[GitHub] [spark] cloud-fan commented on a diff in pull request #40885: [SPARK-43226] Define extractors for file-constant metadata

["cloud-fan (via GitHub)" <gi...@apache.org>]

cloud-fan commented on code in PR #40885:
URL: https://github.com/apache/spark/pull/40885#discussion_r1174277575


##########
sql/core/src/main/scala/org/apache/spark/sql/execution/datasources/FileFormat.scala:
##########
@@ -203,6 +203,21 @@ trait FileFormat {
    * method. Technically, a file format could choose suppress them, but that is not recommended.
    */
   def metadataSchemaFields: Seq[StructField] = FileFormat.BASE_METADATA_FIELDS
+
+  /**
+   * The extractors to use when deriving file-constant metadata columns for this file format.
+   *
+   * By default, the value of a file-constant metadata column is obtained by looking up the column's
+   * name in the file's metadata column value map. However, implementations can override this method
+   * in order to provide an extractor that has access to the entire [[PartitionedFile]] when
+   * deriving the column's value.
+   *
+   * NOTE: Extractors are lazy, invoked only if the query actually selects their column at runtime.
+   *
+   * See also [[FileFormat.getFileConstantMetadataColumnValue]].
+   */
+  def fileConstantMetadataExtractors: Map[String, PartitionedFile => Any] =

Review Comment:
   I have 2 questions about the API design:
   1. File source implementations now need to define constant metadata columns in both `metadataSchemaFields` and `fileConstantMetadataExtractors`. Shall we have a single API to define constant metadata columns? The API should define both column name, type, and extractor.
   2. Should the extractor return a `Literal` instead of `Any`?



-- 
This is an automated message from the Apache Git Service.
To respond to the message, please log on to GitHub and use the
URL above to go to the specific comment.

To unsubscribe, e-mail: reviews-unsubscribe@spark.apache.org

For queries about this service, please contact Infrastructure at:
users@infra.apache.org


---------------------------------------------------------------------
To unsubscribe, e-mail: reviews-unsubscribe@spark.apache.org
For additional commands, e-mail: reviews-help@spark.apache.org