[jira] [Created] (SPARK-23007) Add schema evolution test suite

["Dongjoon Hyun (JIRA)" <ji...@apache.org>]

Dongjoon Hyun created SPARK-23007:
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             Summary: Add schema evolution test suite
                 Key: SPARK-23007
                 URL: https://issues.apache.org/jira/browse/SPARK-23007
             Project: Spark
          Issue Type: Bug
          Components: SQL, Tests
    Affects Versions: 2.2.1
            Reporter: Dongjoon Hyun


A schema can evolve in several ways and the followings are already supported in file-based data sources.

   1. Add a column
   2. Remove a column
   3. Change a column position
   4. Change a column type

This issue aims to guarantee users a backward-compatible schema evolution coverage on file-based data sources and to prevent future regressions by *adding schema evolution test suites explicitly*.

Here, we consider safe evolution without data loss. For example, data type evolution should be from small types to larger types like `int`-to-`long`, not vice versa.

As of today, in the master branch, file-based data sources have schema evolution coverages like the followings.

|| File Format || Coverage     || Note                                                   ||
| TEXT         | N/A          | Schema consists of a single string column.             |
| CSV          | 1, 2, 4      |                                                        |
| JSON         | 1, 2, 3, 4   |                                                        |
| ORC          | 1, 2, 3, 4   | Native vectorized ORC reader has the widest coverage.  |
| PARQUET      | 1, 2, 3      |                                                        |



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