Github user cloud-fan commented on a diff in the pull request:
https://github.com/apache/spark/pull/17289#discussion_r106114894
--- Diff:
sql/core/src/main/scala/org/apache/spark/sql/DataFrameWriter.scala ---
@@ -319,6 +319,10 @@ final class DataFrameWriter[T] private[sql](ds:
Dataset[T]) {
* +---+---+
* }}}
*
+ * The external catalog (Hive metastore) is used to determine if the
table exists.
--- End diff --
```
In this method, save mode is used to determine the behavior if the data
source table exists in Spark catalog.
We will always overwrite the underlying data of data source(e.g. a table in
JDBC data source) if the table
doesn't exist in Spark catalog, and will always append to the underlying
data of data source if the table
already exists.
```
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