ryan-johnson-databricks commented on code in PR #40885:
URL: https://github.com/apache/spark/pull/40885#discussion_r1173758526
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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:
I thought I _did_ describe it explicitly:
1. If you provide an extractor, the extractor has access to all state in the
`PartitionedFile` (including the column value map) and can do any computations
it needs to.
2. Otherwise, the column's value is fetched from the column value map.
Was it not clear [enough] in the comment that the extractor has access to
the entire `PartitionedFile` ?
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