[jira] [Comment Edited] (PARQUET-194) Provide callback to allow user defined key-value metadata merging strategy

["Cheng Lian (JIRA)" <ji...@apache.org>]

    [ https://issues.apache.org/jira/browse/PARQUET-194?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=14395722#comment-14395722 ] 

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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