rdblue commented on a change in pull request #23266: [SPARK-26313][SQL] move
read related methods from Table to read related mix-in traits
URL: https://github.com/apache/spark/pull/23266#discussion_r240726681
##########
File path:
sql/core/src/main/java/org/apache/spark/sql/sources/v2/SupportsBatchRead.java
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@@ -20,14 +20,27 @@
import org.apache.spark.annotation.Evolving;
import org.apache.spark.sql.sources.v2.reader.Scan;
import org.apache.spark.sql.sources.v2.reader.ScanBuilder;
+import org.apache.spark.sql.types.StructType;
/**
- * An empty mix-in interface for {@link Table}, to indicate this table
supports batch scan.
- * <p>
- * If a {@link Table} implements this interface, its {@link
Table#newScanBuilder(DataSourceOptions)}
- * must return a {@link ScanBuilder} that builds {@link Scan} with {@link
Scan#toBatch()}
- * implemented.
- * </p>
+ * A mix-in interface for {@link Table} to provide data reading ability of
batch processing.
*/
@Evolving
-public interface SupportsBatchRead extends Table { }
+public interface SupportsBatchRead extends Table {
+
+ /**
+ * Returns the schema of this table.
+ */
+ StructType schema();
Review comment:
I think that the use case for a table with no schema is extremely narrow, if
not laughably unlikely.
Silly use cases should not cause us to make changes to an API when there is
a reasonable alternative: a source with no schema should return an empty schema
and signal that it wants to disable write validation (using a capability).
There are two advantages to this approach:
1. Implementations need to implement `schema` and validation rules are
enabled by default. If an implementation needs to remember to add a `HasSchema`
interface to turn on validation rules, then the default is wrong. The result
would be that people less familiar with the API will not have validated writes,
and that's a problem.
2. The API is simpler and easier to understand.
I'm -1 on moving this from `Table`.
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