Solve Unsupported PostgreSQL Statements That Start With 'c' for Shardingsphere Parser

The ShardingSphere parser engine can handle various SQL dialects such as MySQL, PostgreSQL, SQLServer, openGauss, and Oracle. Although the parser engine supports numerous PostgreSQL statements, some important PostgreSQL statements that start with ‘c’ require assistance in parsing and getting the SQLStatement (Java object). Therefore, this proposal aims to support those PostgreSQL statements for the parser engine. This will be achieved by: 1. Review the PostgreSQL SQL commands documentation and modify the target ANTLR g4 files (essential). 2. Implement the visit method (essential). 3. Add SQL cases and expected parse results (essential). 4. Run the associated tests to ensure no failures and exceptions (essential). Once the parser engine gets support for the target PostgreSQL statements, the users and developers of Apache ShardingSphere can be benefitted by working on these SQL statements.