ryan-johnson-databricks commented on code in PR #40885:
URL: https://github.com/apache/spark/pull/40885#discussion_r1174397988
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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:
> Shall we have a single API to define constant metadata columns?
Maybe? I couldn't figure out a good way to do it:
> I originally wanted to just embed the extractor in the attribute
definition directly, but attribute/struct metadata cannot store code and IMO
that's a wise design decision we shouldn't change
I guess we could store pairs or a triple, but (a) the extractors are
optional (we'd be forcing every column to either use one or opt out); and (b)
the uses for the attribute/struct definitions are rather separated from the
uses of the extractors (result: a lot of `x.map` and `x.filter` type calls
clutter up the code)
> Should the extractor return a `Literal` instead of `Any`?
I thought about it? But:
* it complicates the definitions of all extractors with boilerplate, which
IMO indicates a flawed design
* we anyway have to wrap it in a literal again to handle the `null` vs.
`lit(null)` case (or require every extractor to think carefully about nulls).
* Currently, `getFileConstantMetadataColumnValue` takes care of converting
to literal, and that's what everyone uses (nobody calls the extractor directly).
* It would be non-symmetric with the default case, that pulls `Any` from the
column value map.
Note that nothing prevents an extractor from returning a literal, because
`Literal.apply` will detect and return it unchanged.
Overall, I worry the above changes would only give the code a bigger surface
area with little/no gain in safety or functionality?
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