WangGuangxin opened a new pull request #24043: [SPARK-11412][SQL]Support merge 
schema for ORC
URL: https://github.com/apache/spark/pull/24043
 
 
   ## What changes were proposed in this pull request?
   
   Currently, ORC's `inferSchema` is implemented as randomly choosing one ORC 
file and reading its schema. 
   
   This PR follows the behavior of Parquet, it implements merge schemas logic 
by reading all ORC files in parallel through a spark job. 
   
   Users can enable merge schema by 
`spark.read.orc("xxx").option("mergeSchema", "true")` or by setting 
`spark.sql.orc.mergeSchema` to `true`, the prior one has higher priority.
   
   ## How was this patch tested?
   tested by UT OrcUtilsSuite.scala

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