Github user cloud-fan commented on a diff in the pull request:

    https://github.com/apache/spark/pull/15046#discussion_r79300964
  
    --- Diff: 
sql/core/src/main/scala/org/apache/spark/sql/execution/datasources/DataSource.scala
 ---
    @@ -318,24 +319,28 @@ case class DataSource(
        */
       def resolveRelation(): BaseRelation = {
         val caseInsensitiveOptions = new CaseInsensitiveMap(options)
    -    val relation = (providingClass.newInstance(), userSpecifiedSchema) 
match {
    +    val relation = (providingClass.newInstance(), inputSchema) match {
           // TODO: Throw when too much is given.
           case (dataSource: SchemaRelationProvider, Some(schema)) =>
             dataSource.createRelation(sparkSession.sqlContext, 
caseInsensitiveOptions, schema)
           case (dataSource: RelationProvider, None) =>
             dataSource.createRelation(sparkSession.sqlContext, 
caseInsensitiveOptions)
           case (_: SchemaRelationProvider, None) =>
             throw new AnalysisException(s"A schema needs to be specified when 
using $className.")
    -      case (_: RelationProvider, Some(_)) =>
    -        throw new AnalysisException(s"$className does not allow 
user-specified schemas.")
    +      case (dataSource: RelationProvider, Some(_)) =>
    +        if (isSchemaFromUsers) {
    +          throw new AnalysisException(s"$className does not allow 
user-specified schemas.")
    +        } else {
    +          dataSource.createRelation(sparkSession.sqlContext, 
caseInsensitiveOptions)
    --- End diff --
    
    If the `RelationProvider` doesn't allow user-specified schema, can we 
assume it's cheap to infer schema for it? Then we can simply check if the given 
schema matches the schema of relation returned by `createRelation`


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