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