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https://issues.apache.org/jira/browse/PARQUET-194?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=14395722#comment-14395722
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Cheng Lian edited comment on PARQUET-194 at 4/4/15 1:41 PM:
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[~rdblue] After double thinking about this, I think providing such a callback
is still useful. Because we also need to merge user defined key-value metadata
in {{ParquetOutputCommitter}} for writing summary files. Currently, if any
conflicting user defined key-value pair is found, Parquet simply throws an
exception and gives up writing the summary file.
I noticed that in PARQUET-179, Parquet developers tried to retire both
{{_metadata}} and {{_common_metadata}} at first, but later on decided to keep
{{_common_metadata}} for frameworks and tools like Pig, where no centralized
schema management service is available.
was (Author: lian cheng):
After double thinking about this, I think providing such a callback is still
useful. Because we also need to merge user defined key-value metadata in
{{ParquetOutputCommitter}} for writing summary files. Currently, if any
conflicting user defined key-value pair is found, Parquet simply throws an
exception and gives up writing the summary file.
I noticed that in PARQUET-179, Parquet developers tried to retire both
{{_metadata}} and {{_common_metadata}} at first, but later on decided to keep
{{_common_metadata}} for frameworks and tools like Pig, where no centralized
schema management service is available.
> Provide callback to allow user defined key-value metadata merging strategy
> --------------------------------------------------------------------------
>
> Key: PARQUET-194
> URL: https://issues.apache.org/jira/browse/PARQUET-194
> Project: Parquet
> Issue Type: Improvement
> Components: parquet-mr
> Affects Versions: 1.6.0
> Reporter: Cheng Lian
>
> When merging footers, Parquet doesn't know how to merge conflicting user
> defined key-value metadata entries, and simply throws. It would be better to
> provide callbacks to let users define metadata merging strategies.
> For example, in Spark SQL, we store our own schema information in Parquet
> files as key-value metadata (similar to parquet-avro). While trying to add
> schema merging support for reading Parquet files with different but
> compatible schemas, {{InitContext.getMergedKeyValueMetaData}} throws because
> we have different Spark SQL schemas stored in different Parquet data files.
> Thus, we have to overwrite {{ParquetInputFormat}} and merge the schema within
> {{getSplits}}, which is kinda hacky and inconvenient.
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