[jira] [Commented] (PARQUET-601) Add support in Parquet to configure the encoding used by ValueWriters

["Piyush Narang (JIRA)" <ji...@apache.org>]

    [ https://issues.apache.org/jira/browse/PARQUET-601?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=15409974#comment-15409974 ] 

Piyush Narang commented on PARQUET-601:
---------------------------------------

Another option suggested by [~alexlevenson], was to allow overriding of the ValuesWriterFactory using reflection:
{code}
parquet.writer.factory-override = "org.apache.parquet.hadoop.MyValuesWriterFactory"
{code}

This creates a factory, MyValuesWriterFactory which is then invoked for every ColumnDescriptor to get a ValueWriter. This provides the flexibility to the user to implement a ValuesWriterFactory that can read configuration for per type / column encoding overrides. Can also be used to plug-in a more sophisticated approach where we choose the appropriate encoding based on the data being seen. A concern raised by [~rdblue] regarding this approach was that ValuesWriters are supposed to be internal classes in Parquet. So we shouldn't be allowing users to configure the ValuesWriter factories via config. 

cc [~julienledem] / [~rdblue] / [~alexlevenson]. We could also explore other ideas based on any other potential use cases. 

> Add support in Parquet to configure the encoding used by ValueWriters
> ---------------------------------------------------------------------
>
>                 Key: PARQUET-601
>                 URL: https://issues.apache.org/jira/browse/PARQUET-601
>             Project: Parquet
>          Issue Type: Improvement
>          Components: parquet-mr
>            Reporter: Piyush Narang
>            Assignee: Piyush Narang
>
> Parquet is currently structured to choose the appropriate value writer based on the type of the column as well as the Parquet version. Value writers are responsible for writing out values with the appropriate encoding. As an example, for Boolean data types, we use BooleanPlainValuesWriter (v1.0) or RunLengthBitPackingHybridValuesWriter (v2.0). The code to take these decisions is in [ParquetProperties|https://github.com/apache/parquet-mr/blob/master/parquet-column/src/main/java/org/apache/parquet/column/ParquetProperties.java#L242]. 
> Thanks to this set up, the writer(s) (and hence encoding) for each data type is hard coded in the Parquet source code. 
> Would be nice to support being able to override the encodings per type via config. That allows users to experiment with various encoding strategies manually as well as enables them to override the hardcoded defaults if they don't suit their use case. 
> We can override encodings per data type (int32 / int64 / ...). 
> Something on the lines of:
> {code}
> parquet.writer.encoding-override.<type> = "encoding1[,encoding2]"
> {code}
> As an example:
> {code}
> "parquet.writer.encoding-override.int32" = "plain"
> (Chooses Plain encoding and hence the PlainValuesWriter).
> {code}
> When a primary + fallback need to be specified, we can do the following:
> {code}
> "parquet.writer.encoding-override.binary" = "rle_dictionary,delta_byte_array"
> (Chooses RLE_DICTIONARY encoding as the initial encoding and DELTA_BYTE_ARRAY encoding as the fallback and hence creates a FallbackWriter(PlainBinaryDictionaryValuesWriter, DeltaByteArrayWriter). 
> {code}
> In such cases we can mandate that the first encoding listed must allow for Fallbacks by implementing [RequiresFallback|https://github.com/apache/parquet-mr/blob/master/parquet-column/src/main/java/org/apache/parquet/column/values/RequiresFallback.java#L31].



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