## What changes were proposed in this pull request?
This patch adds a DDL command `SHOW CREATE TABLE AS SERDE`. It is used to generate Hive DDL for a Hive table.
For original `SHOW CREATE TABLE`, it now shows Spark DDL always. If given a Hive table, it tries to generate Spark DDL.
For Hive serde to data source conversion, this uses the existing mapping inside `HiveSerDe`. If can't find a mapping there, throws an analysis exception on unsupported serde configuration.
It is arguably that some Hive fileformat + row serde might be mapped to Spark data source, e.g., CSV. It is not included in this PR. To be conservative, it may not be supported.
For Hive serde properties, for now this doesn't save it to Spark DDL because it may not useful to keep Hive serde properties in Spark table.
## How was this patch tested?
Added test.
Closes#24938 from viirya/SPARK-27946.
Lead-authored-by: Liang-Chi Hsieh <viirya@gmail.com>
Co-authored-by: Liang-Chi Hsieh <liangchi@uber.com>
Signed-off-by: Xiao Li <gatorsmile@gmail.com>
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