[SPARK-28359][SQL][PYTHON][TESTS] Make integrated UDF tests robust by making UDFs (virtually) no-op

## What changes were proposed in this pull request?

Current UDFs available in `IntegratedUDFTestUtils` are not exactly no-op. It converts input column to strings and outputs to strings.

It causes some issues when we convert and port the tests at SPARK-27921. Integrated UDF test cases share one output file and it should outputs the same. However,

1. Special values are converted into strings differently:

    | Scala      | Python |
    | ---------- | ------ |
    | `null`     | `None` |
    | `Infinity` | `inf`  |
    | `-Infinity`| `-inf` |
    | `NaN`      | `nan`  |

2. Due to float limitation at Python (see https://docs.python.org/3/tutorial/floatingpoint.html), if float is passed into Python and sent back to JVM, the values are potentially not exactly correct. See apache#25128 and apache#25110

To work around this, this PR targets to change the current UDF to be wrapped by cast. So, Input column is casted into string, UDF returns strings as are, and then output column is casted back to the input column.

Roughly:

**Before:**

```
JVM (col1) -> (cast to string within Python) Python (string) -> (string) JVM
```

**After:**

```
JVM (cast col1 to string) -> (string) Python (string) -> (cast back to col1's type) JVM
```

In this way, UDF is virtually no-op although there might be some subtleties due to roundtrip in string cast. I believe this is good enough.

Python native functions and Scala native functions will take strings and output strings as are. So, there will be no potential test failures due to differences of conversion between Python and Scala.

After this fix, for instance, `udf-aggregates_part1.sql` outputs exactly same as `aggregates_part1.sql`:

<details><summary>Diff comparing to 'pgSQL/aggregates_part1.sql'</summary>
<p>

```diff
diff --git a/sql/core/src/test/resources/sql-tests/results/pgSQL/aggregates_part1.sql.out b/sql/core/src/test/resources/sql-tests/results/udf/pgSQL/udf-aggregates_part1.sql.out
index 51ca1d5..801735781c7 100644
--- a/sql/core/src/test/resources/sql-tests/results/pgSQL/aggregates_part1.sql.out
+++ b/sql/core/src/test/resources/sql-tests/results/udf/pgSQL/udf-aggregates_part1.sql.out
 -3,7 +3,7

 -- !query 0
-SELECT avg(four) AS avg_1 FROM onek
+SELECT avg(udf(four)) AS avg_1 FROM onek
 -- !query 0 schema
 struct<avg_1:double>
 -- !query 0 output
 -11,7 +11,7  struct<avg_1:double>

 -- !query 1
-SELECT avg(a) AS avg_32 FROM aggtest WHERE a < 100
+SELECT udf(avg(a)) AS avg_32 FROM aggtest WHERE a < 100
 -- !query 1 schema
 struct<avg_32:double>
 -- !query 1 output
 -19,7 +19,7  struct<avg_32:double>

 -- !query 2
-select CAST(avg(b) AS Decimal(10,3)) AS avg_107_943 FROM aggtest
+select CAST(avg(udf(b)) AS Decimal(10,3)) AS avg_107_943 FROM aggtest
 -- !query 2 schema
 struct<avg_107_943:decimal(10,3)>
 -- !query 2 output
 -27,7 +27,7  struct<avg_107_943:decimal(10,3)>

 -- !query 3
-SELECT sum(four) AS sum_1500 FROM onek
+SELECT sum(udf(four)) AS sum_1500 FROM onek
 -- !query 3 schema
 struct<sum_1500:bigint>
 -- !query 3 output
 -35,7 +35,7  struct<sum_1500:bigint>

 -- !query 4
-SELECT sum(a) AS sum_198 FROM aggtest
+SELECT udf(sum(a)) AS sum_198 FROM aggtest
 -- !query 4 schema
 struct<sum_198:bigint>
 -- !query 4 output
 -43,7 +43,7  struct<sum_198:bigint>

 -- !query 5
-SELECT sum(b) AS avg_431_773 FROM aggtest
+SELECT udf(udf(sum(b))) AS avg_431_773 FROM aggtest
 -- !query 5 schema
 struct<avg_431_773:double>
 -- !query 5 output
 -51,7 +51,7  struct<avg_431_773:double>

 -- !query 6
-SELECT max(four) AS max_3 FROM onek
+SELECT udf(max(four)) AS max_3 FROM onek
 -- !query 6 schema
 struct<max_3:int>
 -- !query 6 output
 -59,7 +59,7  struct<max_3:int>

 -- !query 7
-SELECT max(a) AS max_100 FROM aggtest
+SELECT max(udf(a)) AS max_100 FROM aggtest
 -- !query 7 schema
 struct<max_100:int>
 -- !query 7 output
 -67,7 +67,7  struct<max_100:int>

 -- !query 8
-SELECT max(aggtest.b) AS max_324_78 FROM aggtest
+SELECT udf(udf(max(aggtest.b))) AS max_324_78 FROM aggtest
 -- !query 8 schema
 struct<max_324_78:float>
 -- !query 8 output
 -75,237 +75,238  struct<max_324_78:float>

 -- !query 9
-SELECT stddev_pop(b) FROM aggtest
+SELECT stddev_pop(udf(b)) FROM aggtest
 -- !query 9 schema
-struct<stddev_pop(CAST(b AS DOUBLE)):double>
+struct<stddev_pop(CAST(CAST(udf(cast(b as string)) AS FLOAT) AS DOUBLE)):double>
 -- !query 9 output
 131.10703231895047

 -- !query 10
-SELECT stddev_samp(b) FROM aggtest
+SELECT udf(stddev_samp(b)) FROM aggtest
 -- !query 10 schema
-struct<stddev_samp(CAST(b AS DOUBLE)):double>
+struct<CAST(udf(cast(stddev_samp(cast(b as double)) as string)) AS DOUBLE):double>
 -- !query 10 output
 151.38936080399804

 -- !query 11
-SELECT var_pop(b) FROM aggtest
+SELECT var_pop(udf(b)) FROM aggtest
 -- !query 11 schema
-struct<var_pop(CAST(b AS DOUBLE)):double>
+struct<var_pop(CAST(CAST(udf(cast(b as string)) AS FLOAT) AS DOUBLE)):double>
 -- !query 11 output
 17189.053923482323

 -- !query 12
-SELECT var_samp(b) FROM aggtest
+SELECT udf(var_samp(b)) FROM aggtest
 -- !query 12 schema
-struct<var_samp(CAST(b AS DOUBLE)):double>
+struct<CAST(udf(cast(var_samp(cast(b as double)) as string)) AS DOUBLE):double>
 -- !query 12 output
 22918.738564643096

 -- !query 13
-SELECT stddev_pop(CAST(b AS Decimal(38,0))) FROM aggtest
+SELECT udf(stddev_pop(CAST(b AS Decimal(38,0)))) FROM aggtest
 -- !query 13 schema
-struct<stddev_pop(CAST(CAST(b AS DECIMAL(38,0)) AS DOUBLE)):double>
+struct<CAST(udf(cast(stddev_pop(cast(cast(b as decimal(38,0)) as double)) as string)) AS DOUBLE):double>
 -- !query 13 output
 131.18117242958306

 -- !query 14
-SELECT stddev_samp(CAST(b AS Decimal(38,0))) FROM aggtest
+SELECT stddev_samp(CAST(udf(b) AS Decimal(38,0))) FROM aggtest
 -- !query 14 schema
-struct<stddev_samp(CAST(CAST(b AS DECIMAL(38,0)) AS DOUBLE)):double>
+struct<stddev_samp(CAST(CAST(CAST(udf(cast(b as string)) AS FLOAT) AS DECIMAL(38,0)) AS DOUBLE)):double>
 -- !query 14 output
 151.47497042966097

 -- !query 15
-SELECT var_pop(CAST(b AS Decimal(38,0))) FROM aggtest
+SELECT udf(var_pop(CAST(b AS Decimal(38,0)))) FROM aggtest
 -- !query 15 schema
-struct<var_pop(CAST(CAST(b AS DECIMAL(38,0)) AS DOUBLE)):double>
+struct<CAST(udf(cast(var_pop(cast(cast(b as decimal(38,0)) as double)) as string)) AS DOUBLE):double>
 -- !query 15 output
 17208.5

 -- !query 16
-SELECT var_samp(CAST(b AS Decimal(38,0))) FROM aggtest
+SELECT var_samp(udf(CAST(b AS Decimal(38,0)))) FROM aggtest
 -- !query 16 schema
-struct<var_samp(CAST(CAST(b AS DECIMAL(38,0)) AS DOUBLE)):double>
+struct<var_samp(CAST(CAST(udf(cast(cast(b as decimal(38,0)) as string)) AS DECIMAL(38,0)) AS DOUBLE)):double>
 -- !query 16 output
 22944.666666666668

 -- !query 17
-SELECT var_pop(1.0), var_samp(2.0)
+SELECT udf(var_pop(1.0)), var_samp(udf(2.0))
 -- !query 17 schema
-struct<var_pop(CAST(1.0 AS DOUBLE)):double,var_samp(CAST(2.0 AS DOUBLE)):double>
+struct<CAST(udf(cast(var_pop(cast(1.0 as double)) as string)) AS DOUBLE):double,var_samp(CAST(CAST(udf(cast(2.0 as string)) AS DECIMAL(2,1)) AS DOUBLE)):double>
 -- !query 17 output
 0.0    NaN

 -- !query 18
-SELECT stddev_pop(CAST(3.0 AS Decimal(38,0))), stddev_samp(CAST(4.0 AS Decimal(38,0)))
+SELECT stddev_pop(udf(CAST(3.0 AS Decimal(38,0)))), stddev_samp(CAST(udf(4.0) AS Decimal(38,0)))
 -- !query 18 schema
-struct<stddev_pop(CAST(CAST(3.0 AS DECIMAL(38,0)) AS DOUBLE)):double,stddev_samp(CAST(CAST(4.0 AS DECIMAL(38,0)) AS DOUBLE)):double>
+struct<stddev_pop(CAST(CAST(udf(cast(cast(3.0 as decimal(38,0)) as string)) AS DECIMAL(38,0)) AS DOUBLE)):double,stddev_samp(CAST(CAST(CAST(udf(cast(4.0 as string)) AS DECIMAL(2,1)) AS DECIMAL(38,0)) AS DOUBLE)):double>
 -- !query 18 output
 0.0    NaN

 -- !query 19
-select sum(CAST(null AS int)) from range(1,4)
+select sum(udf(CAST(null AS int))) from range(1,4)
 -- !query 19 schema
-struct<sum(CAST(NULL AS INT)):bigint>
+struct<sum(CAST(udf(cast(cast(null as int) as string)) AS INT)):bigint>
 -- !query 19 output
 NULL

 -- !query 20
-select sum(CAST(null AS long)) from range(1,4)
+select sum(udf(CAST(null AS long))) from range(1,4)
 -- !query 20 schema
-struct<sum(CAST(NULL AS BIGINT)):bigint>
+struct<sum(CAST(udf(cast(cast(null as bigint) as string)) AS BIGINT)):bigint>
 -- !query 20 output
 NULL

 -- !query 21
-select sum(CAST(null AS Decimal(38,0))) from range(1,4)
+select sum(udf(CAST(null AS Decimal(38,0)))) from range(1,4)
 -- !query 21 schema
-struct<sum(CAST(NULL AS DECIMAL(38,0))):decimal(38,0)>
+struct<sum(CAST(udf(cast(cast(null as decimal(38,0)) as string)) AS DECIMAL(38,0))):decimal(38,0)>
 -- !query 21 output
 NULL

 -- !query 22
-select sum(CAST(null AS DOUBLE)) from range(1,4)
+select sum(udf(CAST(null AS DOUBLE))) from range(1,4)
 -- !query 22 schema
-struct<sum(CAST(NULL AS DOUBLE)):double>
+struct<sum(CAST(udf(cast(cast(null as double) as string)) AS DOUBLE)):double>
 -- !query 22 output
 NULL

 -- !query 23
-select avg(CAST(null AS int)) from range(1,4)
+select avg(udf(CAST(null AS int))) from range(1,4)
 -- !query 23 schema
-struct<avg(CAST(NULL AS INT)):double>
+struct<avg(CAST(udf(cast(cast(null as int) as string)) AS INT)):double>
 -- !query 23 output
 NULL

 -- !query 24
-select avg(CAST(null AS long)) from range(1,4)
+select avg(udf(CAST(null AS long))) from range(1,4)
 -- !query 24 schema
-struct<avg(CAST(NULL AS BIGINT)):double>
+struct<avg(CAST(udf(cast(cast(null as bigint) as string)) AS BIGINT)):double>
 -- !query 24 output
 NULL

 -- !query 25
-select avg(CAST(null AS Decimal(38,0))) from range(1,4)
+select avg(udf(CAST(null AS Decimal(38,0)))) from range(1,4)
 -- !query 25 schema
-struct<avg(CAST(NULL AS DECIMAL(38,0))):decimal(38,4)>
+struct<avg(CAST(udf(cast(cast(null as decimal(38,0)) as string)) AS DECIMAL(38,0))):decimal(38,4)>
 -- !query 25 output
 NULL

 -- !query 26
-select avg(CAST(null AS DOUBLE)) from range(1,4)
+select avg(udf(CAST(null AS DOUBLE))) from range(1,4)
 -- !query 26 schema
-struct<avg(CAST(NULL AS DOUBLE)):double>
+struct<avg(CAST(udf(cast(cast(null as double) as string)) AS DOUBLE)):double>
 -- !query 26 output
 NULL

 -- !query 27
-select sum(CAST('NaN' AS DOUBLE)) from range(1,4)
+select sum(CAST(udf('NaN') AS DOUBLE)) from range(1,4)
 -- !query 27 schema
-struct<sum(CAST(NaN AS DOUBLE)):double>
+struct<sum(CAST(CAST(udf(cast(NaN as string)) AS STRING) AS DOUBLE)):double>
 -- !query 27 output
 NaN

 -- !query 28
-select avg(CAST('NaN' AS DOUBLE)) from range(1,4)
+select avg(CAST(udf('NaN') AS DOUBLE)) from range(1,4)
 -- !query 28 schema
-struct<avg(CAST(NaN AS DOUBLE)):double>
+struct<avg(CAST(CAST(udf(cast(NaN as string)) AS STRING) AS DOUBLE)):double>
 -- !query 28 output
 NaN

 -- !query 30
-SELECT avg(CAST(x AS DOUBLE)), var_pop(CAST(x AS DOUBLE))
+SELECT avg(CAST(udf(x) AS DOUBLE)), var_pop(CAST(udf(x) AS DOUBLE))
 FROM (VALUES ('Infinity'), ('1')) v(x)
 -- !query 30 schema
-struct<avg(CAST(x AS DOUBLE)):double,var_pop(CAST(x AS DOUBLE)):double>
+struct<avg(CAST(CAST(udf(cast(x as string)) AS STRING) AS DOUBLE)):double,var_pop(CAST(CAST(udf(cast(x as string)) AS STRING) AS DOUBLE)):double>
 -- !query 30 output
 Infinity       NaN

 -- !query 31
-SELECT avg(CAST(x AS DOUBLE)), var_pop(CAST(x AS DOUBLE))
+SELECT avg(CAST(udf(x) AS DOUBLE)), var_pop(CAST(udf(x) AS DOUBLE))
 FROM (VALUES ('Infinity'), ('Infinity')) v(x)
 -- !query 31 schema
-struct<avg(CAST(x AS DOUBLE)):double,var_pop(CAST(x AS DOUBLE)):double>
+struct<avg(CAST(CAST(udf(cast(x as string)) AS STRING) AS DOUBLE)):double,var_pop(CAST(CAST(udf(cast(x as string)) AS STRING) AS DOUBLE)):double>
 -- !query 31 output
 Infinity       NaN

 -- !query 32
-SELECT avg(CAST(x AS DOUBLE)), var_pop(CAST(x AS DOUBLE))
+SELECT avg(CAST(udf(x) AS DOUBLE)), var_pop(CAST(udf(x) AS DOUBLE))
 FROM (VALUES ('-Infinity'), ('Infinity')) v(x)
 -- !query 32 schema
-struct<avg(CAST(x AS DOUBLE)):double,var_pop(CAST(x AS DOUBLE)):double>
+struct<avg(CAST(CAST(udf(cast(x as string)) AS STRING) AS DOUBLE)):double,var_pop(CAST(CAST(udf(cast(x as string)) AS STRING) AS DOUBLE)):double>
 -- !query 32 output
 NaN    NaN

 -- !query 33
-SELECT avg(CAST(x AS DOUBLE)), var_pop(CAST(x AS DOUBLE))
+SELECT avg(udf(CAST(x AS DOUBLE))), udf(var_pop(CAST(x AS DOUBLE)))
 FROM (VALUES (100000003), (100000004), (100000006), (100000007)) v(x)
 -- !query 33 schema
-struct<avg(CAST(x AS DOUBLE)):double,var_pop(CAST(x AS DOUBLE)):double>
+struct<avg(CAST(udf(cast(cast(x as double) as string)) AS DOUBLE)):double,CAST(udf(cast(var_pop(cast(x as double)) as string)) AS DOUBLE):double>
 -- !query 33 output
 1.00000005E8   2.5

 -- !query 34
-SELECT avg(CAST(x AS DOUBLE)), var_pop(CAST(x AS DOUBLE))
+SELECT avg(udf(CAST(x AS DOUBLE))), udf(var_pop(CAST(x AS DOUBLE)))
 FROM (VALUES (7000000000005), (7000000000007)) v(x)
 -- !query 34 schema
-struct<avg(CAST(x AS DOUBLE)):double,var_pop(CAST(x AS DOUBLE)):double>
+struct<avg(CAST(udf(cast(cast(x as double) as string)) AS DOUBLE)):double,CAST(udf(cast(var_pop(cast(x as double)) as string)) AS DOUBLE):double>
 -- !query 34 output
 7.000000000006E12      1.0

 -- !query 35
-SELECT covar_pop(b, a), covar_samp(b, a) FROM aggtest
+SELECT udf(covar_pop(b, udf(a))), covar_samp(udf(b), a) FROM aggtest
 -- !query 35 schema
-struct<covar_pop(CAST(b AS DOUBLE), CAST(a AS DOUBLE)):double,covar_samp(CAST(b AS DOUBLE), CAST(a AS DOUBLE)):double>
+struct<CAST(udf(cast(covar_pop(cast(b as double), cast(cast(udf(cast(a as string)) as int) as double)) as string)) AS DOUBLE):double,covar_samp(CAST(CAST(udf(cast(b as string)) AS FLOAT) AS DOUBLE), CAST(a AS DOUBLE)):double>
 -- !query 35 output
 653.6289553875104      871.5052738500139

 -- !query 36
-SELECT corr(b, a) FROM aggtest
+SELECT corr(b, udf(a)) FROM aggtest
 -- !query 36 schema
-struct<corr(CAST(b AS DOUBLE), CAST(a AS DOUBLE)):double>
+struct<corr(CAST(b AS DOUBLE), CAST(CAST(udf(cast(a as string)) AS INT) AS DOUBLE)):double>
 -- !query 36 output
 0.1396345165178734

 -- !query 37
-SELECT count(four) AS cnt_1000 FROM onek
+SELECT count(udf(four)) AS cnt_1000 FROM onek
 -- !query 37 schema
 struct<cnt_1000:bigint>
 -- !query 37 output
 -313,7 +314,7  struct<cnt_1000:bigint>

 -- !query 38
-SELECT count(DISTINCT four) AS cnt_4 FROM onek
+SELECT udf(count(DISTINCT four)) AS cnt_4 FROM onek
 -- !query 38 schema
 struct<cnt_4:bigint>
 -- !query 38 output
 -321,10 +322,10  struct<cnt_4:bigint>

 -- !query 39
-select ten, count(*), sum(four) from onek
+select ten, udf(count(*)), sum(udf(four)) from onek
 group by ten order by ten
 -- !query 39 schema
-struct<ten:int,count(1):bigint,sum(four):bigint>
+struct<ten:int,CAST(udf(cast(count(1) as string)) AS BIGINT):bigint,sum(CAST(udf(cast(four as string)) AS INT)):bigint>
 -- !query 39 output
 0      100     100
 1      100     200
 -339,10 +340,10  struct<ten:int,count(1):bigint,sum(four):bigint>

 -- !query 40
-select ten, count(four), sum(DISTINCT four) from onek
+select ten, count(udf(four)), udf(sum(DISTINCT four)) from onek
 group by ten order by ten
 -- !query 40 schema
-struct<ten:int,count(four):bigint,sum(DISTINCT four):bigint>
+struct<ten:int,count(CAST(udf(cast(four as string)) AS INT)):bigint,CAST(udf(cast(sum(distinct cast(four as bigint)) as string)) AS BIGINT):bigint>
 -- !query 40 output
 0      100     2
 1      100     4
 -357,11 +358,11  struct<ten:int,count(four):bigint,sum(DISTINCT four):bigint>

 -- !query 41
-select ten, sum(distinct four) from onek a
+select ten, udf(sum(distinct four)) from onek a
 group by ten
-having exists (select 1 from onek b where sum(distinct a.four) = b.four)
+having exists (select 1 from onek b where udf(sum(distinct a.four)) = b.four)
 -- !query 41 schema
-struct<ten:int,sum(DISTINCT four):bigint>
+struct<ten:int,CAST(udf(cast(sum(distinct cast(four as bigint)) as string)) AS BIGINT):bigint>
 -- !query 41 output
 0      2
 2      2
 -374,23 +375,23  struct<ten:int,sum(DISTINCT four):bigint>
 select ten, sum(distinct four) from onek a
 group by ten
 having exists (select 1 from onek b
-               where sum(distinct a.four + b.four) = b.four)
+               where sum(distinct a.four + b.four) = udf(b.four))
 -- !query 42 schema
 struct<>
 -- !query 42 output
 org.apache.spark.sql.AnalysisException

 Aggregate/Window/Generate expressions are not valid in where clause of the query.
-Expression in where clause: [(sum(DISTINCT CAST((outer() + b.`four`) AS BIGINT)) = CAST(b.`four` AS BIGINT))]
+Expression in where clause: [(sum(DISTINCT CAST((outer() + b.`four`) AS BIGINT)) = CAST(CAST(udf(cast(four as string)) AS INT) AS BIGINT))]
 Invalid expressions: [sum(DISTINCT CAST((outer() + b.`four`) AS BIGINT))];

 -- !query 43
 select
-  (select max((select i.unique2 from tenk1 i where i.unique1 = o.unique1)))
+  (select udf(max((select i.unique2 from tenk1 i where i.unique1 = o.unique1))))
 from tenk1 o
 -- !query 43 schema
 struct<>
 -- !query 43 output
 org.apache.spark.sql.AnalysisException
-cannot resolve '`o.unique1`' given input columns: [i.even, i.fivethous, i.four, i.hundred, i.odd, i.string4, i.stringu1, i.stringu2, i.ten, i.tenthous, i.thousand, i.twenty, i.two, i.twothousand, i.unique1, i.unique2]; line 2 pos 63
+cannot resolve '`o.unique1`' given input columns: [i.even, i.fivethous, i.four, i.hundred, i.odd, i.string4, i.stringu1, i.stringu2, i.ten, i.tenthous, i.thousand, i.twenty, i.two, i.twothousand, i.unique1, i.unique2]; line 2 pos 67
```
</p>
</details>

## How was this patch tested?

Manually tested.

Closes apache#25130 from HyukjinKwon/SPARK-28359.

Authored-by: HyukjinKwon <[email protected]>
Signed-off-by: Wenchen Fan <[email protected]>

Author: HyukjinKwon

sql/core/src/main/scala/org/apache/spark/sql/execution/python/UserDefinedPythonFunction.scala

@@ -32,7 +32,7 @@ case class UserDefinedPythonFunction(
     pythonEvalType: Int,
     udfDeterministic: Boolean) {
 
-  def builder(e: Seq[Expression]): PythonUDF = {
+  def builder(e: Seq[Expression]): Expression = {
     PythonUDF(name, func, dataType, e, pythonEvalType, udfDeterministic)
   }
 

sql/core/src/test/resources/sql-tests/inputs/udf/pgSQL/udf-aggregates_part1.sql

@@ -9,12 +9,10 @@
 -- SET extra_float_digits = 0;
 
 -- This test file was converted from pgSQL/aggregates_part1.sql.
--- Note that currently registered UDF returns a string. So there are some differences, for instance
--- in string cast within UDF in Scala and Python.
 
-SELECT CAST(avg(udf(four)) AS decimal(10,3)) AS avg_1 FROM onek;
+SELECT avg(udf(four)) AS avg_1 FROM onek;
 
-SELECT CAST(udf(avg(a)) AS decimal(10,3)) AS avg_32 FROM aggtest WHERE a < 100;
+SELECT udf(avg(a)) AS avg_32 FROM aggtest WHERE a < 100;
 
 -- In 7.1, avg(float4) is computed using float8 arithmetic.
 -- Round the result to 3 digits to avoid platform-specific results.
@@ -23,32 +21,32 @@ select CAST(avg(udf(b)) AS Decimal(10,3)) AS avg_107_943 FROM aggtest;
 -- `student` has a column with data type POINT, which is not supported by Spark [SPARK-27766]
 -- SELECT avg(gpa) AS avg_3_4 FROM ONLY student;
 
-SELECT CAST(sum(udf(four)) AS int) AS sum_1500 FROM onek;
+SELECT sum(udf(four)) AS sum_1500 FROM onek;
 SELECT udf(sum(a)) AS sum_198 FROM aggtest;
-SELECT CAST(udf(udf(sum(b))) AS decimal(10,3)) AS avg_431_773 FROM aggtest;
+SELECT udf(udf(sum(b))) AS avg_431_773 FROM aggtest;
 -- `student` has a column with data type POINT, which is not supported by Spark [SPARK-27766]
 -- SELECT sum(gpa) AS avg_6_8 FROM ONLY student;
 
 SELECT udf(max(four)) AS max_3 FROM onek;
-SELECT max(CAST(udf(a) AS int)) AS max_100 FROM aggtest;
-SELECT CAST(udf(udf(max(aggtest.b))) AS decimal(10,3)) AS max_324_78 FROM aggtest;
+SELECT max(udf(a)) AS max_100 FROM aggtest;
+SELECT udf(udf(max(aggtest.b))) AS max_324_78 FROM aggtest;
 -- `student` has a column with data type POINT, which is not supported by Spark [SPARK-27766]
 -- SELECT max(student.gpa) AS max_3_7 FROM student;
 
-SELECT CAST(stddev_pop(udf(b)) AS decimal(10,3)) FROM aggtest;
-SELECT CAST(udf(stddev_samp(b)) AS decimal(10,3)) FROM aggtest;
-SELECT CAST(var_pop(udf(b)) AS decimal(10,3)) FROM aggtest;
-SELECT CAST(udf(var_samp(b)) AS decimal(10,3)) FROM aggtest;
+SELECT stddev_pop(udf(b)) FROM aggtest;
+SELECT udf(stddev_samp(b)) FROM aggtest;
+SELECT var_pop(udf(b)) FROM aggtest;
+SELECT udf(var_samp(b)) FROM aggtest;
 
-SELECT CAST(udf(stddev_pop(CAST(b AS Decimal(38,0)))) AS decimal(10,3)) FROM aggtest;
-SELECT CAST(stddev_samp(CAST(udf(b) AS Decimal(38,0))) AS decimal(10,3)) FROM aggtest;
-SELECT CAST(udf(var_pop(CAST(b AS Decimal(38,0)))) AS decimal(10,3)) FROM aggtest;
-SELECT CAST(var_samp(udf(CAST(b AS Decimal(38,0)))) AS decimal(10,3)) FROM aggtest;
+SELECT udf(stddev_pop(CAST(b AS Decimal(38,0)))) FROM aggtest;
+SELECT stddev_samp(CAST(udf(b) AS Decimal(38,0))) FROM aggtest;
+SELECT udf(var_pop(CAST(b AS Decimal(38,0)))) FROM aggtest;
+SELECT var_samp(udf(CAST(b AS Decimal(38,0)))) FROM aggtest;
 
 -- population variance is defined for a single tuple, sample variance
 -- is not
-SELECT CAST(udf(var_pop(1.0)) AS int), var_samp(udf(2.0));
-SELECT CAST(stddev_pop(udf(CAST(3.0 AS Decimal(38,0)))) AS int), stddev_samp(CAST(udf(4.0) AS Decimal(38,0)));
+SELECT udf(var_pop(1.0)), var_samp(udf(2.0));
+SELECT stddev_pop(udf(CAST(3.0 AS Decimal(38,0)))), stddev_samp(CAST(udf(4.0) AS Decimal(38,0)));
 
 
 -- verify correct results for null and NaN inputs
@@ -76,9 +74,9 @@ FROM (VALUES ('-Infinity'), ('Infinity')) v(x);
 
 
 -- test accuracy with a large input offset
-SELECT CAST(avg(udf(CAST(x AS DOUBLE))) AS int), CAST(udf(var_pop(CAST(x AS DOUBLE))) AS decimal(10,3))
+SELECT avg(udf(CAST(x AS DOUBLE))), udf(var_pop(CAST(x AS DOUBLE)))
 FROM (VALUES (100000003), (100000004), (100000006), (100000007)) v(x);
-SELECT CAST(avg(udf(x)) AS long), CAST(udf(var_pop(CAST(x AS DOUBLE))) AS decimal(10,3))
+SELECT avg(udf(CAST(x AS DOUBLE))), udf(var_pop(CAST(x AS DOUBLE)))
 FROM (VALUES (7000000000005), (7000000000007)) v(x);
 
 -- SQL2003 binary aggregates [SPARK-23907]
@@ -89,8 +87,8 @@ FROM (VALUES (7000000000005), (7000000000007)) v(x);
 -- SELECT regr_avgx(b, a), regr_avgy(b, a) FROM aggtest;
 -- SELECT regr_r2(b, a) FROM aggtest;
 -- SELECT regr_slope(b, a), regr_intercept(b, a) FROM aggtest;
-SELECT CAST(udf(covar_pop(b, udf(a))) AS decimal(10,3)), CAST(covar_samp(udf(b), a) as decimal(10,3)) FROM aggtest;
-SELECT CAST(corr(b, udf(a)) AS decimal(10,3)) FROM aggtest;
+SELECT udf(covar_pop(b, udf(a))), covar_samp(udf(b), a) FROM aggtest;
+SELECT corr(b, udf(a)) FROM aggtest;
 
 
 -- test accum and combine functions directly [SPARK-23907]
@@ -122,7 +120,7 @@ SELECT CAST(corr(b, udf(a)) AS decimal(10,3)) FROM aggtest;
 SELECT count(udf(four)) AS cnt_1000 FROM onek;
 SELECT udf(count(DISTINCT four)) AS cnt_4 FROM onek;
 
-select ten, udf(count(*)), CAST(sum(udf(four)) AS int) from onek
+select ten, udf(count(*)), sum(udf(four)) from onek
 group by ten order by ten;
 
 select ten, count(udf(four)), udf(sum(DISTINCT four)) from onek

sql/core/src/test/resources/sql-tests/inputs/udf/pgSQL/udf-aggregates_part2.sql

@@ -6,8 +6,6 @@
 -- https://github.com/postgres/postgres/blob/REL_12_BETA2/src/test/regress/sql/aggregates.sql#L145-L350
 --
 -- This test file was converted from pgSQL/aggregates_part2.sql.
--- Note that currently registered UDF returns a string. So there are some differences, for instance
--- in string cast within UDF in Scala and Python.
 
 create temporary view int4_tbl as select * from values
   (0),

sql/core/src/test/resources/sql-tests/inputs/udf/pgSQL/udf-case.sql

@@ -7,8 +7,6 @@
 -- Test the CASE statement
 --
 -- This test file was converted from pgSQL/case.sql.
--- Note that currently registered UDF returns a string. So there are some differences, for instance
--- in string cast within UDF in Scala and Python.
 
 CREATE TABLE CASE_TBL (
   i integer,
@@ -38,7 +36,7 @@ INSERT INTO CASE2_TBL VALUES (NULL, -6);
 
 SELECT '3' AS `One`,
   CASE
-    WHEN CAST(udf(1 < 2) AS boolean) THEN 3
+    WHEN udf(1 < 2) THEN 3
   END AS `Simple WHEN`;
 
 SELECT '<NULL>' AS `One`,
@@ -60,7 +58,7 @@ SELECT udf('4') AS `One`,
 
 SELECT udf('6') AS `One`,
   CASE
-    WHEN CAST(udf(1 > 2) AS boolean) THEN 3
+    WHEN udf(1 > 2) THEN 3
     WHEN udf(4) < 5 THEN 6
     ELSE 7
   END AS `Two WHEN with default`;
@@ -70,7 +68,7 @@ SELECT '7' AS `None`,
   END AS `NULL on no matches`;
 
 -- Constant-expression folding shouldn't evaluate unreachable subexpressions
-SELECT CASE WHEN CAST(udf(1=0) AS boolean) THEN 1/0 WHEN 1=1 THEN 1 ELSE 2/0 END;
+SELECT CASE WHEN udf(1=0) THEN 1/0 WHEN 1=1 THEN 1 ELSE 2/0 END;
 SELECT CASE 1 WHEN 0 THEN 1/udf(0) WHEN 1 THEN 1 ELSE 2/0 END;
 
 -- [SPARK-27923] PostgreSQL throws an exception but Spark SQL is NULL
@@ -142,7 +140,7 @@ SELECT udf('') AS Five, NULLIF(a.i,b.i) AS `NULLIF(a.i,b.i)`,
 
 SELECT '' AS `Two`, *
   FROM CASE_TBL a, CASE2_TBL b
-  WHERE CAST(udf(COALESCE(f,b.i) = 2) AS boolean);
+  WHERE udf(COALESCE(f,b.i) = 2);
 
 -- We don't support update now.
 --

sql/core/src/test/resources/sql-tests/inputs/udf/udf-having.sql

@@ -1,6 +1,4 @@
 -- This test file was converted from having.sql.
--- Note that currently registered UDF returns a string. So there are some differences, for instance
--- in string cast within UDF in Scala and Python.
 
 create temporary view hav as select * from values
   ("one", 1),

sql/core/src/test/resources/sql-tests/inputs/udf/udf-natural-join.sql

@@ -4,8 +4,6 @@
 --SET spark.sql.autoBroadcastJoinThreshold=-1,spark.sql.join.preferSortMergeJoin=false
 
 -- This test file was converted from natural-join.sql.
--- Note that currently registered UDF returns a string. So there are some differences, for instance
--- in string cast within UDF in Scala and Python.
 
 create temporary view nt1 as select * from values
   ("one", 1),

sql/core/src/test/resources/sql-tests/inputs/udf/udf-special-values.sql

@@ -0,0 +1,8 @@
+-- This file tests special values such as NaN, Infinity and NULL.
+
+SELECT udf(x) FROM (VALUES (1), (2), (NULL)) v(x);
+SELECT udf(x) FROM (VALUES ('A'), ('B'), (NULL)) v(x);
+SELECT udf(x) FROM (VALUES ('NaN'), ('1'), ('2')) v(x);
+SELECT udf(x) FROM (VALUES ('Infinity'), ('1'), ('2')) v(x);
+SELECT udf(x) FROM (VALUES ('-Infinity'), ('1'), ('2')) v(x);
+SELECT udf(x) FROM (VALUES 0.00000001, 0.00000002, 0.00000003) v(x);