### What changes were proposed in this pull request?
For a literal number with an exponent(e.g. 1e-45, 1E2), we'd parse it to Double by default rather than Decimal. And user could still use `spark.sql.legacy.exponentLiteralToDecimal.enabled=true` to fall back to previous behavior.
### Why are the changes needed?
According to ANSI standard of SQL, we see that the (part of) definition of `literal` :
```
<approximate numeric literal> ::=
<mantissa> E <exponent>
```
which indicates that a literal number with an exponent should be approximate numeric(e.g. Double) rather than exact numeric(e.g. Decimal).
And when we test Presto, we found that Presto also conforms to this standard:
```
presto:default> select typeof(1E2);
_col0
--------
double
(1 row)
```
```
presto:default> select typeof(1.2);
_col0
--------------
decimal(2,1)
(1 row)
```
We also find that, actually, literals like `1E2` are parsed as Double before Spark2.1, but changed to Decimal after #14828 due to *The difference between the two confuses most users* as it said. But we also see support(from DB2 test) of original behavior at #14828 (comment).
Although, we also see that PostgreSQL has its own implementation:
```
postgres=# select pg_typeof(1E2);
pg_typeof
-----------
numeric
(1 row)
postgres=# select pg_typeof(1.2);
pg_typeof
-----------
numeric
(1 row)
```
We still think that Spark should also conform to this standard while considering SQL standard and Spark own history and majority DBMS and also user experience.
### Does this PR introduce any user-facing change?
Yes.
For `1E2`, before this PR:
```
scala> spark.sql("select 1E2")
res0: org.apache.spark.sql.DataFrame = [1E+2: decimal(1,-2)]
```
After this PR:
```
scala> spark.sql("select 1E2")
res0: org.apache.spark.sql.DataFrame = [100.0: double]
```
And for `1E-45`, before this PR:
```
org.apache.spark.sql.catalyst.parser.ParseException:
decimal can only support precision up to 38
== SQL ==
select 1E-45
at org.apache.spark.sql.catalyst.parser.AbstractSqlParser.parse(ParseDriver.scala:131)
at org.apache.spark.sql.execution.SparkSqlParser.parse(SparkSqlParser.scala:48)
at org.apache.spark.sql.catalyst.parser.AbstractSqlParser.parsePlan(ParseDriver.scala:76)
at org.apache.spark.sql.SparkSession.$anonfun$sql$1(SparkSession.scala:605)
at org.apache.spark.sql.catalyst.QueryPlanningTracker.measurePhase(QueryPlanningTracker.scala:111)
at org.apache.spark.sql.SparkSession.sql(SparkSession.scala:605)
... 47 elided
```
after this PR:
```
scala> spark.sql("select 1E-45");
res1: org.apache.spark.sql.DataFrame = [1.0E-45: double]
```
And before this PR, user may feel super weird to see that `select 1e40` works but `select 1e-40 fails`. And now, both of them work well.
### How was this patch tested?
updated `literals.sql.out` and `ansi/literals.sql.out`
Closes#26595 from Ngone51/SPARK-29956.
Authored-by: wuyi <ngone_5451@163.com>
Signed-off-by: Wenchen Fan <wenchen@databricks.com>
87ebfaf003