AvroReadSupport does not support Avro schema resolution

Given multiple "different-yet-compatible" Avro-backed Parquet files, a runtime exception will be encountered when trying to merge the metadata values across the files if they are used as input sources for a MapReduce job.

A contrived example of this problem is provided, along with a derived version of `AvroReadSupport` that can correctly handle valid schema resolution/evolution scenarios.

**Illustration of Problem**
A simple Avro schema exists, which contains a single record type that consists of a required String member.

```
{"type":"record","name":"Foo","namespace":"com.tapad.avro","fields":[{"name":"my_field","type":{"type":"string","avro.java.string":"String"}}]}
```

When stored as Parquet-Avro the resulting schema is:

```
message com.tapad.avro.Foo {
required binary my_field (UTF8);
}
```

Data is written to a Parquet-Avro file with the following contents:

```
my_field = aaa
my_field = bbb
```

The schema for the Foo record is then changed so that its String member is made optional, with a default value of null now provided for the String member.

```
{"type":"record","name":"Foo","namespace":"com.tapad.avro","fields":[{"name":"my_field","type":["null",{"type":"string","avro.java.string":"String"}],"default":null}]}

message com.tapad.avro.Foo {
optional binary my_field (UTF8);
}
```

This change adheres to the Avro Schema Resoution rules as found in http://avro.apache.org/docs/current/spec.html#Schema+Resolution.

Data is then written to a new Parquet-Avro file.

```
my_field = ccc
```

When both Parquet-Avro files are used as input to a MapReduce job, wherein the schemas in the data files are considered to be the "writer" schemas and the schema on our job's classpath – in this case, the updated schema – is used as the "reader" schema, the following `RuntimeException` is encountered:

```
Caused by: java.lang.RuntimeException: could not merge metadata: key avro.schema has conflicting values: [{"type":"record","name":"Foo","namespace":"com.tapad.avro","fields":[{"name":"my_field","type":{"type":"string","avro.java.string":"String"}}]}, {"type":"record", "name":"Foo","namespace":"com.tapad.avro","fields":[{"name":"my_field","type":["null",{"type":"string","avro.java.string":"String"}],"default":null}]}]
42 at parquet.hadoop.api.InitContext.getMergedKeyValueMetaData(InitContext.java:67)
43 at parquet.hadoop.api.ReadSupport.init(ReadSupport.java:84)
44 at parquet.hadoop.ParquetInputFormat.getSplits(ParquetInputFormat.java:263)
...
```

**Solution**

Each schema in every data file (the "writer" schemas) should check for schema compatibility with the "reader" schema. If all "writer" schemas are compatible with the "reader" schema, all records in all data files can be migrated to the "reader" schema.

The Apache Avro library provides utilities for performing compatibility checks across schemas and provided is a derived version of `AvroReadSupport` which uses these utilities to successfully process the records in the aforementioned data files when they are both used as input to a MapReduce job.

~~_NOTE: Solution will be provided as a hyperlink to a Github Pull Request_~~ https://github.com/apache/incubator-parquet-mr/pull/107

**Reporter**: [Jeffrey Olchovy](https://issues.apache.org/jira/secure/ViewProfile.jspa?name=jeffo)

_{**Note**: *This issue was originally created as [PARQUET-171](https://issues.apache.org/jira/browse/PARQUET-171). Please see the [migration documentation](https://issues.apache.org/jira/browse/PARQUET-2502) for further details.*}

Iniziare con AvroReadSupport e con il percorso di inizializzazione dei metadati di ReadSupport mostrato nello stack trace, quindi esaminare gli schema writer e reader compatibili forniti. Riprodurre il caso di input MapReduce con entrambi i file Parquet-Avro; l’attività è completata quando gli schema writer compatibili vengono accettati e i record vengono risolti nello schema reader senza la RuntimeException durante l’unione dei metadati.

Scritto dal modello di indicizzazione a partire dal testo della issue.