[SPARK-21338][SQL]implement isCascadingTruncateTable() method in AggregatedDialect

[huaxingao]

What changes were proposed in this pull request?

org.apache.spark.sql.jdbc.JdbcDialect's method:
def isCascadingTruncateTable(): Option[Boolean] = None
is not overriden in org.apache.spark.sql.jdbc.AggregatedDialect class.
Because of this issue, when you add more than one dialect Spark doesn't truncate table because isCascadingTruncateTable always returns default None for Aggregated Dialect.
Will implement isCascadingTruncateTable in AggregatedDialect class in this PR.

How was this patch tested?

In JDBCSuite, inside test("Aggregated dialects"), will add one line to test AggregatedDialect.isCascadingTruncateTable

[gatorsmile]

ok to test

[gatorsmile]

Why using the first one?

[huaxingao]

Both getCatalystType and getJDBCType(dt: DataType) use the first one. Also, in the class header, it has the following:
/**

• AggregatedDialect can unify multiple dialects into one virtual Dialect.
• Dialects are tried in order, and the first dialect that does not return a
• neutral element will will.
• @param dialects List of dialects.
  */
  It has a typo, I guess it means "the first dialect that does not return a
  neutral element will return"

[gatorsmile]

This one is different from getCatalystType and getJDBCType . Instead, we should follow canHandle.

[SparkQA]

Test build #81848 has finished for PR 19256 at commit 7b9d6fc.

• This patch fails due to an unknown error code, -9.
• This patch merges cleanly.
• This patch adds no public classes.

[gatorsmile]

retest this please

[gatorsmile]

BTW, could you update the PR title?

[SparkQA]

Test build #81859 has finished for PR 19256 at commit 7b9d6fc.

• This patch fails Spark unit tests.
• This patch merges cleanly.
• This patch adds no public classes.

[huaxingao]

Thanks @gatorsmile
I will change both the implementation and the PR title.

[gatorsmile]

Let me correct what I said above. The logics should be

If any dialect's isCascadingTruncateTable returns true, we should return true.

[huaxingao]

Thanks @gatorsmile
Does the following logic look good to you?

if(any dialect's isCascadingTruncateTable returns true)
  return Some(true)
else
  if (any dialect's isCascadingTruncateTable returns false)
     return Some(false)
  else // None of the dialect implements this method, return the superclass default value
     return None

[gatorsmile]

It looks good, but the actual code should be very simple if you are writing using the Scala way

[gatorsmile]

LGTM pending Jenkins

[SparkQA]

Test build #81898 has finished for PR 19256 at commit bd8e2b9.

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

[gatorsmile]

Thanks! Merged to master

[huaxingao]

@gatorsmile Thanks a lot for your help!!!