chore: fix datafusion-spark substring

datafusion/spark/src/function/string/substring.rs

@@ -159,15 +159,17 @@ fn spark_substring(args: &[ArrayRef]) -> Result<ArrayRef> {
 /// - Zero start: treated as 1
 /// - Negative start: counts from end of string
 ///
-/// Returns the converted 1-based start position for use with `get_true_start_end`.
+/// The result may be `<= 0` when a negative start lands before the string
+/// (e.g. `start=-10` on a 3-char string gives `-6`). Such values are passed
+/// through to `get_true_start_end`, which clamps them and yields an empty
+/// slice — matching Spark's behavior for out-of-range negative positions.
 #[inline]
 fn spark_start_to_datafusion_start(start: i64, len: usize) -> i64 {
     if start >= 0 {
         start.max(1)
     } else {
         let len_i64 = i64::try_from(len).unwrap_or(i64::MAX);
-        let start = start.saturating_add(len_i64).saturating_add(1);
-        start.max(1)
+        start + len_i64 + 1
     }
 }
 

datafusion/sqllogictest/test_files/spark/string/substring.slt

@@ -146,7 +146,7 @@ ark SQL
 (empty)
 (empty)
 (empty)
-Spa
+(empty)
 NULL
 NULL
 NULL
@@ -197,7 +197,14 @@ ark SQL
 (empty)
 (empty)
 (empty)
-Spa
+(empty)
 NULL
 NULL
 NULL
+
+query T
+SELECT substr(column1, -10, 3)
+FROM VALUES
+('abc'::string)
+----
+(empty)