[SPARK-17237][SPARK-17458][SQL][Backport-2.0] Preserve aliases that are given for pivot aggregations

[maropu]

What changes were proposed in this pull request?

This pr is to preserve aliases that are given for pivot aggregations to solve the issue reported in SPARK-17237. This pivoting adds backticks (e.g. 3_count(`c`)) in column names and, in some cases,
thes causes analysis exceptions like;

scala> val df = Seq((2, 3, 4), (3, 4, 5)).toDF("a", "x", "y")
scala> df.groupBy("a").pivot("x").agg(count("y"), avg("y")).na.fill(0)
org.apache.spark.sql.AnalysisException: syntax error in attribute name: `3_count(`y`)`;
  at org.apache.spark.sql.catalyst.analysis.UnresolvedAttribute$.e$1(unresolved.scala:134)
  at org.apache.spark.sql.catalyst.analysis.UnresolvedAttribute$.parseAttributeName(unresolved.scala:144)
...

So, this pr also removes these backticks from column names.

How was this patch tested?

Added a test in DataFrameAggregateSuite.

[SparkQA]

Test build #71281 has finished for PR 16565 at commit 875e2c0.

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

[gatorsmile]

Could you change the PR title to [SPARK-17237][SQL][Backport-2.0] Remove backticks in a pivot result schema

[maropu]

okay! I'm now looking into the test failure, so just a sec, thanks

[SparkQA]

Test build #71290 has finished for PR 16565 at commit e2c2fae.

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

[gatorsmile]

I checked the change history. Actually, you also backported #15111. Could you please update your PR description and PR title?

[gatorsmile]

LGTM except one comment

[maropu]

@gatorsmile oh, I see. Is it okay to mix this pr with the fix of #15111? Would it be better to backport #15111 first then, backport this?

[maropu]

@gatorsmile ping

[gatorsmile]

I think it is fine to do it together. Basically, your PR is to fix the bug of #15111

[maropu]

okay! I'll update them

[maropu]

@gatorsmile How about this fix? plz check this again?

[gatorsmile]

LGTM

[gatorsmile]

Thanks! Merging to 2.0

Could you please close it?

[maropu]

Okay and thanks!