fix str_slice

Author: chelsea-lin

bigframes/core/compile/sqlglot/expressions/string_ops.py

@@ -366,10 +366,16 @@ def string_slice(
                     column_length + sge.convert(start + 1),
                 ]
             )
-            length_expr = sge.convert(op_end) - sge.Greatest(
+            length_expr = sge.Greatest(
                 expressions=[
                     sge.convert(0),
-                    column_length + sge.convert(start),
+                    sge.convert(op_end)
+                    - sge.Greatest(
+                        expressions=[
+                            sge.convert(0),
+                            column_length + sge.convert(start),
+                        ]
+                    ),
                 ]
             )
         else:

tests/unit/core/compile/sqlglot/expressions/snapshots/test_string_ops/test_str_slice/out.sql

@@ -13,6 +13,6 @@ SELECT
   SUBSTRING(
     `string_col`,
     GREATEST(1, LENGTH(`string_col`) + -2),
-    5 - GREATEST(0, LENGTH(`string_col`) + -3)
+    GREATEST(0, 5 - GREATEST(0, LENGTH(`string_col`) + -3))
   ) AS `m3_5`
 FROM `bigframes-dev`.`sqlglot_test`.`scalar_types` AS `bft_0`