Current Spark SQL parser can have pretty confusing error messages when parsing an incorrect SELECT SQL statement. The proposed fix has the following effect.
BEFORE:
```
spark-sql> SELECT * FROM test WHERE x NOT NULL;
Error in query:
mismatched input 'FROM' expecting {<EOF>, 'CLUSTER', 'DISTRIBUTE', 'EXCEPT', 'GROUP', 'HAVING', 'INTERSECT', 'LATERAL', 'LIMIT', 'ORDER', 'MINUS', 'SORT', 'UNION', 'WHERE', 'WINDOW'}(line 1, pos 9)
== SQL ==
SELECT * FROM test WHERE x NOT NULL
---------^^^
```
where in fact the error message should be hinted to be near `NOT NULL`.
AFTER:
```
spark-sql> SELECT * FROM test WHERE x NOT NULL;
Error in query:
mismatched input 'NOT' expecting {<EOF>, 'AND', 'CLUSTER', 'DISTRIBUTE', 'EXCEPT', 'GROUP', 'HAVING', 'INTERSECT', 'LIMIT', 'OR', 'ORDER', 'MINUS', 'SORT', 'UNION', 'WINDOW'}(line 1, pos 27)
== SQL ==
SELECT * FROM test WHERE x NOT NULL
---------------------------^^^
```
In fact, this problem is brought by some problematic Spark SQL grammar. There are two kinds of SELECT statements that are supported by Hive (and thereby supported in SparkSQL):
* `FROM table SELECT blahblah SELECT blahblah`
* `SELECT blah FROM table`
*Reference* [HiveQL single-from stmt grammar](https://github.com/apache/hive/blob/master/ql/src/java/org/apache/hadoop/hive/ql/parse/HiveParser.g)
It is fine when these two SELECT syntaxes are supported separately. However, since we are currently supporting these two kinds of syntaxes in a single ANTLR rule, this can be problematic and therefore leading to confusing parser errors. This is because when a SELECT clause was parsed, it can't tell whether the following FROM clause actually belongs to it or is just the beginning of a new `FROM table SELECT *` statement.
## What changes were proposed in this pull request?
1. Modify ANTLR grammar to fix the above-mentioned problem. This fix is important because the previous problematic grammar does affect a lot of real-world queries. Due to the previous problematic and messy grammar, we refactored the grammar related to `querySpecification`.
2. Modify `AstBuilder` to have separate visitors for `SELECT ... FROM ...` and `FROM ... SELECT ...` statements.
3. Drop the `FROM table` statement, which is supported by accident and is actually parsed in the wrong code path. Both Hive and Presto do not support this syntax.
## How was this patch tested?
Existing UTs and new UTs.
Closes#24809 from yeshengm/parser-refactor.
Authored-by: Yesheng Ma <kimi.ysma@gmail.com>
Signed-off-by: Xingbo Jiang <xingbo.jiang@databricks.com>
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