[GitHub] [spark] cloud-fan opened a new pull request #24041: [SPARK-27119][SQL] Do not infer schema when reading Hive serde table with native data source

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cloud-fan opened a new pull request #24041: [SPARK-27119][SQL] Do not infer schema when reading Hive serde table with native data source
URL: https://github.com/apache/spark/pull/24041
 
 
   ## What changes were proposed in this pull request?
   
   In Spark 2.1, we hit a correctness bug. When reading a Hive serde parquet table with the native parquet data source, and the actual file schema doesn't match the table schema in Hive metastore(only upper/lower case difference), the query returns 0 results.
   
   The reason is that, the parquet reader is case sensitive. If we push down filters with column names that don't match the file physical schema case-sensitively, no data will be returned.
   
   To fix this bug, there were 2 solutions proposed at that time:
   1. Add a config to optionally disable parquet filter pushdown, and make parquet column pruning case insensitive.
   https://github.com/apache/spark/pull/16797
   
   2. Infer the actual schema from data files, when reading Hive serde table with native data source. A config is provided to disable it.
   https://github.com/apache/spark/pull/17229
   
   Solution 2 was accepted and merged to Spark 2.1.1
   
   In Spark 2.4, we refactored the parquet data source a little:
   1. do parquet filter pushdown with the actual file schema.
   https://github.com/apache/spark/pull/21696
   
   2. make parquet filter pushdown case insensitive.
   https://github.com/apache/spark/pull/22197
   
   3. make parquet column pruning case insensitive.
   https://github.com/apache/spark/pull/22148
   
   With these patches, the correctness bug in Spark 2.1 no longer exists, and the schema inference becomes unnecessary.
   
   To be safe, this PR just changes the default value to NEVER_INFER, so that users can set it back to INFER_AND_SAVE. If we don't receive any bug reports for it, we can remove the related code in the next release.
   
   ## How was this patch tested?
   
   existing tests

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