fix(resolver): correctly resolve unnest alias shadowing for BigQuery

sqlglot/optimizer/resolver.py

@@ -305,6 +305,21 @@ def _get_unambiguous_columns(
             # Performance optimization - avoid copying first_columns if there is only one table.
             return SingleValuedMapping(first_columns, first_table)
 
+        # For BigQuery UNNEST_COLUMN_ONLY, build a mapping of original UNNEST aliases
+        # from alias.columns[0] to their source names. This is used to resolve shadowing
+        # where an UNNEST alias shadows a column name from another table.
+        unnest_original_aliases: t.Dict[str, str] = {}
+        if self.dialect.UNNEST_COLUMN_ONLY:
+            unnest_original_aliases = {
+                alias_arg.columns[0].name: source_name
+                for source_name, source in self.scope.sources.items()
+                if (
+                    isinstance(source.expression, exp.Unnest)
+                    and (alias_arg := source.expression.args.get("alias"))
+                    and alias_arg.columns
+                )
+            }
+
         unambiguous_columns = {col: first_table for col in first_columns}
         all_columns = set(unambiguous_columns)
 
@@ -314,6 +329,10 @@ def _get_unambiguous_columns(
             all_columns.update(columns)
 
             for column in ambiguous:
+                if column in unnest_original_aliases:
+                    unambiguous_columns[column] = unnest_original_aliases[column]
+                    continue
+
                 unambiguous_columns.pop(column, None)
             for column in unique.difference(ambiguous):
                 unambiguous_columns[column] = table

tests/test_optimizer.py

@@ -1917,6 +1917,30 @@ def test_struct_annotation_bigquery(self):
 
         assert annotated.selects[0].type == exp.DataType.build("VARCHAR")
 
+    def test_bigquery_unnest_alias_shadowing(self):
+        """Test that BigQuery UNNEST table alias shadows column names from other tables."""
+        sql = """
+            SELECT timeline_date
+            FROM UNNEST(GENERATE_DATE_ARRAY('2020-01-01', '2020-01-03')) AS timeline_date
+            LEFT JOIN production_tier ON production_tier.timeline_date = timeline_date
+        """
+        schema = {"production_tier": {"timeline_date": "DATE", "id": "INT"}}
+
+        result = optimizer.qualify.qualify(
+            parse_one(sql, dialect="bigquery"),
+            schema=schema,
+            dialect="bigquery",
+        )
+
+        result_sql = result.sql(dialect="bigquery")
+        self.assertEqual(
+            result_sql,
+            "SELECT `timeline_date` AS `timeline_date` "
+            "FROM UNNEST(GENERATE_DATE_ARRAY('2020-01-01', '2020-01-03', INTERVAL '1' DAY)) AS `timeline_date` "
+            "LEFT JOIN `production_tier` AS `production_tier` "
+            "ON `production_tier`.`timeline_date` = `timeline_date`",
+        )
+
     def test_annotate_object_construct(self):
         sql = "SELECT OBJECT_CONSTRUCT('foo', 'bar', 'a b', 'c d') AS c"