## What changes were proposed in this pull request?
[HIVE-12063](https://issues.apache.org/jira/browse/HIVE-12063) improved pad decimal numbers with trailing zeros to the scale of the column. The following description is copied from the description of HIVE-12063.
> HIVE-7373 was to address the problems of trimming tailing zeros by Hive, which caused many problems including treating 0.0, 0.00 and so on as 0, which has different precision/scale. Please refer to HIVE-7373 description. However, HIVE-7373 was reverted by HIVE-8745 while the underlying problems remained. HIVE-11835 was resolved recently to address one of the problems, where 0.0, 0.00, and so on cannot be read into decimal(1,1).
However, HIVE-11835 didn't address the problem of showing as 0 in query result for any decimal values such as 0.0, 0.00, etc. This causes confusion as 0 and 0.0 have different precision/scale than 0.
The proposal here is to pad zeros for query result to the type's scale. This not only removes the confusion described above, but also aligns with many other DBs. Internal decimal number representation doesn't change, however.
**Spark SQL**:
```sql
// bin/spark-sql
spark-sql> select cast(1 as decimal(38, 18));
1
spark-sql>
// bin/beeline
0: jdbc:hive2://localhost:10000/default> select cast(1 as decimal(38, 18));
+----------------------------+--+
| CAST(1 AS DECIMAL(38,18)) |
+----------------------------+--+
| 1.000000000000000000 |
+----------------------------+--+
// bin/spark-shell
scala> spark.sql("select cast(1 as decimal(38, 18))").show(false)
+-------------------------+
|CAST(1 AS DECIMAL(38,18))|
+-------------------------+
|1.000000000000000000 |
+-------------------------+
// bin/pyspark
>>> spark.sql("select cast(1 as decimal(38, 18))").show()
+-------------------------+
|CAST(1 AS DECIMAL(38,18))|
+-------------------------+
| 1.000000000000000000|
+-------------------------+
// bin/sparkR
> showDF(sql("SELECT cast(1 as decimal(38, 18))"))
+-------------------------+
|CAST(1 AS DECIMAL(38,18))|
+-------------------------+
| 1.000000000000000000|
+-------------------------+
```
**PostgreSQL**:
```sql
postgres=# select cast(1 as decimal(38, 18));
numeric
----------------------
1.000000000000000000
(1 row)
```
**Presto**:
```sql
presto> select cast(1 as decimal(38, 18));
_col0
----------------------
1.000000000000000000
(1 row)
```
## How was this patch tested?
unit tests and manual test:
```sql
spark-sql> select cast(1 as decimal(38, 18));
1.000000000000000000
```
Spark SQL Upgrading Guide:
Closes#26697 from wangyum/SPARK-28461.
Authored-by: Yuming Wang <yumwang@ebay.com>
Signed-off-by: HyukjinKwon <gurwls223@apache.org>
708ab57f37
