PostgreSQL: support right-deep join chains (deferred ON clauses)

PostgreSQL accepts `t0 JOIN t1 JOIN t2 ON c1 ON c2` where ON clauses
follow all JOIN keywords, associating right-to-left. Enable this by
overriding supports_left_associative_joins_without_parens() to false
for PostgresqlDialect, matching Snowflake's existing behavior.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

Author: moshap-firebolt

src/dialect/postgresql.rs

@@ -290,6 +290,12 @@ impl Dialect for PostgreSqlDialect {
         true
     }
 
+    /// PostgreSQL supports right-deep join chains: `t0 JOIN t1 JOIN t2 ON c1 ON c2`
+    /// See: <https://www.postgresql.org/docs/current/queries-table-expressions.html#QUERIES-JOIN>
+    fn supports_left_associative_joins_without_parens(&self) -> bool {
+        false
+    }
+
     /// Postgres supports `NOTNULL` as an alias for `IS NOT NULL`
     /// See: <https://www.postgresql.org/docs/17/functions-comparison.html>
     fn supports_notnull_operator(&self) -> bool {

tests/sqlparser_postgres.rs

@@ -9261,3 +9261,20 @@ fn parse_lock_table() {
         }
     }
 }
+
+#[test]
+fn parse_right_deep_join_chain() {
+    // PostgreSQL supports right-deep join syntax where ON clauses follow all JOIN keywords:
+    //   t0 JOIN t1 JOIN t2 ON c1 ON c2
+    // which is equivalent to (and serialized as) t0 JOIN (t1 JOIN t2 ON c1) ON c2.
+    pg().one_statement_parses_to(
+        "SELECT * FROM t0 INNER JOIN t1 INNER JOIN t2 ON true ON true",
+        "SELECT * FROM t0 INNER JOIN (t1 INNER JOIN t2 ON true) ON true",
+    );
+    pg().one_statement_parses_to(
+        "SELECT * FROM t0 INNER JOIN t1 INNER JOIN t2 INNER JOIN t3 ON true ON true ON true",
+        "SELECT * FROM t0 INNER JOIN (t1 INNER JOIN (t2 INNER JOIN t3 ON true) ON true) ON true",
+    );
+    // NATURAL JOIN followed by a constrained join must stay left-associative.
+    pg().verified_stmt("SELECT * FROM t0 NATURAL JOIN t1 INNER JOIN t2 ON true");
+}