Github user liancheng commented on a diff in the pull request:
https://github.com/apache/spark/pull/4373#discussion_r24142085
--- Diff:
sql/core/src/main/scala/org/apache/spark/sql/sources/interfaces.scala ---
@@ -158,6 +158,21 @@ trait CatalystScan extends BaseRelation {
}
@DeveloperApi
+/**
+ * ::DeveloperApi::
+ * A BaseRelation that can be used to insert data into it through the
insert method.
+ * If overwrite in insert method is true, the old data in the relation
should be overwritten with
+ * the new data. If overwrite in insert method is false, the new data
should be appended.
+ *
+ * InsertableRelation has the following three assumptions.
+ * 1. It assumes that the data (Rows in the DataFrame) provided to the
insert method
+ * exactly matches the ordinal of fields in the schema of the BaseRelation.
+ * 2. It assumes that the schema of this relation will not be changed.
+ * Even if the insert method updates the schema, the new schema cannot be
used.
--- End diff --
Would be good to mention the schema evolving cases in JSON and Parquet
here. I guess it's not super intuitive for most people because stable schema is
a basic assumption for traditional RDBMS.
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