support IF EXISTS in COMMENT statements

src/ast/mod.rs

@@ -1487,6 +1487,8 @@ pub enum Statement {
         object_type: CommentObject,
         object_name: ObjectName,
         comment: Option<String>,
+        /// An optional `IF EXISTS` clause. (Non-standard.)
+        if_exists: bool,
     },
     /// `COMMIT [ TRANSACTION | WORK ] [ AND [ NO ] CHAIN ]`
     Commit { chain: bool },
@@ -2637,8 +2639,13 @@ impl fmt::Display for Statement {
                 object_type,
                 object_name,
                 comment,
+                if_exists,
             } => {
-                write!(f, "COMMENT ON {object_type} {object_name} IS ")?;
+                write!(f, "COMMENT ")?;
+                if *if_exists {
+                    write!(f, "IF EXISTS ")?
+                };
+                write!(f, "ON {object_type} {object_name} IS ")?;
                 if let Some(c) = comment {
                     write!(f, "'{c}'")
                 } else {

src/dialect/postgresql.rs

@@ -49,6 +49,8 @@ impl Dialect for PostgreSqlDialect {
 }
 
 pub fn parse_comment(parser: &mut Parser) -> Result<Statement, ParserError> {
+    let if_exists = parser.parse_keywords(&[Keyword::IF, Keyword::EXISTS]);
+
     parser.expect_keyword(Keyword::ON)?;
     let token = parser.next_token();
 
@@ -74,5 +76,6 @@ pub fn parse_comment(parser: &mut Parser) -> Result<Statement, ParserError> {
         object_type,
         object_name,
         comment,
+        if_exists,
     })
 }

tests/sqlparser_postgres.rs

@@ -1790,10 +1790,12 @@ fn parse_comments() {
             object_type,
             object_name,
             comment: Some(comment),
+            if_exists,
         } => {
             assert_eq!("comment", comment);
             assert_eq!("tab.name", object_name.to_string());
             assert_eq!(CommentObject::Column, object_type);
+            assert!(!if_exists);
         }
         _ => unreachable!(),
     }
@@ -1803,22 +1805,26 @@ fn parse_comments() {
             object_type,
             object_name,
             comment: Some(comment),
+            if_exists,
         } => {
             assert_eq!("comment", comment);
             assert_eq!("public.tab", object_name.to_string());
             assert_eq!(CommentObject::Table, object_type);
+            assert!(!if_exists);
         }
         _ => unreachable!(),
     }
 
-    match pg().verified_stmt("COMMENT ON TABLE public.tab IS NULL") {
+    match pg().verified_stmt("COMMENT IF EXISTS ON TABLE public.tab IS NULL") {
         Statement::Comment {
             object_type,
             object_name,
             comment: None,
+            if_exists,
         } => {
             assert_eq!("public.tab", object_name.to_string());
             assert_eq!(CommentObject::Table, object_type);
+            assert!(if_exists);
         }
         _ => unreachable!(),
     }