## What changes were proposed in this pull request?
The Antlr lexer we use to tokenize a SQL string may wrongly tokenize a fully qualified identifier as a decimal number token. For example, table identifier `default.123_table` is wrongly tokenized as
```
default // Matches lexer rule IDENTIFIER
.123 // Matches lexer rule DECIMAL_VALUE
_TABLE // Matches lexer rule IDENTIFIER
```
The correct tokenization for `default.123_table` should be:
```
default // Matches lexer rule IDENTIFIER,
. // Matches a single dot
123_TABLE // Matches lexer rule IDENTIFIER
```
This PR fix the Antlr grammar so that it can tokenize fully qualified identifier correctly:
1. Fully qualified table name can be parsed correctly. For example, `select * from database.123_suffix`.
2. Fully qualified column name can be parsed correctly, for example `select a.123_suffix from a`.
### Before change
#### Case 1: Failed to parse fully qualified column name
```
scala> spark.sql("select a.123_column from a").show
org.apache.spark.sql.catalyst.parser.ParseException:
extraneous input '.123' expecting {<EOF>,
...
, IDENTIFIER, BACKQUOTED_IDENTIFIER}(line 1, pos 8)
== SQL ==
select a.123_column from a
--------^^^
```
#### Case 2: Failed to parse fully qualified table name
```
scala> spark.sql("select * from default.123_table")
org.apache.spark.sql.catalyst.parser.ParseException:
extraneous input '.123' expecting {<EOF>,
...
IDENTIFIER, BACKQUOTED_IDENTIFIER}(line 1, pos 21)
== SQL ==
select * from default.123_table
---------------------^^^
```
### After Change
#### Case 1: fully qualified column name, no ParseException thrown
```
scala> spark.sql("select a.123_column from a").show
```
#### Case 2: fully qualified table name, no ParseException thrown
```
scala> spark.sql("select * from default.123_table")
```
## How was this patch tested?
Unit test.
Author: Sean Zhong <seanzhong@databricks.com>
Closes#15006 from clockfly/SPARK-17364.
a6b8182006