Create table with nullable column for default (#2256)

Change the default column null property to nullable

docs/documentation/cn/getting-started/basic-usage.md

@@ -228,6 +228,7 @@ MySQL> DESC table2;
 > 3. 数据导入可以导入指定的 Partition。详见 `HELP LOAD`。
 > 4. 可以动态修改表的 Schema。
 > 5. 可以对 Table 增加上卷表（Rollup）以提高查询性能，这部分可以参见高级使用指南关于 Rollup 的描述。
+> 6. 表的列的Null属性默认为true，会对查询性能有一定的影响。
 
 ### 2.4 导入数据
 

docs/documentation/en/getting-started/basic-usage_EN.md

@@ -229,6 +229,7 @@ MySQL> DESC table2;
 > 3. Data import can import the specified Partition. See `HELP LOAD'.
 > 4. Schema of table can be dynamically modified.
 > 5. Rollup can be added to Table to improve query performance. This section can be referred to the description of Rollup in Advanced Usage Guide.
+> 6. The default value of Null property for column is true, which may result in poor scan performance.
 
 ### 2.4 Import data
 
@@ -371,4 +372,4 @@ MySQL> SELECT SUM(pv) FROM table2 WHERE siteid IN (SELECT siteid FROM table1 WHE
 |         8 |
 +-----------+
 1 row in set (0.13 sec)
-```
+```

fe/src/main/cup/sql_parser.cup

@@ -1702,7 +1702,7 @@ column_definition ::=
 
 opt_is_allow_null ::=
     {:
-        RESULT = false;
+        RESULT = true;
     :}
     | KW_NULL
     {: