fix: explode_outer to unnest closes #2941

sqlglot/transforms.py

@@ -213,6 +213,16 @@ def new_name(names: t.Set[str], name: str) -> str:
                     is_posexplode = isinstance(explode, exp.Posexplode)
                     explode_arg = explode.this
 
+                    if isinstance(explode, exp.ExplodeOuter):
+                        explode_arg = exp.func(
+                            "IF",
+                            exp.func(
+                                "ARRAY_SIZE", exp.func("COALESCE", explode_arg, exp.Array())
+                            ).eq(0),
+                            exp.Array(expressions=[exp.null()]),
+                            explode_arg,
+                        )
+
                     # This ensures that we won't use [POS]EXPLODE's argument as a new selection
                     if isinstance(explode_arg, exp.Column):
                         taken_select_names.add(explode_arg.output_name)

tests/dialects/test_bigquery.py

@@ -620,6 +620,10 @@ def test_bigquery(self):
                 "spark": "WITH cte AS (SELECT ARRAY(1, 2, 3) AS arr) SELECT EXPLODE(arr) FROM cte"
             },
         )
+        self.validate_all(
+            "SELECT IF(pos = pos_2, col, NULL) AS col FROM UNNEST(GENERATE_ARRAY(0, GREATEST(ARRAY_LENGTH(IF(ARRAY_LENGTH(COALESCE([], [])) = 0, [NULL], []))) - 1)) AS pos CROSS JOIN UNNEST(IF(ARRAY_LENGTH(COALESCE([], [])) = 0, [NULL], [])) AS col WITH OFFSET AS pos_2 WHERE pos = pos_2 OR (pos > (ARRAY_LENGTH(IF(ARRAY_LENGTH(COALESCE([], [])) = 0, [NULL], [])) - 1) AND pos_2 = (ARRAY_LENGTH(IF(ARRAY_LENGTH(COALESCE([], [])) = 0, [NULL], [])) - 1))",
+            read={"spark": "select explode_outer([])"},
+        )
         self.validate_all(
             "SELECT AS STRUCT ARRAY(SELECT AS STRUCT b FROM x) AS y FROM z",
             write={