allow DateTimeField::Custom with EXTRACT in Postgres (#1394)

src/dialect/generic.rs

@@ -78,4 +78,12 @@ impl Dialect for GenericDialect {
     fn support_map_literal_syntax(&self) -> bool {
         true
     }
+
+    fn allow_extract_custom(&self) -> bool {
+        true
+    }
+
+    fn allow_extract_single_quotes(&self) -> bool {
+        true
+    }
 }

src/dialect/mod.rs

@@ -499,6 +499,16 @@ pub trait Dialect: Debug + Any {
     fn describe_requires_table_keyword(&self) -> bool {
         false
     }
+
+    /// Returns true if this dialect allows the `EXTRACT` function to words other than [`Keyword`].
+    fn allow_extract_custom(&self) -> bool {
+        false
+    }
+
+    /// Returns true if this dialect allows the `EXTRACT` function to use single quotes in the part being extracted.
+    fn allow_extract_single_quotes(&self) -> bool {
+        false
+    }
 }
 
 /// This represents the operators for which precedence must be defined

src/dialect/postgresql.rs

@@ -154,6 +154,14 @@ impl Dialect for PostgreSqlDialect {
             Precedence::Or => OR_PREC,
         }
     }
+
+    fn allow_extract_custom(&self) -> bool {
+        true
+    }
+
+    fn allow_extract_single_quotes(&self) -> bool {
+        true
+    }
 }
 
 pub fn parse_comment(parser: &mut Parser) -> Result<Statement, ParserError> {

src/dialect/snowflake.rs

@@ -158,6 +158,14 @@ impl Dialect for SnowflakeDialect {
     fn describe_requires_table_keyword(&self) -> bool {
         true
     }
+
+    fn allow_extract_custom(&self) -> bool {
+        true
+    }
+
+    fn allow_extract_single_quotes(&self) -> bool {
+        true
+    }
 }
 
 /// Parse snowflake create table statement.

src/parser/mod.rs

@@ -1970,15 +1970,14 @@ impl<'a> Parser<'a> {
                 Keyword::TIMEZONE_HOUR => Ok(DateTimeField::TimezoneHour),
                 Keyword::TIMEZONE_MINUTE => Ok(DateTimeField::TimezoneMinute),
                 Keyword::TIMEZONE_REGION => Ok(DateTimeField::TimezoneRegion),
-                _ if dialect_of!(self is SnowflakeDialect | GenericDialect) => {
+                _ if self.dialect.allow_extract_custom() => {
                     self.prev_token();
                     let custom = self.parse_identifier(false)?;
                     Ok(DateTimeField::Custom(custom))
                 }
                 _ => self.expected("date/time field", next_token),
             },
-            Token::SingleQuotedString(_) if dialect_of!(self is SnowflakeDialect | GenericDialect) =>
-            {
+            Token::SingleQuotedString(_) if self.dialect.allow_extract_single_quotes() => {
                 self.prev_token();
                 let custom = self.parse_identifier(false)?;
                 Ok(DateTimeField::Custom(custom))