definition levels is smaller.
> [Python] Support spark parquet array and map types
> --------------------------------------------------
>
> Key: ARROW-3247
> URL: https://issues.apache.org/jira/browse/ARROW-3247
> Project: Apache Arrow
> Issue Type: Improvement
> Components: Python
> Reporter: Martin Durant
> Priority: Minor
> Labels: parquet
>
> As far I understand, there is already some support for nested
> array/dict/structs in arrow. However, spark Map and List types are structured
> one level deeper (I believe to allow for both NULL and empty entries).
> Surprisingly, fastparquet can load these. I do not know the plan for
> arbitrary nested object support, but it should be made clear.
> Schema of spark-generated file from the fastparquet test suite:
> {code:java}
> - spark_schema:
> | - map_op_op: MAP, OPTIONAL
> | - key_value: REPEATED
> | | - key: BYTE_ARRAY, UTF8, REQUIRED
> | - value: BYTE_ARRAY, UTF8, OPTIONAL
> | - map_op_req: MAP, OPTIONAL
> | - key_value: REPEATED
> | | - key: BYTE_ARRAY, UTF8, REQUIRED
> | - value: BYTE_ARRAY, UTF8, REQUIRED
> | - map_req_op: MAP, REQUIRED
> | - key_value: REPEATED
> | | - key: BYTE_ARRAY, UTF8, REQUIRED
> | - value: BYTE_ARRAY, UTF8, OPTIONAL
> | - map_req_req: MAP, REQUIRED
> | - key_value: REPEATED
> | | - key: BYTE_ARRAY, UTF8, REQUIRED
> | - value: BYTE_ARRAY, UTF8, REQUIRED
> | - arr_op_op: LIST, OPTIONAL
> | - list: REPEATED
> | - element: BYTE_ARRAY, UTF8, OPTIONAL
> | - arr_op_req: LIST, OPTIONAL
> | - list: REPEATED
> | - element: BYTE_ARRAY, UTF8, REQUIRED
> | - arr_req_op: LIST, REQUIRED
> | - list: REPEATED
> | - element: BYTE_ARRAY, UTF8, OPTIONAL
> - arr_req_req: LIST, REQUIRED
> - list: REPEATED
> - element: BYTE_ARRAY, UTF8, REQUIRED
> {code}
> (please forgive that some of this has already been mentioned elsewhere; this
> is one of the entries in the list at
> [https://github.com/dask/fastparquet/issues/374] as a feature that is useful
> in fastparquet)
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