Feat: include comments in gen (#4535)

sqlglot/optimizer/simplify.py

@@ -1328,28 +1328,35 @@ def _flat_simplify(expression, simplifier, root=True):
     return expression
 
 
-def gen(expression: t.Any) -> str:
+def gen(expression: t.Any, comments: bool = False) -> str:
     """Simple pseudo sql generator for quickly generating sortable and uniq strings.
 
     Sorting and deduping sql is a necessary step for optimization. Calling the actual
     generator is expensive so we have a bare minimum sql generator here.
+
+    Args:
+        expression: the expression to convert into a SQL string.
+        comments: whether to include the expression's comments.
     """
-    return Gen().gen(expression)
+    return Gen().gen(expression, comments=comments)
 
 
 class Gen:
     def __init__(self):
         self.stack = []
         self.sqls = []
 
-    def gen(self, expression: exp.Expression) -> str:
+    def gen(self, expression: exp.Expression, comments: bool = False) -> str:
         self.stack = [expression]
         self.sqls.clear()
 
         while self.stack:
             node = self.stack.pop()
 
             if isinstance(node, exp.Expression):
+                if comments and node.comments:
+                    self.stack.append(f" /*{','.join(node.comments)}*/")
+
                 exp_handler_name = f"{node.key}_sql"
 
                 if hasattr(self, exp_handler_name):

tests/test_optimizer.py

@@ -551,6 +551,10 @@ def test_simplify(self):
 SELECT :with,WITH :expressions,CTE :this,UNION :this,SELECT :expressions,1,:expression,SELECT :expressions,2,:distinct,True,:alias, AS cte,CTE :this,SELECT :expressions,WINDOW :this,ROW(),:partition_by,y,:over,OVER,:from,FROM ((SELECT :expressions,1):limit,LIMIT :expression,10),:alias, AS cte2,:expressions,STAR,a + 1,a DIV 1,FILTER("B",LAMBDA :this,x + y,:expressions,x,y),:from,FROM (z AS z:joins,JOIN :this,z,:kind,CROSS) AS f(a),:joins,JOIN :this,a.b.c.d.e.f.g,:side,LEFT,:using,n,:order,ORDER :expressions,ORDERED :this,1,:nulls_first,True
 """.strip(),
         )
+        self.assertEqual(
+            optimizer.simplify.gen(parse_one("select item_id /* description */"), comments=True),
+            "SELECT :expressions,item_id /* description */",
+        )
 
     def test_unnest_subqueries(self):
         self.check_file("unnest_subqueries", optimizer.unnest_subqueries.unnest_subqueries)