ryan-johnson-databricks commented on code in PR #40885:
URL: https://github.com/apache/spark/pull/40885#discussion_r1175437133
##########
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 made a dummy PR https://github.com/apache/spark/pull/40930 to show what
the other approach actually looks like. I still think it is uglier code for
everyone involved, with no benefit to safety, but at least this way we have
something concrete to compare.
The change for the other PR vs master is:
```
sql/core/src/main/scala/org/apache/spark/sql/execution/DataSourceScanExec.scala
| 33 +++++++++++++---------------
sql/core/src/main/scala/org/apache/spark/sql/execution/datasources/FileFormat.scala
| 145
++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++--------------------------------------
sql/core/src/main/scala/org/apache/spark/sql/execution/datasources/FileScanRDD.scala
| 70
+++++++++++++++--------------------------------------------
sql/core/src/main/scala/org/apache/spark/sql/execution/datasources/PartitioningAwareFileIndex.scala
| 10 ++++-----
sql/core/src/main/scala/org/apache/spark/sql/execution/datasources/parquet/ParquetFileFormat.scala
| 4 ++--
sql/core/src/test/scala/org/apache/spark/sql/execution/datasources/FileSourceCustomMetadataStructSuite.scala
| 71 ++++++++++++++++++++++++++++++++++++++++++++++++------------
6 files changed, 196 insertions(+), 137 deletions(-)
```
The change of this PR vs. master touches fewer files and is ~50LoC smaller:
```
sql/core/src/main/scala/org/apache/spark/sql/execution/DataSourceScanExec.scala
| 33 +++++++++++++++------------------
sql/core/src/main/scala/org/apache/spark/sql/execution/datasources/FileFormat.scala
| 102
++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++------------------------------
sql/core/src/main/scala/org/apache/spark/sql/execution/datasources/FileScanRDD.scala
| 70
+++++++++++++++++-----------------------------------------------------
sql/core/src/main/scala/org/apache/spark/sql/execution/datasources/PartitioningAwareFileIndex.scala
| 10 +++++-----
sql/core/src/test/scala/org/apache/spark/sql/execution/datasources/FileSourceCustomMetadataStructSuite.scala
| 35 ++++++++++++++++++++++++++++++++++-
5 files changed, 143 insertions(+), 107 deletions(-)
```
The other PR vs. this one is changes 3 files and ~100LoC:
```
sql/core/src/main/scala/org/apache/spark/sql/execution/datasources/FileFormat.scala
| 99
++++++++++++++++++++++++++++++++++++++++++++++++++++++++-------------------------------------------
sql/core/src/main/scala/org/apache/spark/sql/execution/datasources/parquet/ParquetFileFormat.scala
| 4 ++--
sql/core/src/test/scala/org/apache/spark/sql/execution/datasources/FileSourceCustomMetadataStructSuite.scala
| 70 ++++++++++++++++++++++++++++++++++++++++------------------------------
3 files changed, 98 insertions(+), 75 deletions(-)
```
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