johanl-db commented on code in PR #40885:
URL: https://github.com/apache/spark/pull/40885#discussion_r1173803115
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sql/core/src/main/scala/org/apache/spark/sql/execution/datasources/FileFormat.scala:
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@@ -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.
+ *
+ * A scanner must derive each file-constant metadata field's value from each
[[PartitionedFile]]
+ * it processes. By default, the value is obtained by a direct lookup of the
column's name on
+ * [[PartitionedFile.otherConstantMetadataColumnValues]] (see
+ * [[FileFormat.getFileConstantMetadataColumnValue]]). However,
implementations can override this
+ * method in order to provide more sophisticated lazy extractors (e.g. in
case the column value is
+ * complicated or expensive to compute).
Review Comment:
The way I read it is that using `otherConstantMetadataColumnValues` is the
default and an extractor should only be used for more advanced use cases. I
think we rather want to encourage using extractors whenever possible and only
fallback to manually providing the values when necessary.
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