[SPARK-29734][SQL] Datasource V2: Support SHOW CURRENT NAMESPACE

[imback82]

What changes were proposed in this pull request?

This PR introduces a new SQL command: SHOW CURRENT NAMESPACE.

Why are the changes needed?

Datasource V2 supports multiple catalogs/namespaces and having SHOW CURRENT NAMESPACE to retrieve the current catalog/namespace info would be useful.

Does this PR introduce any user-facing change?

Yes, the user can perform the following:

scala> spark.sql("SHOW CURRENT NAMESPACE").show
+-------------+---------+
|      catalog|namespace|
+-------------+---------+
|spark_catalog|  default|
+-------------+---------+

scala> spark.sql("USE testcat.ns1.ns2").show
scala> spark.sql("SHOW CURRENT NAMESPACE").show
+-------+---------+
|catalog|namespace|
+-------+---------+
|testcat|  ns1.ns2|
+-------+---------+

How was this patch tested?

Added unit tests.

[imback82]

cc: @cloud-fan @rdblue

[imback82]

I was planning to expose SHOW CURRENT NAMESPACE as a separate command. But, how about just exposing SHOW CURRENT NAMESPACE which prints both current catalog and namespace? In this case, we don't need to expose SHOW CURRENT CATALOG. Please let me know your thoughts. Thanks!

[SparkQA]

Test build #113181 has finished for PR 26379 at commit 7df7574.

• This patch passes all tests.
• This patch merges cleanly.
• This patch adds the following public classes (experimental):
• case class AlterTableRenamePartitionStatement(
• case class AlterTableDropPartitionStatement(

[cloud-fan]

SGTM. SHOW CURRENT NAMESPACE always put the current catalog name in the first part, which should be good enough.

[imback82]

Thanks @cloud-fan. I updated it to SHOW CURRENT NAMESPACE and updated title/description of PR.

[cloud-fan]

let's be consistent. either catalog/namespace or catalogName/namespacName

[imback82]

updated to catalog/namespace

[SparkQA]

Test build #113215 has finished for PR 26379 at commit 506ce2a.

• This patch passes all tests.
• This patch merges cleanly.
• This patch adds no public classes.

[dongjoon-hyun]

+1, LGTM (except three minor comments about test cases.)
In general, unit test case should not have a side-effect after the test. Please confirm if the new UT follow that principle.

Thanks, @imback82 !

[SparkQA]

Test build #113218 has finished for PR 26379 at commit aed9bc8.

• This patch passes all tests.
• This patch merges cleanly.
• This patch adds no public classes.

[dongjoon-hyun]

Merged to master. The last touched test case already passed~

[imback82]

Thanks everyone for review and help!

[SparkQA]

Test build #113227 has finished for PR 26379 at commit a667321.

• This patch passes all tests.
• This patch merges cleanly.
• This patch adds no public classes.