Implements DROP POLICY syntax for PostgreSQL

```SQL
DROP POLICY [ IF EXISTS ] name ON table_name [ CASCADE | RESTRICT ]
```

For the documentation, please refer:

https://www.postgresql.org/docs/current/sql-createpolicy.html

This closes apache#1404.

Author: git-hulk

src/ast/mod.rs

@@ -2546,6 +2546,16 @@ pub enum Statement {
         name: Ident,
         storage_specifier: Option<Ident>,
     },
+    ///```sql
+    /// DROP POLICY
+    /// ```
+    /// See [PostgreSQL](https://www.postgresql.org/docs/current/sql-droppolicy.html)
+    DropPolicy {
+        if_exists: bool,
+        name: Ident,
+        table_name: ObjectName,
+        option: Option<ReferentialAction>,
+    },
     /// ```sql
     /// DECLARE
     /// ```
@@ -4258,6 +4268,22 @@ impl fmt::Display for Statement {
                 }
                 Ok(())
             }
+            Statement::DropPolicy {
+                if_exists,
+                name,
+                table_name,
+                option,
+            } => {
+                write!(f, "DROP POLICY")?;
+                if *if_exists {
+                    write!(f, " IF EXISTS")?;
+                }
+                write!(f, " {name} ON {table_name}")?;
+                if let Some(option) = option {
+                    write!(f, " {option}")?;
+                }
+                Ok(())
+            }
             Statement::Discard { object_type } => {
                 write!(f, "DISCARD {object_type}")?;
                 Ok(())

src/parser/mod.rs

@@ -4887,6 +4887,8 @@ impl<'a> Parser<'a> {
             ObjectType::Stage
         } else if self.parse_keyword(Keyword::FUNCTION) {
             return self.parse_drop_function();
+        } else if self.parse_keyword(Keyword::POLICY) {
+            return self.parse_drop_policy();
         } else if self.parse_keyword(Keyword::PROCEDURE) {
             return self.parse_drop_procedure();
         } else if self.parse_keyword(Keyword::SECRET) {
@@ -4928,38 +4930,56 @@ impl<'a> Parser<'a> {
         })
     }
 
+    fn parse_optional_referential_action(&mut self) -> Option<ReferentialAction> {
+        match self.parse_one_of_keywords(&[Keyword::CASCADE, Keyword::RESTRICT]) {
+            Some(Keyword::CASCADE) => Some(ReferentialAction::Cascade),
+            Some(Keyword::RESTRICT) => Some(ReferentialAction::Restrict),
+            _ => None,
+        }
+    }
+
     /// ```sql
     /// DROP FUNCTION [ IF EXISTS ] name [ ( [ [ argmode ] [ argname ] argtype [, ...] ] ) ] [, ...]
     /// [ CASCADE | RESTRICT ]
     /// ```
     fn parse_drop_function(&mut self) -> Result<Statement, ParserError> {
         let if_exists = self.parse_keywords(&[Keyword::IF, Keyword::EXISTS]);
         let func_desc = self.parse_comma_separated(Parser::parse_function_desc)?;
-        let option = match self.parse_one_of_keywords(&[Keyword::CASCADE, Keyword::RESTRICT]) {
-            Some(Keyword::CASCADE) => Some(ReferentialAction::Cascade),
-            Some(Keyword::RESTRICT) => Some(ReferentialAction::Restrict),
-            _ => None,
-        };
+        let option = self.parse_optional_referential_action();
         Ok(Statement::DropFunction {
             if_exists,
             func_desc,
             option,
         })
     }
 
+    /// ```sql
+    /// DROP POLICY [ IF EXISTS ] name ON table_name [ CASCADE | RESTRICT ]
+    /// ```
+    ///
+    /// [PostgreSQL Documentation](https://www.postgresql.org/docs/current/sql-droppolicy.html)
+    fn parse_drop_policy(&mut self) -> Result<Statement, ParserError> {
+        let if_exists = self.parse_keywords(&[Keyword::IF, Keyword::EXISTS]);
+        let name = self.parse_identifier(false)?;
+        self.expect_keyword(Keyword::ON)?;
+        let table_name = self.parse_object_name(false)?;
+        let option = self.parse_optional_referential_action();
+        Ok(Statement::DropPolicy {
+            if_exists,
+            name,
+            table_name,
+            option,
+        })
+    }
+
     /// ```sql
     /// DROP PROCEDURE [ IF EXISTS ] name [ ( [ [ argmode ] [ argname ] argtype [, ...] ] ) ] [, ...]
     /// [ CASCADE | RESTRICT ]
     /// ```
     fn parse_drop_procedure(&mut self) -> Result<Statement, ParserError> {
         let if_exists = self.parse_keywords(&[Keyword::IF, Keyword::EXISTS]);
         let proc_desc = self.parse_comma_separated(Parser::parse_function_desc)?;
-        let option = match self.parse_one_of_keywords(&[Keyword::CASCADE, Keyword::RESTRICT]) {
-            Some(Keyword::CASCADE) => Some(ReferentialAction::Cascade),
-            Some(Keyword::RESTRICT) => Some(ReferentialAction::Restrict),
-            Some(_) => unreachable!(), // parse_one_of_keywords does not return other keywords
-            None => None,
-        };
+        let option = self.parse_optional_referential_action();
         Ok(Statement::DropProcedure {
             if_exists,
             proc_desc,

tests/sqlparser_common.rs

@@ -11089,3 +11089,44 @@ fn test_create_policy() {
         "sql parser error: Expected: (, found: EOF"
     );
 }
+
+#[test]
+fn test_drop_policy() {
+    let sql = "DROP POLICY IF EXISTS my_policy ON my_table RESTRICT";
+    match all_dialects().verified_stmt(sql) {
+        Statement::DropPolicy {
+            if_exists,
+            name,
+            table_name,
+            option,
+        } => {
+            assert_eq!(if_exists, true);
+            assert_eq!(name.to_string(), "my_policy");
+            assert_eq!(table_name.to_string(), "my_table");
+            assert_eq!(option, Some(ReferentialAction::Restrict));
+        }
+        _ => unreachable!(),
+    }
+
+    // omit IF EXISTS is allowed
+    all_dialects().verified_stmt("DROP POLICY my_policy ON my_table CASCADE");
+    // omit option is allowed
+    all_dialects().verified_stmt("DROP POLICY my_policy ON my_table");
+
+    // missing table name
+    assert_eq!(
+        all_dialects()
+            .parse_sql_statements("DROP POLICY my_policy")
+            .unwrap_err()
+            .to_string(),
+        "sql parser error: Expected: ON, found: EOF"
+    );
+    // Wrong option name
+    assert_eq!(
+        all_dialects()
+            .parse_sql_statements("DROP POLICY my_policy ON my_table WRONG")
+            .unwrap_err()
+            .to_string(),
+        "sql parser error: Expected: end of statement, found: WRONG"
+    );
+}