[SPARK-21119][SQL] unset table properties should keep the table comment

[cloud-fan]

What changes were proposed in this pull request?

Previous code mistakenly use table.properties.get("comment") to read the existing table comment, we should use table.comment

How was this patch tested?

new regression test

[cloud-fan]

cc @gatorsmile @wzhfy @viirya

[viirya]

Does this work for Hive table? When CatalogTable's comment is None, we will ignore it when converting it to Hive table metadata. So seems it will not be unset?

spark/sql/hive/src/main/scala/org/apache/spark/sql/hive/client/HiveClientImpl.scala

Line 935 in 9d0db5a

table.comment.foreach { c => hiveTable.setProperty("comment", c) }

[viirya]

Oh. I see. It goes to override the original table metadata if no comment is set.

[gatorsmile]

LGTM

[viirya]

In above code not shown in this diff, we check if a prop key exists in table.properties or not when ifExists is false. As comment is not in table.properties, I think it will cause AnalysisException when unset comment?

[SparkQA]

Test build #78162 has finished for PR 18325 at commit c84582a.

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

[SparkQA]

Test build #78180 has finished for PR 18325 at commit dd7e1fa.

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

[viirya]

LGTM

[gatorsmile]

Thanks! Merging to master.