introduce dialect nested joins flag

Author: Dimchikkk

src/dialect/generic.rs

@@ -52,6 +52,10 @@ impl Dialect for GenericDialect {
         true
     }
 
+    fn supports_left_associative_joins_without_parens(&self) -> bool {
+        true
+    }
+
     fn supports_connect_by(&self) -> bool {
         true
     }

src/dialect/mod.rs

@@ -278,6 +278,34 @@ pub trait Dialect: Debug + Any {
         false
     }
 
+    /// Indicates whether the dialect supports left-associative join parsing
+    /// by default when parentheses are omitted in nested joins.
+    ///
+    /// Most dialects (like MySQL or Postgres) assume **left-associative** precedence,
+    /// so a query like:
+    ///
+    /// ```sql
+    /// SELECT * FROM t1 NATURAL JOIN t5 INNER JOIN t0 ON ...
+    /// ```
+    /// is interpreted as:
+    /// ```sql
+    /// ((t1 NATURAL JOIN t5) INNER JOIN t0 ON ...)
+    /// ```
+    /// and internally represented as a **flat list** of joins.
+    ///
+    /// In contrast, some dialects (e.g. **Snowflake**) assume **right-associative**
+    /// precedence and interpret the same query as:
+    /// ```sql
+    /// (t1 NATURAL JOIN (t5 INNER JOIN t0 ON ...))
+    /// ```
+    /// which results in a **nested join** structure in the AST.
+    ///
+    /// If this method returns `false`, the parser must build nested join trees
+    /// even in the absence of parentheses to reflect the correct associativity
+    fn supports_left_associative_joins_without_parens(&self) -> bool {
+        true
+    }
+
     /// Returns true if the dialect supports the `(+)` syntax for OUTER JOIN.
     fn supports_outer_join_operator(&self) -> bool {
         false

src/dialect/snowflake.rs

@@ -283,6 +283,10 @@ impl Dialect for SnowflakeDialect {
         true
     }
 
+    fn supports_left_associative_joins_without_parens(&self) -> bool {
+        false
+    }
+
     fn is_reserved_for_identifier(&self, kw: Keyword) -> bool {
         // Unreserve some keywords that Snowflake accepts as identifiers
         // See: https://docs.snowflake.com/en/sql-reference/reserved-keywords

src/parser/mod.rs

@@ -12486,7 +12486,11 @@ impl<'a> Parser<'a> {
                 };
                 let mut relation = self.parse_table_factor()?;
 
-                if dialect_of!(self is SnowflakeDialect) && self.peek_parens_less_nested_join() {
+                if !self
+                    .dialect
+                    .supports_left_associative_joins_without_parens()
+                    && self.peek_parens_less_nested_join()
+                {
                     let joins = self.parse_joins()?;
                     relation = TableFactor::NestedJoin {
                         table_with_joins: Box::new(TableWithJoins { relation, joins }),

tests/sqlparser_common.rs

@@ -15359,7 +15359,8 @@ fn check_enforced() {
 
 #[test]
 fn join_precedence() {
-    all_dialects_except(|d| d.is::<SnowflakeDialect>()).verified_query_with_canonical(
+    all_dialects_except(|d| !d.supports_left_associative_joins_without_parens())
+        .verified_query_with_canonical(
         "SELECT *
          FROM t1
          NATURAL JOIN t5
@@ -15368,7 +15369,7 @@ fn join_precedence() {
         // canonical string without parentheses
         "SELECT * FROM t1 NATURAL JOIN t5 INNER JOIN t0 ON (t0.v1 + t5.v0) > 0 WHERE t0.v1 = t1.v0",
     );
-    TestedDialects::new(vec![Box::new(SnowflakeDialect {})]).verified_query_with_canonical(
+    all_dialects_except(|d| d.supports_left_associative_joins_without_parens()).verified_query_with_canonical(
         "SELECT *
          FROM t1
          NATURAL JOIN t5