Github user ericl commented on a diff in the pull request:
https://github.com/apache/spark/pull/16424#discussion_r94170967
--- Diff: docs/sql-programming-guide.md ---
@@ -526,11 +526,18 @@ By default `saveAsTable` will create a "managed
table", meaning that the locatio
be controlled by the metastore. Managed tables will also have their data
deleted automatically
when a table is dropped.
-Currently, `saveAsTable` does not expose an API supporting the creation of
an "External table" from a `DataFrame`,
-however, this functionality can be achieved by providing a `path` option
to the `DataFrameWriter` with `path` as the key
-and location of the external table as its value (String) when saving the
table with `saveAsTable`. When an External table
+Currently, `saveAsTable` does not expose an API supporting the creation of
an "external table" from a `DataFrame`,
+however. This functionality can be achieved by providing a `path` option
to the `DataFrameWriter` with `path` as the key
+and location of the external table as its value (a string) when saving the
table with `saveAsTable`. When an External table
is dropped only its metadata is removed.
+Starting from Spark 2.1, persistent datasource tables have per-partition
metadata stored in the Hive metastore. This brings several benefits:
+
+- Since full information of all partitions can be retrieved from
metastore, excessive partition discovery is no longer needed. This greatly
saves query planning time for partitioned tables with a large number of
partitions.
+- Hive DDLs such as `ALTER TABLE PARTITION ... SET LOCATION` are now
available for tables created with the Datasource API.
+
+Note that partition information is not gathered by default when creating
an external datasource tables (those with a `path` option). You may want to
invoke `MSCK REPAIR TABLE` to trigger partition discovery and persist
per-partition information into metastore before querying a created external
table.
--- End diff --
s/an external/external
To sync the partition information in the metastore, you can invoke `MSCK
REPAIR TABLE`.
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